Copyright 0 1989 by the Society of America

Duplication Mutation as anSOS Response in Escherichia coli: Enhanced Duplication Formationby a Constitutively Activated RecA

Joan Dimpfl and Department of , University of CaE$ornia, Berkeley, California 94720 Manuscript received March 10, 1989 Accepted for publication July 6, 1989

ABSTRACT The SOS response in Escherichia coli involves the induction of a multioperon regulatory system, which copes withthe presence of DNA lesions that interfere with DNA replication. Induction depends on activation of the RecA protein to cleave the LexA repressor of SOS . In addition to inducible DNA repair, the SOS system producesa large increasein the frequency of point mutations. To examine the possibility that other types of mutations are induced as partof the SOS response, we have studied the production of tandem duplications.To avoid the complications of indirect effectsof the DNA lesions, we have activated the SOS response by a constitutive mutation in the recA gene, recA730. The introduction of the recA730 mutation results in an increase in duplications in the range of tenfold or greater, as judged by two different criteria. Based on its genetic requirements, the pathway for induced duplication formation is distinct from the point mutation pathway and also differs from the major normal recombination pathway.The induction of pathways for both duplica- tions and point mutationsshows that the SOS system produces a broad mutagenic response.We have suggested previously that many typesof mutations might be inducedby severe environmental stress, thereby enhancing genetic variationin an endangered population.

HE introduction of a replication-inhibitinglesion deletions, translocations) (ECHOLS198 1 ; 1982; Mc- T (by UV light or a chemical mutagen) into the DONALD1984). Indeed, UV light and chemical mu- DNA of Escherichia coli results in the induction of a tagens increase the frequency of duplications (HILL multioperon regulatory system which rescues the bac- and COMBRIATO1973; HOFFMANNet al. 1985). To terialpopulation from its threatened demise (SOS investigate whether this duplication mutagenesis is a response)(RADMAN 1975; WITKIN 1976; ECHOLS direct consequence of SOS induction, we have exam- 198 1; WALKER 1984).The SOS response is normally ined the effect of a RecA protein constitutively acti- turned on by the activation of the RecA protein to vated by mutation for the SOS response in the absence mediate cleavage of the LexA repressor of SOS ope- of external treatment (RecA730). rons (LITTLEand MOUNT 1982). This activation is Tandem gene duplications are an importanttype of probablyaccomplished by the association of RecA mutation in E. coli, occurring at high frequency with single-strandedDNA (ROBERTSand DEVORET to lo-’ per cell) and often including large segments 1983), or distortions in double-stranded DNA gener- of the chromosome (ANDERSONand ROTH 1977; ated by the DNA lesions (Lu, SCHEUERMANNand PETESand HILL 1988). These frequent large dupli- ECHOLS1986; Lu and ECHOLS1987). Many of the cations are primarilyRecA-dependent events, with induced genes code for proteins involved in excision endpoints corresponding to regions of homology.Ac- and recombinational repair, but two genes, umuC and cording to one model, unequal crossover events at umuD, are specifically required for a large, SOS-de- regions of homology might lead to tandem copies of pendent increase in the frequency of point mutations the intervening genes, via RecA-mediated recombi- (WALKER1984, 1985). Thus, the SOS response can nation (ANDERSONand ROTH 1977; PETESand HILL be considered to have two general features favoring 1988). Duringthe emergency state ofSOS, additional survival of the stressed bacterial population: increased duplications might occurby this mechanism (e.g.,from capacity for DNA repair and enhanced geneticvaria- overproduction of RecA) or by others (e.g., recombi- tion (RADMAN 1980;ECHOLS 1981, 1982). national repair around lesions). We have studied the The genetic variation hypothesis for SOS mutagen- effect of a constitutive SOS response on duplications esis suggests that other types of mutations might also at several loci, using two measurementsbased on gene be inducible (e.g., tandem duplications,inversions, dosage. The first assay measures duplications of the glyS gene. The glyS gene lies within a segment of DNA The publication costs of this article were partly defrayed by the payment of page charges. This article must therefore be hereby marked “oduertisement” that is highly susceptible to duplication (approximately in accordance with 18 U.S.C. 91734 solely to indicate this fact. per cell), presumably because of two large ho-

Genetics 123: 255-260 (October, 1989) 256 J. Dimpfl and H. Echols Segregation analysis of duplications: These measure- ments were based on techniques developed for Salmonella

+ c (ANDERSON,MILLER and ROTH 1976). To remove selective rhsB e pressurefavoring glySL duplications,individual colonies weregrown at 37" inLB medium containing0.1 mM tryptophan for 10 to15 generations before plating logphase cultures for individualcolonies on the original selective medium lacking tryptophan. Similarly, to remove selective rhsB glyS xyl A > 13 glyS xyl rhsA rntl pressure favoring pur duplications, isolated colonies were grown at 37" in LB medium with 5 mM adenine. Loss of the duplication was assayed by replica plating to score loss FIGURE1 .-Map of the glyS region in Escherichia coli (after LIN, of either tetracycline resistanceor adenine prototrophy. CAPACEand HILL1984). The glyS gene is flanked by homologous regions B and A. Unequal crossover events between daughter chromosomes could result in tandem copies of glyS. RESULTS

mologous regions (rhsB and rhsA) flanking the 140- Induction of gZySL duplications by introduction of kbp segment (Figure 1) (LIN,CAPAGE and HILL 1984). recA730: Of the two assays used to measure duplica- We have found that thefrequency of glyS duplications tions, we have selected the glySL system as the method rises about ten-fold when a wild-type recA gene is ofchoice because the measurements arethe most replaced with the recA730 allele. We have also used a direct and have given the most consistent results. second type of measurement basedon transduction of Duplications of the glySL gene are recognized as large an insertion mutation into a recipient strain (ANDER- colonies after growth under low temperature condi- SON, MILLER and ROTH 1976). Using this assay, we tions. To determine the effect of a constitutive SOS confirmed the occurrenceof induced duplication for- response on the frequency of glySL duplication, we mation at the purC, purE and PurF genes. Based on introduced recA730 and sulA- (to prevent filamenta- additional genetic analysis, the induced pathway for tion) into the glySL trpA36 glyU (Sup) background duplication mutations appears to be distinct from the (GEORGE, CASTELLAZIand BUTTIN 1975; WITKIN et pathway for point mutationsand also differs from the al. 1982). We then compared the relative frequency major normal recombination pathway. of large colonies with that found in strains with wild- type recA after growth under selective plating condi- MATERIALS AND METHODS tions (Table 2). Strains with wild-type recA yielded a Bacterial strains: Bacterial strains are listed in Table 1 frequency of largecolonies of approximately 10-4, in with their genotypes. New strains were prepared using P1 agreement with previous reports (CAPAGEand HILL transduction of transposon insertionsin or near the gene of 1979; LIN, CAPAGEand HILL 1984). Two independ- interest (MILLER 1972). ent isolates containing recA730, however, produced a Assay of gZy& duplications: These measurements were ninefold increase in frequency of large colonies, ap- performed essentially as described (LIN,CAPAGE and HILL 1984). The selection method requires a cold-sensitive mu- proaching lo-' (Table 2, lines 2 and 3). From these tant glycyl-tRNA synthetasegene, glySL, in conjunction with data, we conclude that aconstitutively activated RecA trpA36 glyU (Sup) (FOLKand BERG 197 1; CAPAGEand HILL protein is sufficient for an increase in putative glyS 1979; LIN, CAPAGEand HILL1984). At 20", the glycine- duplication events, and that the introduction of rep- inserting missense suppressors are charged more efficiently lication-blocking lesions by external treatment is not in a glySL duplication mutant, leading tobetter growth and large colony formationin the absence of tryptophan. South- necessary for this increase. ern blot analysis has shownthat this selection methodis very Segregationexperiments: Large colonies repre- specific for duplications ofthe glySL gene (LIN,CAPAGE and senting possible glyS duplication events could also be HILL1984). In our assays, cells in log phase growth were due to mutations overcoming the inefficient suppres- plated at 20" and 37" on minimal plates supplementedwith sion of the trpA36 missense mutation (although the glucose, casamino acids and thiamine. At day 4, the 20" plates were examinedfor large colonies, representing poten- glySL test shows good correlation between large colony tial duplications of the glySL gene. The expression of the size and glySL duplications in a wild-type recA strain). SOS pathway was tested by a plaque assay for cleavage of For example, reversion of trpA36 to wild-type &PA, the X CZ repressor (MOUNT1977). mutations at the gZyU gene, or mutations increasing Assay of pur duplications: Duplications inpurine synthe- the intracellular levels of glycine could overcome this sis genes (purc,purE, purF) were assayed by transduction, as previously tested in Salmonella typhimurium (ANDERSON block to growth. Because the recA730 mutation gen- and ROTH 1981). A TnlO transposon insertion in a pur erates an SOS-constitutive response, there is an in- gene was transduced via Pluir phage; the frequency of crease in the frequency of point mutations (WITKIN adenine prototrophs in the tetracycline-resistant transduc- and KOGOMA1984). Therefore,in order to determine tants measured the frequencyofpur duplications preexisting whether largecolonies represent duplications or point in the recipients. For thermal inductionof the SOS response mutations, we performed segregation tests; duplica- in RecA441 strains, cells were grown at 42". As a control for TnlO stability, the reversion rate of pur- to pur' in all tion mutations are distinguished by their instability donor strains was found to be alO"O. when the selection pressurefor them is removed, D uplications as an as Duplications SOS Response 257

TABLE 1 E. coli K-12 strains

Strain designation Genotype Source or reference AT370 malE::Tn9 lexA51pyrD sulA::Tn5 lac pro strAthiendA sbcB15 K. PETERSON hsdR4 supE/F’ traD36 proAB + ladQ ram15 CH1504 trpA36glyU(Sup) glySLxyl tsx tyrT(Sup) LIN, CAPACEand HILL (1984) DE391 recA730 srlC300::TnlO lexA51 umuC122::TnZsupD- D. ENNIS DE463 same as DM2550, but recA730lexA51 D. ENNIS DE582 recA730 srlC300::TnlO lexA51umuD44 supD- D. ENNIS DM1187 his4 strA31 recA441 sulAIl lexA51 MOUNT (1977) NO54 sulA::Tn5 pyrD A(1ac-pro) his D. ENNIS DM2550 A(1ac-pro) strRmtl supD43 D. ENNIS GW 1000 recA441 sulAl I Alac leu thr arg his ile valBl BAGG,KENYON and WALKER(1 98 1) JClO990 recF::Tn? tna::TnlO (AB1 157 background) A. J. CLARK JCDlOOO. same as CH1504, but recA730 srlC300::TnlO sulA::Tn5 pyrD This laboratory JCD1041’ same asJCDlOOO, but sulA-pyrD+ umuC::Tn5 This laboratory JCD1057” same JCDlOOO,as but recF::Tn? This laboratory JCD1060d same asJCDlOOO, but malB::Tn9 lexA? This laboratory JM-1 thr leuthi lacy1 galK ara xylmtl proA his argE strBl tsx G. BUTTIN KP324 malB::Tn9 lexA3A(1ac-pro) rpsL sulA211 thi ara xy1 mtltsx ilv K. PETERSON ~~pD43/pSK192/pDE-FL54

a Constructed by P1 transduction of sulA::Tn5 from DM1790 and recA730 srlC300::TnlO from DE582 into CH 1504. ’ Constructed by P1 transduction of sulAl1 purD’ from GW1000 and umuC::Tn5 from DE391 into JCDlOOO. ‘ Constructed by P1 transduction of recF::Tn? from JClO990 into JCDlOOO. Constructed by P1 transduction of malB::Tn9 lexA? from KP324 into JCDlOOO.

TABLE 2 TABLE 3 Induction of putative glySL duplications by recA730 Segregation of gly& duplications

Frequency Percent of large segre tion of Relevant colonies Standard Relevant No. of unstable unstagecole Strain“ genotype (ZOO)* deviation Strain genotype colonies/total” nies, range

CH 1504 recA+glySL 8.6 X k4.2 X CH 1504 recA+glySL 5/13 8-90 JCD 1OOOA recA 730 glySL 7.7 X 10-4 k2.2 X 10-~ JCDlOOOA recA7?OglySL 911 5 14-99 JCDlOOOB recA730 glySL 7.8 X 10-4 k1.9 X 10-4 JCDlOOOB recA730glyS~ 9/14 20-94 1.1 X IO-’ k0.5 X lo-’ JCDl041 recA730glySLumuC Isolated colonies were grown non-selectively and segregants recA7?0glySL recF 1.9 X k0.4 X JCD1057 were identified as described in MATERIALS AND METHODS. JCDl060 recA730glySL lexA3 4.5 X k0.4 X Strains JCDlOOOA and JCDlOOOB represent independent iso- the constitutively activated RecA markedly increases lates derived from CH 1504. Values represent the average of 2 or 3 experiments; frequencies the frequency of duplication mutations. of large colonies were determined as described in MATERIALS AND Genes affecting induced duplication formation: METHODS. The SOS pathway for point mutations requires the umuC and umuD gene products (KATO and SHINOURA presumably because of homologousrecombination 1977; STEINBORN 1978; BAGG,KENYON and WALKER within the duplicated region (CAMPBELL1965; AN- 1981). We introduceda umuC mutationinto the DERSON and ROTH 1977). recA730 glySL strain; the frequency of large colony After the selective pressurefor glySL duplication mutants remained high (Table 2, line 4). Thus SOS- was removed, moreof the large colonies derived from induced duplication mutations are likely to involve a recA730 yielded progeny with an unstable phenotype different pathway fromthat responsible forpoint (60%) than the recA+-derived colonies (40%) (Table mutations. 3). These data indicate that at least the majority of Under normalconditions, the RecF pathway for the recA 730-derived large colonies represent duplica- recombination is not involved in most of the homol- tions rather than point mutations. The large colony ogous recombination in E. coli, which occurs by the phenotype segregates out with varying frequencies in RecBC pathway (CLARK1973; SMITH 1987). How- different isolates. Some of the large colonies with ever, genes of the RecF pathway are induced in the “stable”phenotypes could represent duplications SOS response (SHURVINTONand LLOYD1982; PICK- which have not formed segregants at detectable fre- SLEY 1984), an observation which indicates that the quency in the growthperiod provided. From the RecF pathway might have a special role under SOS results of Table 3, we conclude that the presence of conditions. To examine this possibility, we introduced 258 J. Dimpfl and H. Echols TABLE 4 Induction of putative pur duplications by recA730 or recA441

Frequency of tetRprototrophs” for

Strain Relevant Strain genotype purC purE purF

DM2550 recA+ lexA+ 3.9 f 0.8 x 4.6 k 2.7 X 6.0 f 3.0 X DM1187 lexA5IrecA44I 2.2 f 1.8 x 10-3 2.4 2 0.5 X IO-’ DE463 lexA5IrecA730 1.9 * 0.9 x 7.6 f 4.9 X 10-~ Each value represents the average of two or threeexperiments. a recF mutation intothe recA730 glySL strain; the of the tetracycline-resistant, pur+ transductants at frequency of large colony mutants dropped to a level thesetwo loci. It couldbe argued thatthe act of nearly that of recA+ (Table 2, line 5). We conclude generalized transduction itself induces duplications, that the RecF pathway is likely to be important for by providing DNA ends; however, there is a clear SOS-induced duplication. effect of constitutively activated recA alleles on dupli- We wanted to know whether thealtered RecA cation frequency compared to wild-type recA. Thus protein generated by the recA730 mutation was suffi- we conclude that the SOS-induced duplication muta- cient for enhanced duplication formation in the ab- genesis occurs over several regions of the E. coli ge- sence of derepression of SOS genes. For this purpose, nome. we introduced the lexA3 mutation, which produces an altered LexA repressor that cannot be cleaved by DISCUSSION activated RecA (LITTLE1983). The induced duplica- Mechanism for SOS-inducedduplications: Pre- tion formation was eliminated (Table 2, line 6). Thus, vious work hasdemonstrated that UV light and chem- SOS-induced gene(s)are required, consistent with the ical mutagens increase the frequency of duplications; possible involvement ofthe RecF pathway. However, the suggestedmechanism involved recombinational it is possible that only induced levels of RecA730 are repair around the lesion (HILL andCOMBRIATO 1973; needed. SOS-inducedlevels of wild-type RecA are HOFFMANNet al. 1985). The work reportedhere probably not sufficient, becauseintroduction of a lexA- shows that induced duplication formation is a direct inactivating mutation did not yield induced duplica- consequence of SOS induction. Our experiments also tion mutagenesis withthe pur system described below indicate that theSOS-induced pathwayfor duplication (data not shown). (The 2exA-defective experiment mutations is distinct from the pathway for point mu- could not be done in the gZrS, system because intro- tations (UmuDC). duction of the mutation causes a cold-sensitive phe- What are the special features of the SOS-induced notype by itself.) duplication pathway? One possible distinction arises Induced duplication formation for other regions from the previously defined properties of the RecA of the E. coli genome: To test for the general occur- protein. RecA binds very efficiently to single-strand rence of induced duplication mutations, weused a DNA and thenmediates heteroduplex formation with second assay to examine the formation of duplications a double-strand substrate (Cox and LEHMAN1987). involving the purC, purE, and purF genes. In this The initiating event in most normal bacterial recom- method, duplications are recognized as pur’ colonies bination is therefore probably the association of RecA in recipient bacteria following transduction of with a single-strand region of DNA, either presented pur::TnlOinsertion mutations (ANDERSONand ROTH directly (in transformation) or rendered locally single- 1981). For these experiments, two different recA al- strand by the actionof RecBC (COX and LEHMAN leles were used, the SOS constitutive recA730 and the 1987; SMITH1987). RecA protein does not normally thermally induciblerecA441. A lexA mutation was also associate efficiently withdouble-strand DNA, but can used to provide maximal SOS induction. For both be induced to do so by a UV lesion in the DNA or by strains with activated RecA, the frequency of tetra- mutational activation of RecA for SOS induction (Lu, cycline-resistant (TnlO), pur+ transductants increased SCHEUERMANNand ECHOLS 1986; Lu and ECHOLS markedly (Table 4). Segregation analyses showedthat 1987). Thus the special feature of induced duplication mostof the presumed purC and purF duplications formation might be the activation of a pathway for induced by recA730 or recA441 did indeed yield tet- recombination initiated by the association of RecA racycline-sensitive or adenine-requiring colonies (1 1 with double-strand DNA. A double-strand initiation of 13 for purC and 7 of 8 for purF ). The segregation pathway would likely not require RecBC, but is likely data argue that TnlO “hopping” induced by transduc- to require alternative proteins such as those of the tion (KLECKNERet aE. 1978) was not the major source RecF pathway. Duplications as an SOS Response 259 Biological significance of SOS-induced duplica- ANNTEMPLIN for various strains and advice on strain construction. tions: The enhanced frequency of point mutations We also thank RICHARDEISNER for editorial help, and CHARLES HILL and JOHN ROTH for helpful comments on the manuscript. associated with SOS induction can be considered from This research was supported in part by grants from the National two (not necessarily exclusive) points ofview. The Cancer Institute (CA 41655) and from the National Institute of UmuC/D mutagenic pathway is an important com- Environmental Health Sciences (ES 07075). ponent of aninduced cellular survival mechanism, and mutations are a “side-effect”; or, the UmuC/D LITERATURE CITED pathway exists as apopulation survival mechanism ANDERSON,R. P., C. G. MILLER and J. R. ROTH, 1976 Tandem that enhancesgenetic variation, increasing the duplications of the histidine observed following gener- chances for a fortunatevariant adapted to survive the alized transduction in Salmonella typhimurium. J. Mol. Biol. 105: environmental stress. The UmuC/D pathway contrib- 201-218. utes essentially all of the mutations and also contrib- ANDERSON,R. P., and J. R. ROTH, 1977 Tandem gene duplica- utes to survival, but to a small extent compared to tions in phage and bacteria. Annu. Rev. Microbiol. 31: 473- 505. error-free mechanisms (WALKER1984, 1985). The ANDERSON,R. P., andJ. R. ROTH, 1981 Spontaneoustandem genetic variation hypothesis and its possible wider genetic duplications in Salmonella typhimurium arise by unequal application has been considered in detail previously recombination between rRNA (rrn)cistrons. Proc. Natl. Acad. (ECHOLS198 1, 1982).The genetic variation hypoth- Sci. USA 78: 31 13-3 117. BAGG,A., C. J. KENYONand G. C. WALKER,1981 Inducibility of esis suggests that SOS mutagenesis might be expected a gene product required for UV and chemical mutagenesis in to include other types of mutations, such as duplica- Escherichia coli. Proc. Natl. Acad. Sci. USA 78: 5749-5753. tions andrearrangements, which might have pro- CAIRNS,J., J. OVERBAUGH andJ. MILLER, 1988 The origin of found phenotypic effects. In this work, we have dem- mutants. Nature 335: 142-145. onstrated that tandem duplications are indeed SOS CAMPBELL,A., 1965 The steric effect in lysogenization by bacte- inducible. riophage lambda. I. Lysogenization of a partially diploid strain of Escherichia coli K12. Virology 27: 329-339. Recent work has provided clear evidence formech- CAPAGE,M., and C. W. HILL, 1979 Preferential unequal recom- anisms that accelerate the appearance of adaptively bination in the glyS region of the Escherichia coli chromosome. favorablemutations (CAIRNS, OVERBAUCHand J. Mol. Biol. 127: 73-87. MILLER 1988; HALL 1988). These authors have sug- CLARK,A. J., 1973 Recombination deficient mutants of E. coli gested the exciting possibility that the favorable mu- and other bacteria. Annu. Rev. Genet. 7: 67-86. CONNOLLY,D. M., and M. E. WINKLER,1989 Genetic and phys- tations are somehow directed to the appropriatelocus. iological relationships between the miaA gene, ms2i6A-37tRNA The experiments clearly show spectacular increases in modification, and spontaneous mutagenesis in Escherichia coli mutation rate for certain mutations under conditions K-12. J. Bacteriol. (in press). of extreme starvation. However, the fact that SOS Cox, M. M., and J. R. LEHMAN,1987 Enzymes of general recom- mutagenesis is a broad spectrum inducible response bination. Annu. Rev. Biochem. 56 229-262. CROW,J. F., 1984 The P-factor: a transposable element in Dro- involving different types of mutations and pathways sophila, pp. 257-273 in Mutation,Cancer, and Malformation, leaves openthe possibility thata similar response edited by E. H. Y.CHU and W. M. GENEROSO.Plenum Press, might be operativein these systems, followed by selec- New York. tion of the fortunate variant. Certain classes of muta- ECHOLS,H., 1981 SOS functions, cancer, and inducible evolution. tions might increase in response to specific stress sig- Cell 25: 1-2. ECHOLS,H., 1982 Mutation rate: some biological and biochemical nals, rather than to a needy locus. Recently, evidence considerations. Biochimie 64: 571-575. has been presented for a mutational response coupled FOLK,W. R., and P. BERG,1971 Duplication of the structural to base modification of tRNA (CONNOLLYand WINK- gene for glycyl-transfer RNA synthetase in Escherichia coli. J. LER 1989). Mol. Biol. 58: 595-610. The possibility that inducible systems for genetic GEORGE,J., M. CASTELLAZIand G. BUTTIN, 1975 Prophage in- duction and cell division in E. coli. 111. Mutations sfiA and sfiB variation exist in higher organisms has been consid- restore division in tifand lon strains and permit the expression ered recently in view of evidence for inducible trans- of mutator properties oft$ Mol. Gen. Genet. 140 309-332. position systems (ECHOLS198 1, 1982; MCCLINTOCK HALL,B. G., 1988 Adaptive evolution thatrequires multiple 1984; MCDONALD1984; WINTERSBERCER1984). Al- spontaneous mutations. I. Mutations involving an insertion though the possible importance of such mechanisms sequence. Genetics 120 887-897. HILL, C. W., and G. C. COMBRIATO,1973 Genetic duplications for evolution has been questioned (CROW 1984), in- induced at very high frequency by ultraviolet irradiation in duction of a broad spectrum mutationalresponse does Escherichia coli. Mol. Gen. Genet. 127: 197-214. have the potential for accelerating evolutionary ad- HOFFMANN,G. R., L. S. CATOUGNO,J. F. LINNANEand L. A. aptation.Additional work on inducedmutation in PARENTE,1985 Effects of DNA-repair processes onthe in- higher organisms will obviously be required before duction of genetic duplications in bacteria by ultraviolet light. Mutat. Res. 151: 25-33. any definitive statements canbe madeon these points. KATO, T., and Y.SHINOURA, 1977 Isolation and characterization of mutants ofEscherichia coli deficient in induction of mutations We thank CHARLES HILLfor the mutant glyS strain and advice by ultraviolet light. Mol. Gen. Genet. 156: 121-131. on its use, and DON ENNIS,KEN PETERSON,SURESH MAHAJAN and KLECKNER, N., D.F. BARKER,D. G. Ross and D. BOTSTEIN, 260 J. Dimpfl and H. Echols 1978 Properties of the translocatable tetracycline-resistance inducible DNA repair which is accompanied by mutagenesis, element TnlO in Escherichia coli and lambda. pp. 355-367 in Molecular Mechanisms for the Repair of DNA, Genetics 90: 427-461. Part A, edited by P. HANAWALTand R. B. SETLOW.Plenum LIN, R.-J., M. CAPAGEand C.W. HILL, 1984 A repetitive DNA Press, New York. sequence, rhs, responsible for duplications within the Esche- RADMAN,M., 1980 Is there SOS induction in mammalian cells? richia coli K-12 chromosome. J. Mol. Biol. 177: 1-18. Photochem. Photobiol. 32: 823-830. LITTLE,J. W., 1983 The SOS regulatory system: control of its ROBERTS,J. W., and R. DEVORET,1983 Lysogenic induction, pp. state by the level of RecA protease. J. Mol. Biol. 167: 791- 123-144 in Lambda II, edited by R. HENDRIX,J. ROBERTS,F. 808. STAHLand R.WEISBERG. Cold Spring Harbor Laboratory LITTLE, J. W., and D.W. MOUNT,1982 The SOS regulatory Press, Cold Spring Harbor, N.Y. system of Escherichia coli. Cell 29: 11-22. SHURVINTON,C. E., and R. G. LLOYD,1982 Damage to DNA Lu, C., and H. ECHOLS, 1987 RecA protein and SOS: correlation induces expression of the ruv gene of Escherichia coli. Mol. of mutagenesis phenotype with binding of mutant RecAs to Gen. Genet. 185: 352-355. SMITH,G. R., 1987 Mechanism and control of homologous recom- duplex DNA and LexA cleavage. J. Mol. Biol. 196 497-504. bination in Escherichia coli. Annu. Rev. Genet. 21: 179-20 1. Lu, C., H. SCHEUERMANN andH. ECHOLS,1986 Capacity of R. STEINBORN,G., 1978 Uvm mutants of Escherichia coli K12 deficient RecA protein to bind preferentially to UV lesions and inhibit in UV mutagenesis. Mol. Gen. Genet. 165: 87-93. the editing subunit (c) ofDNA polymerase 111: A possible WALKER,G. C., 1984 Mutagenesis and inducible responses to mechanism for SOS-induced targeted mutagenesis. Proc. Natl. deoxyribonucleic acid damage in Escherichia coli. Microbiol. Acad. Sci. USA 83: 619-623. Rev. 48: 60-93. MCCLINTOCK,B., 1984 The significance of responses of the ge- WALKER,G. C., 1985 Inducible DNA repair systems. Annu. Rev. nome to challenge. Science 226 792-801. Biochem. 54 425-457. MCDONALD,J. F., 1984 The molecular basisof adaptation:a WINTERSBERGER,U., 1984 The selective advantage of cancer critical review of relevant ideas and observations. Annu. Rev. cells: a consequence of genome mobilization in the course of EcoI. Syst. 14 77-102. the induction of DNA repair processes? (Model studies on MILLER,J. H.,1972 Experiments in Molecular Genetics.Cold Spring yeast.) Adv. Enzyme Regul. 22: 31 1-323. Harbor Laboratory, Cold Spring Harbor, N.Y. WITKIN,E. M., 1976 Ultraviolet mutagenesis and inducible DNA MOUNT,D. W., 1977 A mutant of Escherichia coli showing consti- repair in Escherichia coli. Bacteriol. Rev. 40 869-907. tutive expression of the lysogenic induction and error-prone WITKIN,E. M., and T. KOGOMA, 1984 Involvement of the acti- DNA repair pathways. Proc. Natl. Acad. Sci. USA 74 300- vated form of RecA protein in SOS mutagenesis and stable 304. DNA replication in Escherichia coli. Bacteriol. Rev. 81: 7539- PETES, P. D., and C. W. HILL, 1988 Recombination between 7543. repeated genes in microorganisms. Annu. Rev. Genet. 22: 147- WITKIN,E. M., J. 0. MCCALL,M. R. VOLKERTand I. E. WERMUND- 168. SEN, 1982 Constitutive expression of SOS functions and mod- PICKSLEY,S. M., 1984 Repair of DNA double-strand breaks in ulation of mutagenesis resulting from resolution of genetic Escherichia coli K12 requires a functional recN product. Mol. instability at or near the recA locus ofEscherichia coli. Mol. Gen. Gen. Genet. 195 267-274. Genet. 185: 43-50. RADMAN,M., 1975 SOS repair hypothesis: phenomenology of an Communicating editor: J. R. ROTH