Regulation of RNA Polymerase II Transcription

Home , Class II gene, Promoter activity, SON (gene)

Regulation of RNA polymerase II transcription

Ronny Drapkin, Alejandro Merino and Danny Reinberg

Robert Wood Johnson Medical School, University of Medicine and Dentistry of New Jersey, Piscataway, USA

Transcription initiation plays a central role in the regulation of gene expression. Exciting developments in the last year have furthered our understanding of the interactions between general transcription factors and how these factors respond to modulators of transcription.

Current Opinion in Cell Biology 1993, 5:469-476

Introduction TFIIJ. Formation of the DAB--polFEHJ complex, in the presence of each of four ribonucleoside triphosphates, Cellular growth and differentiation employ precise mech- enables RNAPII to clear the promoter region and initiate anisms to regulate the expression of various genes. One RNA synthesis from a specific start site [ 51. of the most rudimentary mechanisms for a cell to control The past year has seen intense activity aimed at elucidat- the functional levels of a protein is to modulate the lev- ing the molecular mechanisms underlying transcription els of mRNA encoding that polypeptide. It is therefore not initiation. In particular, the interactions that GTFs can surprising that most of the genetic programs that main- mediate, the GTF requirements for initiation, the role tain the cell in a constant state of flux mediate their effects of RNAPII phosphorylation, and the phenomenon of by impinging on mechanisms that control transcription antirepression in the process of activation have been initiation. the subject of many studies. These most recent develop- In contrast to prokaryotic RNA polymerase, eukaryotic ments are the focus of this review. enzymes require multiple accessory proteins to acquire promoter specificity. The synthesis of mRNA in eukary- otes is carried out by RNA polymerase II (RNAPII), a multisubunit enzyme that, to date, requires the presence Minicomplexes of seven auxiliary factors, commonly known as basal or general factors, for the accurate initiation of transcription Contrary to the dogma that all promoters require the full from class II promoters. These seven general transcrip- set of GTFs for basal transcription, the immunoglobulin tion factors (GTFs) are TFIIA, IIB, IID, IIE, IIF, IIH and heavy chain (IgH) gene promoter can be transcribed with IIJ. Early studies [l] demonstrated that formation of a sta- a subset of GTFs. Initially, Parvin et al [6] reported that ble initiation complex occurs through the highly ordered transcription from the IgH promoter was independent of assembly of the GTFs and RNAPII on class II promoter se- TFIIE. Recently, however, Parvin and Sharp [7**] found quences. TFIID is the only GTF shown to have sequence- that the 1gI-f promoter could be transcribed to high lev- specific DNA-binding activity. The binding of TFIID to the els by RNAPII in the presence of only TBP and TFIIEL TATA box, typically located 30 nucleotides upstream of The ability of this subset of GTFs to transcribe the IgH the transcription start site, constitutes the initial step in promoter appears to be dependent on the DNA being the formation of a transcription-competent complex [ 21. negatively supercoiled. When the DNA was relaxed or Mammalian TFIID is a multisubunit complex composed linearized, transcription from the IgH promoter required of the TATA-binding protein (TBP) and additional, tightly the entire array of GTFs [7**]. This observation appears complexed, TBP-associated factors (TAFs) [3,4]. TFIIA to be specific to the 1gH promoter, as transcription associates with TBP and stabilizes the DNA-TFIID com- from the adenovirus major late promoter (Ad-MLP) re- plex. TFIIB then enters the complex, resulting in the for- quired all the previously described factors, Independent mation of a TFIID, IIA, IIB (DAB) complex. This pre-initi- of the state of the DNA Kadonaga and colleagues [8] ation complex intermediate serves as the nucleation site have also analyzed factor requirements for transcription for the entry of the remaining GTFs and RNAPII. TFIIF of several Drosophila promoters as well as the Ad-MLP. mediates the entry of RNAPII into the complex, fol- Using TBP they found that a subset of promoters could lowed by the ordered association of TFIIE, TFIIH and be transcribed in the presence of TBP, TFIB, RNAPII and

Abbreviations Ad-MLP-adenovirus major late promoter; CTD-carboxyl-terminal domain; DAB-TFIID, HA, IIB complex; DNA-PK-DNA-dependent protein kinase; CTF-general transcription factor; IgH-immunoglobulin heavy chain; RNAP-RNA polymerase; SRB-suppressor of RNA polymerase B; TAF-TBP-associated factor; TBP-TATA-binding protein; topo-topoisomerase.

@ Current Biology Ltd ISSN 0955-0674 470 Nucleus and gene expression

the small subunit of TFIIF, RAP30. Recent studies in our was found that dTAF250 interacts directly with dTBP. Al- laboratory resulted in the following observations: first, the though Drosophila TAFllO cannot bind directly to dTBP, requirement for some basal factors may be a function it is tightly associated with dTAF250 and is also capable of the length of the transcript analyzed; second, whereas of interacting with the Spl activator. The human TAF250 minimal subsets of factors may be capable of mediating appears to be similar to the Drosophila TAF250 as it can transcription, this process is very inefficient, especially directly interact with TBP in vitro as well as in duo for transcripts exceeding 70 nucleotides in length. Pro- [I6*]. Interestingly, the cDNA encoding hTAF250 was duction of transcripts up to 70 nucleotides in length found to be identical to the cell cycle regulator CCGl. is extremely efficient; the complete set of factors was This intriguing observation suggests that hTAF250/CCGl capable of producing RNA molecules approaching the may be involved in regulation of cell cycle progression. number of template molecules. Interestingly, a subset Based on this result, one can speculate that TFIID is a of factors, namely TBP, TFIIB, RNAPII and TFIIH, was dynamic complex whose TAF composition varies during found to be sufficient for transcription and production different physiological stages. Zhou et al. [17*] contend of an RNA molecule 70 nucleotides in length. However, that, unlike dTAF110, hTAF125 also interacts directly with the extent of the reaction with this subset of factors hTBP. These results may not necessarily be controversial was less than 5% of that observed with all the fac- as they may be a consequence of differences between tors. The activity of this minicomplex, DB-polH, could species. Tanese et al. [4] demonstrated that there are not be demonstrated when analyzing for production of three human TAFs in the 100 kDa range. It is possible that RNA molecules 400 nucleotides in length. The existence hTAF125 is not the homologue of dTAFll0 but that one of a minicomplex capable of transcribing the Ad-MLP al- of the two remaining hTAFs in this range is. Interestingly, lowed us to analyze the effect of the other factors inde- it was demonstrated that the divergent amino terminus of pendently. It was found that TFIIE and the small subunit TBP is not required for the association of the TAFs. This of TFIIF, RAP30, could independently stimulate transcrip- implies that the evolutionarily conserved carboxyl ter- tion of the DB-polH minicomplex, while the large sub- minus is sufficient not only for DNA binding, but also for unit, RAP74, had no effect. Nonetheless, transcription of a the interaction with TAFs [ 18.1. 400 nucleotide RNAwas dependent on RAP74. These pre- liminary lindings agree with previous observations sug- One of the most exciting developments of the last year gesting a dual role for TFIIF in initiation and elongation has been the determination of the X-ray crystal structure [9,10]. Moreover, it was also observed that TFIIJ was not of TBP. The crystal structure of TBP-2 from Arubidopsis required for short transcript synthesis but drastically stim- thaliunu revealed a new, highly symmetrical DNA-bind- ulates transcription of longer RN& suggesting a role in ing fold resembling a saddle. The concave DNA-binding elongation. This finding is consistent with our failure to surface of the saddle is a curved antiparallel g-sheet, demonstrate TFIIJ commitment in template competition which may mediate specific and non-specific contacts assays (L Zawel, P Kumar, D Reinberg, unpublished data). with the DNA. Upon binding to DNA, the convex surface of the saddle is present for interaction with TAFs, Although these Iindings are exciting, the minicomplexes GTFs and other regulatory proteins. We look forward to may only be competent to transcribe when TBP is used future crystallographic studies that will further our under- as a source of TFIID. The presence of TAFs may expand standing of the DNA-TBP interaction, and explain how the requirement for seven GTFs. As TBP appears to al- TBP can accommodate the vast number of interactions ways be associated with TAFs in vivo, the significance of that biochemical studies have demonstrated [ 19.01. these observations is questionable. As previously mentioned, the association of RNAPR and the other GTFs on TATA-containing promoters requires the presence of a DAB complex. These observations have Advances in general transcription factor been extended to demonstrate that TFIIB can interact with three components of the basal transcription machin- interactions ery, namely TBP, RNAPII and the small subunit of TFIIF. Discrete domains in TFIIB mediate these protein-protein Chromatographic studies indicate that endogenous TFIID interactions and may help to anchor RNAPII to the pro- consists of a multiprotein complex containing TBP and moter. The amino terminus of TFIIB contains a putative associated factors [3,4,11,12]. Although some of these zinc finger motif and the carboxyl terminus contains two factors can be dissociated from the TFIID complex imperfect direct repeats and a putative amphipathic a- under conditions of high ionic strength, a number of helix, which is an important domain for contacting TBP. polypeptides, originally referred to as TAFs, remain tightly Specific residues mapping to the carboxyi terminus of bound and require denaturing conditions for removal the second direct repeat were found to be crucial for (3,131. This convention accommodates the notion that the interaction of TFIIB and RNAPII. Additionally, the under physiological conditions, some TAF proteins are interaction with the small subunit of TFIIF, was mapped always associated with TBP while others transiently in- to the amino terminus of TFIIB, which includes the zinc teract with the TFIID complex to regulate its function finger [20]. in response to physiological cues. Progress in this area has resulted in the isolation of two Drosophila cDNA McCracken and Greenblatt [21] have demonstrated that clones encoding dTAFll0 [14*=] and dTAF250 [15*]. RNAPII can interact with the small subunit of TFIIF. In ad- These studies support the initial concept of TAFs, as it dition, RNAPII has also been shown to interact with TFIIE Regulation of transcription by RNA polymerase II Drapkin, Merino and Reinberg 471

[9] and TFIIH [22] (Fig. 1). Recent studies suggest that ation complex [26], while RNAPIIO could be isolated the carboxy-terminal domain (CID) of yeast RNAPII in- from actively elongating complexes [27]. The conver- teracts with a large multisubunit complex containing TBP sion of RNAPIIA to RNAPIIO was found to occur before and a number of tightly associated polypeptides termed the formation of the first phosphodiester bond. Collec- suppressors of RNA polymerase B (SRBs; R Young, per- tively, these experiments are consistent with a model in sonal communication). SRB2 can interact directly with which RNAPIIA enters the preinitiation complex and sub- TBP, yet the signikance of this interaction and the func- sequent phosphotylation of the CTD displaces the poly- tion of SRBs remains unknown. Although yeast TBP was merase from the promoter and triggers elongation. This originally thought not to contain TAPS, future studies may model is also favored by the linding that TBP cannot perhaps reveal that SRBsare the functional equivalents of interact with RNAPIIO, but can associate with RNAPIIA TAFs in the yeast system. [28*], presumably an interaction that occurs within the preinitiation complex. Recent experiments have indicated that TFW can phosphorylate the CID. TFIIH (BTF2) activity co-purifies with Phosphorylation of the RNAPII CTD five polypeptides of 35,41,43,62 and 89 kDa and with a unique kinase activity that mediates the phosphorylation The CTD of the largest subunit of RNAPII is highly con- of RNAPII at the CTD [ 291. The TFIIH kinase activity is served and contains multiple tandem repeats of the con- greatly stimulated in the context of a complete preini- sensus sequence, Tyr-Ser-Pro-Thr-Ser-Pro-Ser. The num- tiation complex. In particular, TFIIE, whose association ber of these repeats is important for viability [23]. with the DAB-PolF complex precedes that of TFIIH, was Due to phosphorylation of the heptapeptide repeats found to stimulate the activity of TFIIH kinase both qual- in ZU%O,RNAPII can be resolved into two forms by gel itatively and quantitatively. The p62 subunit of TFIIH electrophoresis, RNAPIIA and RNAPIIO: RNAPIIO is the was recently cloned [30]. No putative kinase domains phosphorylated form and RNAPIIA is unphosphorylated are present in p62 but monoclonal antibodies directed [24]. The functional significance of CTD phosphorylation against p62 were found to inhibit CTD phosphorylation was first analyzed by Laybourn and Dahmus [ 251 who re- and transcription in vitro. ported that changes in CTD phosphorylation occur during the transcription cycle. Subsequent studies showed A very recent and exciting development has been the that RNAPIIA preferentially associates with the preiniti- cloning of the ~89 subunit of TFIM. Amino acid se-

Fig. 1. Schematic model representing the currently demonstrated interactions between CTFs and RNAPII. These numerous interactions help explain both the cooperativity in preinitiation complex formation and the formation of a transcription complex on TATA-less promoters. The ability of RNAPII to recognize the initiator and accurately initiate transcription is thought to be enhanced by interactions with the CTFs. Solid arrows represent stronger interactions than the dotted arrows. The tail of RNA polymerase represents the unphosphorylated form of the CTD. In the coming year, we look forward to replacing the arrows denoting interactions with numbers representing the strength of these interactions. 472 Nucleus and gene expression

quence analysis revealed that this polypeptide corre- CTF’s response to activation: the phenomenon sponds to the ERCC3 gene product, a presumed helicase of antirepressin and true activation implicated in the human DNA excision repair disorders xeroderma pigmentosum and Cockayne’s syndrome. In Transcription of class II genes is controlled by myriad fact, Schaeffer et al [31**] demonstrated that a highly protein factors that include specific DNA-binding pro- puriiied preparation of TFIIH contains an ATP-dependent teins analogous to those that bind within the operon DNA helicase activity, in addition to the CTD kinase ac- region in bacteria [42,43]. Some of these proteins pos- tivity. The ,relevance of this Ending is made compelling sess the ability to activate transcription. by the studies of Selby and Sancar [ 32**], who demonstrated that the Escbericbia coli mfd (mutation frequency Functionally, transcriptional activators bear two domains: decline) gene product functions as a transcription-repair a DNA-binding domain that confers specificity and an coupling factor (TRCF). TRCF functions by recognizing activator domain that directly or indirectly promotes and displacing RNAPs stalled at the DNA lesions in an transcription initiation 1441. Activation domains fall into ATP-dependent fashion. Furthermore, TRCF binds to the several broad classes, including proline-rich, glutamine- damage recognition subunit (UvrA) of the excision nu- rich and acidic. Of these, the acidic activators appear clease and stimulates repair of the transcribed strand. to be universal in that they can function in all eukary- Both TRCF and ERCC-3 are DNA-dependent ATPases, otes tested. While the mechanism of activation remains contain helicase motifs, and confer ultraviolet sensitivity unclear it is presumed that activators work by commu- when mutated. We anxiously await studies defining the nicating with and iniluencing the basal transcription ma- role of the TFIIH helicase in transcription. chinery [ 21. Direct and specific interactions between regulatory factors and some of the GTFs have been demon- Studies in the yeast Saccbaromyces cerevisiae have iden- strated. A direct interaction between an acidic activator tified factor b as the homologue of TFIIH [ 331. This fac- and TFIIB [45], TFIIH (j Greenblatt, D Reinberg, un- tor purifies as a heterotrimer with polypeptides of 85,73, published data) and TBP [46] has been demonstrated and 50 kDa and also contains a CID-specific kinase activ- (Fig. 2). These results are made more compelling by the ity. The 73 kDa protein, TFBl, shows sequence similarity iinding that mutations in the acidic domain of an acti- with human p62 and, on this basis, is considered to be vator, which weaken or eliminate activation of transcrip- the homologue of ~62. Interestingly, an ATP-binding site tion, also reduce the extent of interaction with the par- has been identified in the 85 kDa protein, suggesting that ticular GTF [45-47]. Conversely, mutations on critical either this subunit contains a catalytic kinase domain or residues of TFIIB that crucially affect the interaction with a helicase activity homologous to human ~89. acidic activators (GAL4-AH, GAL4-VP16) abolish activated Numerous other kinases have been shown to phosphory- transcription, but only slightly decrease basal levels [ 481. late the CTD of RNAPII, suggesting that phosphorylation Despite these findings, physiological levels of activation of RNAPII at the CTD may indeed be a key regulatory cannot be demonstrated in a reconstituted transcription step in the initiation of transcription. A number of these system composed of an activator and the basal factors. putative CTD kinases, however, remain ill-defined, and It is becoming clear that in addition to activators, another some of these studies have not ruled out the possibil- class of factors exists that regulates gene expression by ity of TFIIH conramination [34,35]. Other investigators repression. Evidence for this class came from initial stud- have purified a kinase using a DNA-targeted CID sub- ies with histone Hl and chromatin. Kadonaga and co- strate and found that the DNA-dependent protein ki- workers [49] have shown that histone Hl interacts with nase (DNA-PK) can phosphorylate the CTD [36,37]. It naked DNA and renders it transcriptionally silent. Acti- is not surprising that such an abundant nuclear kinase vators Spl and GAL~-VPI~, among others, were shown can phosphorylate a DNA-bound substrate. Though the role of DNA-PK may be important in regulating certain to counteract Hl repression, a phenomenon now called antirepression [ 491. transcription factors, like Spl [38,39], its relevance in basal transcription remains speculative. Of novel inter- In vitro studies by Workman and colleagues [ 501 showed est, however, is the Ending that DNA-PK co-purifies with a that the presence of nucleosomes at the promoter pre- 350 kDa cataIytic subunit and the previously characterized cludes the binding of TBP to the TATA motif and inhibits Ku autoimmune antigen 137,381. other DNA-binding proteins from recognizing their re- The fact that the CTD can accommodate such a vast spective sites. Some activators, however, like the yeast number of interactions suggests that it may have addi- GAL.4, can bind to nucleosomal templates [50]. In fact, tional roles in gene expression [23]. Studies by Allison the activation domain of GAL4 can alleviate nucleosomal and Ingles [40] have alluded to a role for the CTD repression of promoters presumably by altering some as- in response to certain activators. The rapid cycling that pects of the chromatin architecture [50,51]. These data RNAPII undergoes during multiple rounds of transcrip- indicate that the acidic activation domains of activators tion suggests the existence of a phosphatase that can like GAL.4 stimulate transcription, in part, by enhancing modulate CTD phosphorylation. To date, none of the the ability of basal transcription factors to compete with seven GTFs is thought to contain a phosphatase activity. nucleosomes for occupancy of the promoter. However, Dahmus and co-workers [41] have identified A growing family of negative regulators is also thought such an activity in HeLa celI extracts. The functional sig- to mediate effects by interaction with the basal transcrip- nificance of this phosphatase in the recycling of RNAPII tion machinery. Some of these transcriptional repressors remains to be delineated. include the recently characterized TBP-binding factors Regulation of transcription by RNA polymerase II Drapkin, Merino and Reinberg 473

Fig. 2. Model for a competent transcription initiation complex at the promoter region. Solid arrows represent direct specific interaction between acidic activators and CTFs as determined by affinity columns. Dotted arrows indicate genetic evidence for adaptor molecules that bridge the interaction between TFIIB and acidic activators. inr, initiator.

Drl and Dr2 (A Merino, D Reinberg, unpublished data) is translocated to the elongation apparatus and functions [52-l and tl1 e previously described NC1 and NC2 frac- to remove superhelical torsion imposed by the elongat- tions [53,54]. Drl is a 19 kDa nuclear phosphoprotein ing ternary complex. The association of NCl, NC2 and that can specifically associate with TBP as a monomer, Dr2 with TBP represses transcription and, in the ab- dimer, trimer or tetramer. When Drl interacts with TBP sence of an activator, TFIIA is capable of overcoming as a monomer it precludes the association of TFIIB with this repression (A Merino, D Reinberg, unpublished data) TBP, but not that of TFIIA. This is presumably a result [53,54]. This finding agrees with results from Cortes et al. of Drl binding to the same site in TBP as TFIIB. When [55], who postulated that the hlnction of TFIIA is to re- Drl associates with TBP as a tetramer, both TFIIB and move negative components in the TFIID complex. From TFIIA are prevented from interacting with TBP. In each these and other studies, it is clear that the process of scenario, basal and activated transcription are repressed. transcriptional activation involves two independent, yet A search for activities capable of regulating Drl has re- inter-related processes. The first step involves removal sulted in the observation that the adenovirus ElA protein of factors that maintain genes transcriptionally silent, a and the SV40 large T antigen are capable of prevent- process known as antirepression. The second step rep- ing and/or displacing TBP-Drl interactions (J Nevins, D resents true activation where the levels of expression of Reinberg, personal communication). Similar observations a particular gene are increased well above basal levels. have been published by Meisterernst ef al. [53,54] de- In addition to antirepressing the effects of NC 1, NC2, Drl, scribing two negative regulators, NC1 and NC2 that also Dr2 and chromatin, some transcription activators must seem to interact directly with TBP and form a stable com- overcome more specific forms of repression. There are plex. The effect of NC1 and NC2 can be overcome by several examples of specific regulatory pathways: the nu- activators such a% USF and Spl. Recently, Merino et a/. clear exclusion of NF-xB by IxB (see Liou and Baltimore, purified Dr2 to homogeneity and found that it is iden- this issue pp 477487); competition for DNA binding be- tical to human DNA topoisomerase (topo) I. DrZ/topo I tween the WTl tumor suppressor and the EGR family of specifically associates with TBP and prevents DA complex activators [ 561; attenuation of the DNA-binding activity of formation (A Merino, D Reinberg, unpublished data). Im- MyoD by the inhibitorof DNA binding. Id, and by protein portantly, it was shown that the effect of Dr2 on basal kinase C phosphoqkition [ 57,581; or modulation of the transcription is independent of the DNA relaxation ac- transactivation domain of GAL-i by GAL80 [ 591. tivity of topo I. The hlnctional duality of Dr2/topo I fits well with a model in which Dr2/topo I, in the absence True activation is the ability of an activator to stimu- of activators, represses transcription by association with late transcription above levels conferred by antirepres- TBP. However, in the presence of activators, Dr?/topo I sion. Transcriptional activators, known to occupy spe- 474 Nucleus and gene expression

cific up&-em DNA elements, stimulate transcription in levels of activation is most likely because they only score viva & well as in v&m Biochemical as &ll as genetic for activation above basal levels and not above silent lev- evidence has postulated that such factors may mediate els. The exact role that repressors play during the three transcriptional activation by bridging the activator do- phases of transcription (silent, basal and activated) awaits mains with the general transcription machinery. Further future investigation in vim The coming year promises to studies have shown that excessive amounts of these spe- yield a rich harvest of information that will enhance our tic transcription factors decreased the activated levels understanding of the complex process of transcriptional of transcription, a phenomenon known as squelching regulation. [44]. It has been postulated that the non-DNA bound molecules of transcriptional activators may sequester factors required for optimal levels of activation from the promoter vicinity. Taking advantage of the squelching Acknowledgments phenomenon, Guarente and colleagues [6O] identiiied a gene in yeast, ADA.2 that can suppress the negative We thank Dr Ramin Shiekhattar and Leigh Zawel for comments on eifect of VP16 overexpression. ADAZ is speculated to the manuscript and Drs L Guarente, JM Egly, J Kadonaga, PA Sharp, R participate as a transcriptional adaptor for some acidic Young and R Tjian for communicating results before publication. We extend sincere apologies to those who, due to the enormous amount activators, such as GCN4 and GAL.~-V’PI~, but not the of material that had to be condensed, we failed to acknowledge. RD is acidic activator ~4 [6I*=]. Interestingly, suppressors supported by a NIH Training Grant in Molecular and Ceiktlar Biology of ADA.2 mutants have been isolated in the SUA-7 gene GM 08360. AM is the rapient of the Kirin Brewety Fellowship. DR [62], the yeast TFIIB homologue (R Klaus, L Guarente, is supported by the NIH and is the recipient of an American Cancer personal communication). This exciting iinding supports Society Faculty Research Award. the existence of a functional link between an activator and members of the basal transcription machinery (Fig. 2). References and recommended reading It has been observed that eukaryotic promoters contain a mosaic of regulatory DNA elements for several differ- Papers of particular interest, published within the annual period of ent activators/repressors. The effect of multiple regula- review, have been highlighted- - as: . of special interest tory proteins acting on the same promoter may lead to a . . of outstanding interest synergistic effect on transcription initiation. Such an effect could result if transcriptional activators act in the 1. BURATOWSIUS, HAHN S, GLIAREN~EL, SHARP PA Fiie Inter- mediate Complexes in Transcription Initiation by RNA same pathway, or in pathways that merge on a single Polymerase II. cell 1989, 56:549-561. tmnscriptional event. These pathways include alteration 2. ZAWEL L, REINBERGD: Initiation of Transcription by RNA of chromatin structure that permits transcription fac- Polymerase II: A Multi-Step Process. Prog Nucleic Acid Res tors to engage the promoter region, displacement of Mol Biol 1993, 44:67-108. general and speciIic negative regulators, and favorable 3. PUGH BF, TIJAN R: Transcription from TAT-Less Promoter interactions with co-activators and members of the basal Requires a Multisubunit TFIID Complex. Genes Dev 1991, transcription machinery to enhance transcription [ 631. 5:1935-1945. 4. TANESE N, PUGH BF, TJLAN R: Coactivators for a. RoIine- Rich Activator Purified from the Multisubunit Human TFIID. Genes Dev 1991, 5:2212-2224. 5. ZAWEL I+ REINBERG D: Advances in RNA PoIymerase II Transcription. Curr Opin Cell Biol 1992, 4488-495. Numerous advances during the last year have shed new 6. hRVlN DP, TIMMER~HTM, SHARPPA: Promoter SpeciIicity of Basal Transcription Factors. Cell 1992, 6R113~1144. insight into the mechanisms of transcription initiation. The developments summarized make it increasingly clear 7. PAtrvl~ DP, SHARPPA DNA Topology and a Minimal Set of . . Basal Factors for Transcription by RNA Polyrnerase II. Cell that the regulatory factors modulating this process en- 1993, in press. 73: 533- 540. tail a complex network of interactions between DNA, This article indicates that a supercoiled DNA plasmid containing the chromatin, GTFs, repressors, activators and coactivators. IgH promoter can be transcribed by a subset of GTFs, namely TFIIB, Although a number of factors have been identified that TBP and RNAPII. Antibodies against RAP30 (TFIIF) and p56 (TFIIE) repress basal transcription in vitro, the physiological rele- had no effect on transcription levels from the supercoiled IgH promoter but abolished transcription from the Ad-MLP. Moreover, independent vance of these repressors remains to be determined. It is omission of TFIIH and TFIIJ had no deleterious effects on transcription well established that the GTFs have the ability to direct from this promoter. However, when this template is linearized, ail seven basal transcription in vitro. However, their interactions GTFs are required for the accurate initiation of transcription from the in vivo are most likely limited by general and speciiic IgH promoter. negative regulators. The resulting transcriptionally silent 8. TYREE CM, GEORGECP, IJRA-D!wro LM, WAMPIERSL, DAHMUS phenotype of many genes in vivo can be overcome in ME, ZAWEL L, KAIXNAGA JT: A Minimal Set of Proteins that are SuIBcient for Accurate Initiation of Transcription by two steps. Certain activators can antirepress the effects RNA Polymerase II. Genes Dev 1993, in press. of the negative regulators, permitting basal levels of 9. FLORES0, MUDONADO E, RHNBERGD: Factors Involved in transcription. Other activators not only antirepress but Specific Transcription by MammaIia RNA Polymerase Il. also activate transcription above basal levels. The rea- Factors IIE and IIF Independently Interact with RNA Poly son why in vitro systems fail to simulate physiological merase Il. J Biol &em 1989, 264:8913-t3921. Regulation of transcription by RNA polymerase II Drapkin, Merino and Reinberg 475

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