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And Abdominal-B in Drosqbhilu Melanogaster Copyright 0 1995 by the Genetics Society of America and Trans Interactions Between the iab Regulatory Regions and abdominal-A and Abdominal-B in Drosqbhilu melanogaster Jd Eileen Hendrickson and Shigeru Sakonju Department of Human Genetics, Howard Hughes Medical Institute, University of Utah, Salt Lake City, Utah 841 12 Manuscript received August 15, 1994 Accepted for publication November 8, 1994 ABSTRACT The infra-abdominal ( iab) elements in the bithorax complex of Drosophila melanogaster regulate the transcription of the homeotic genesabdominal-A ( abd-A) and Abdominal-B ( Abd-B) in cis. Here we describe two unusual aspects of regulation by the iab elements, revealed by an analysis of an unexpected comple- mentation between mutations in the Abd-B transcription unit and these regulatory regions. First,we find that iab-6 and iab7 can regulate Abd-B in trans. This iab trans regulation is insensitive to chromosomal rearrangements that disrupt transvection effects at the nearby Ubx locus. In addition, we show that a transposed Abd-B transcription unitand promoter on the Ychromosomecan be activatedby iabelements located on the third chromosome. These results suggest that the iab regions can regulate their target promoter located ata distant sitein the genomein a manner that is much less dependent on homologue pairing than other transvection effects.The iab regulatory regionsmay have a very strong affinity for the target promoter, allowing them to interactwith each other despite the inhibitory effectsof chromosomal rearrangements. Second, by generating abd-A mutations on rearrangement chromosomes that break in the iab-7 region, we show that these breaks induce theiab elements to switch their target promoter from Abd-B to abd-A. These two unusual aspects of iab regulation are related by the iab7 breakpoint chromo- somes that prevent iab elements from acting onAbd-B and allow them to act on abd-A. We propose that the iab-7 breaks prevent both iab trans regulation and target specificity by disrupting a mechanism that targets the iab regions to the Abd-B promoter. ROPER development of an organism requires precise other chromosome ( ZACHARet al. 1985; CASTELLI-GAIR P regulation of gene expression. Enhancer and si- et al. 1990; GEYERet al. 1990) . lencer elements provide one level of this regulation, The homeotic genes of the bithorax complex, Ubx, activating and suppressing, respectively, the transcrip- abd-A and Abd-B, specify segmental identitiesof overlap- tion of genes in precise temporal and spatial patterns. ping body segments of the fly from parasegments (PS) Enhancer/ silencer elements can regulate the activity 5 through PS 14, corresponding to the third thoracic of specific target promoters from a relatively long dis- (T3) throughthe ninth abdominal segment (A9) tance in cis on the same chromosome (for review see (LEWIS1978; DUNCAN 1987).Genetic analyses have re- ATCHISON1988) . In Drosophila melanogaster a genetic vealed two typesof complementation groups, lethal and phenomenon calledtransvection suggests thaten- nonlethal, within thebithorax complex (Figure 1; hancers and silencers can also regulate promoters in LEWIS1978; SANCHEZ-HERREROet al. 1985) . Lethal com- trans on a homologous chromosome (see JUDD 1988; plementation groups identify the Ubx, abd-A and Abd-B WU and GOLDBERC1989; PIRROTTA1990; TARTOFand transcription units that encode essential DNA-binding HENIKOFF1991; WU 1993 for reviews). This effect was proteins. Abd-Bcomprises two subfunctions, Abd-Bm and originally described for regulatory mutations for the Abd-Br, provided by four overlappingtranscripts encod- Ultrabithmax ( Ubx) gene (LEWIS1954). In a transvec- ing Abd-BI and Abd-BII proteins in PS10-13 and PS14, tion effect chromosomal rearrangements that disrupt respectively (CASANOVA et al. 1986; CASANOVA and pairing between homologous chromosomesenhance or WHITE 1987; DELORENZIet al. 1988; KUZIORA and suppress the phenotype caused by certain regulatory MCGINNIS1988; SANCHEZ-HERREROand CROSBY1988; mutations. Thus, it has been proposed that homologue CELNIKERet al. 1989; ZAVORTINK and SAKONJU1989; pairing allows interaction between a regulatory element DELORENZIand BIENZ1990; BOULETet al. 1991 ) . Non- on one chromosome and its target promoter on an- lethal complementation groups,which affect subsets of Ubx, abd-A and Abd-B expression domains, identify regu- We dedicate this paper to E. B. Lewis, who began it d, on the latory elements for Ubx, abd-A and Abd-B transcription of his 77th birthday. occasion units. The abx, bx, pbx and bxd elements regulate Ubx Corresponding author: Shigeru Sakonju, Howard Hughes Medical Institute, 5200 Eccles Institute, Bldg. 533, University of Utah, Salt transcriptionin PS 5 (T2/T3)and PS 6 (T3/A1) Lake City, UT 841 12. E-mail: [email protected] ( BEACHYet al. 1985; HOGNESSet al. 1985; WHITEand Genetics 139: 835-848 (February, 1995) 836 J. E. Hendrickson and S. Sakonju WILCOX1985) . iab-2, -3 and -4 elements regulate abd- 137 Cs source and crossing them to females carryingthe double A in PS7 (A2), PS8 (A3) and PS9 (A4), respectively mutation Cbx'Ubx'. Cbx'Ubx' heterozygotes show a dominant phenotype of warped and held out wings because Cbx' acts ( KARCH et al. 1990; MACIAS et al. 1990) iab-5-iab-7 ele- . to misexpress Ubx+ on the homologous chromosome via a ments regulate theAbd-B class A transcription unit that transvection effect (LEWIS1955; CABRERAet al. 1985; WHITE codes for the Abd-BI protein in PSlO (A5) through and AKAM 1985; WHITEand WILCOX1985; WTELLI-GAIRet al. PS13 (A8) (CELNIKERet al. 1990; BOULETet al. 1991; 1990). This transvection effect is disrupted by chromosomal SANCHEZ-HERRERO1991 ) . Although specific regulatory rearrangements that break once between Ubxand the centro- mere and once outsidethis region. We recoveredre- mutations affecting the expression of the class A Abd-B arrangement mutations on the Abd-BDi4chromosome that dis- transcriptin PS13 andthe three longer transcripts rupt transvection between Cbx' and Ubx+ by screening for (class B, C and y) that code for the Abd-BII protein Abd-Bfli4/ Cbx'Ubx' F1 progeny with wild-type wings. The re- in PS14 and PS15 have not been found, they are ex- arrangements were mapped by following segregationwith au- pected to identify iab8 and iab9, respectively. tosomal markers. Some mutations were more finely mapped by cytogenetic analysis of larval polytene salivary gland chro- Genetic and molecular evidence indicate thatreg- the mosomes. ulatory elements in these iab regions act in cis. When To isolate abd-A mutations on an iab7 breakpoint chromo- mutations mapping to the transcription units aremade some, i~b-7~~'/TM3 Sbmale flies, aged1-7 days, were irradi- heterozygous with mutationsin regulatory elements, ated with 4000 rad using a I3'Cs source. They were immedi- they partially fail to complement each other. Further- ately mated en masse to TM? Sb/ TM6 Tb 1-7-day-old virgin females. After 7 days the parents were removed. Individual more, expression patterns of Ubx, abd-A and Abd-B tran- i~b7~~'*/TM6 Tb F1 males were allowed to mate with abd- scripts are altered in embryos mutant for these regula- A['"/ TM6 Tb females. i~b7~~'*/TM6 TbF2 progeny from tory elements ( BEACHYet al. 1985; HOGNESSet al. 1985; vials containin no Tb' upae were crossed separately to abd- A MXI , abd-AMx', abd-AMyand abd-AflZ4to confirm that they WHITEand WILCOX 1985;CELNIKER et al. 1990; KARCH carried an abd-A mutation. From 9735 individual matings we et al. 1990; MACIAS et al. 1990; BOULETet al. 1991; Ski- recovered two abd-A mutations, abd-AP' and abdAF2. CHEZ-HERRERO1991 ) , and aspects of correct expression abd-AP' and abd-A.F2 are null for abd-A function based on patterns can be recapitulated by a lacZ reporter under the following criteria. Cuticle preparations of abd-AF'/ abd- the controlof these regulatory elements in transformed ~D24and abd-A./Xz/ ~bd-A')'~embryos show transformation of flies (SIMONet al. 1990; MULLERand BIENZ1991; QIAN ventral denticle belts towardAl, typical of abd-A mutants (Sh- et ul. 1991; BUSTURIAand BIENZ1993). CHEZ-HERREROet al. 1985). abd-AP' homozygotes also show transformations typical of abd-A mutants, but abd-A,'xz homozy- We describe in this paper an analysis of an unex- gotes show a transformation of denticle belts toward T3. abd- pected complementation between mutations in Abd-the AP2/ Ubx embryos show a transformation of A1 toward T3 B transcription unit andits regulatory regions.We show and die before hatching, but abd-AJX2/bx34' animals live to that the iab regulatory elements can regulate the target adulthood and show no obvious transformation. We suspect promoter intrans. Unlike previously described transvec- that the abd-APz deletion includes pbx and bxd but not bx. The deletion may include part of the Ubx promoter, causing tion effects, this trans regulation by the iab elements, an enhanced transvection from bxt that suppresses the b~'~' which we call ITR, is unusually resistant to disruption phenotype ( MARTINEZ-LABORDA et al. 1992). Analysis of abd- by chromosomal rearrangements. We alsoshow that A-"' or abd-AP2 chromosomes reveals no chromosomal abnor- chromosomal rearrangements that separate theiab ele- malities other than the parental i~b7~"inversions. Southern ments from their normal Abd-B target promoter cause blot analysis using fragmentsof genomic clonesL2235, L2255, L2265, L2279, R8004 and R8019 (KARCH et al. 1985) reveals abd-A to substitute for someAbd-B functions, mostlikely that abd-AJX' contains a deletion of 4-6 kb including the abd- by allowing the iab elements that normally act on Abd- A homebox and that abd-AP' is associated witha deletion with B to act on abd-A instead. the right endpoint between abd-A exons 2 and 4. Neither mutation contains lesions in the iab regions to the right of the abd-A transcriptional unit. Neither mutant expresses abd- MATERIALS AND METHODS A transcripts in embryos as assayed by whole mount in situ Fly stocks. Fly stocks were maintained at 25" in a humidi- hybridization.
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