UV Light Induces IS10 Transposition in Escherichia coli

Corresponding author: Zvi Livneh, Dept. of Biological Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel, bclivneh{at}weizmann.weizmann.ac.il (E-mail).

Communicating editor: R. MAURER

	ABSTRACT

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

A new mutagenesis assay system based on the phage 434 cI gene carried on a low-copy number plasmid was used to investigate the effect of UV light on intermolecular transposition of IS10. Inactivation of the target gene by IS10 insertion was detected by the expression of the tet gene from the phage 434 P_R promoter, followed by Southern blot analysis of plasmids isolated from Tet^R colonies. UV irradiation of cells harboring the target plasmid and a donor plasmid carrying an IS10 element led to an increase of up to 28-fold in IS10 transposition. Each UV-induced transposition of IS10 was accompanied by fusion of the donor and acceptor plasmid into a cointegrate structure, due to coupled homologous recombination at the insertion site, similar to the situation in spontaneous IS10 transposition. UV radiation also induced transposition of IS10 from the chromosome to the target plasmid, leading almost exclusively to the integration of the target plasmid into the chromosome. UV induction of IS10 transposition did not depend on the umuC and uvrA gene product, but it was not observed in lexA3 and recA strains, indicating that the SOS stress response is involved in regulating UV-induced transposition. IS10 transposition, known to increase the fitness of Escherichia coli, may have been recruited under the SOS response to assist in increasing cell survival under hostile environmental conditions. To our knowledge, this is the first report on the induction of transposition by a DNA-damaging agent and the SOS stress response in bacteria.

TRANSPOSABLE elements are widespread among organisms and fulfill an important role in evolution of the genome (GINZBURG et al. 1984 ; MACKAY 1986 ; FINNEGAN 1989 ; and KIDWELL and LISCH 1997 ). The rate of transposition is usually very low, and it is tightly regulated by several mechanisms, to prevent genomic chaos and inactivation (SHAPIRO 1983 ; BERG and HOWE 1989 ). In several organisms transposition was found to be responsive to environmental agents that cause DNA damage. This includes the Ty transposon in Saccharomyces cerevisiae (ROLFE et al. 1986 ; BRADSHAW and MCENTEE 1989 ), the copia element in Drosophila melanogaster (STRAND and MACDONALD 1985 ), and Mutator in maize (MCCLINTOCK 1984 ; WALBOT 1992 ). It is therefore somewhat puzzling that the movements of bacterial transposable elements were found in several surveys to be quite insensitive to DNA-damaging agents.

IS10R, the right module of the bacterial transposon Tn10, can function either as an individual insertion sequence (IS) or it can mediate transposition of the whole Tn10 element. The transposase, the only protein encoded by IS10, catalyzes both Tn10 and IS10 transposition in a nonreplicative manner (FOSTER et al. 1981 ; HALLING et al. 1982 ). Transposition of IS10 is relatively rare (about 10^-4 per element per generation) and tightly regulated. Several control mechanisms act to reduce expression of the transposase gene, a key factor that determines transposition frequency. However, IS10 transposition is also influenced by other factors such as the cell cycle: IS10 transposition is negatively regulated by dam methylation and occurs preferentially after DNA replication (ROBERTS et al. 1985 ). Signals coming from the bacterial host or its environment can also affect IS10 transposition. Thus, continuous incubation at the stationary phase of growth was found to cause an increase in IS10 transposition frequency, in a process that was dependent on lexA and recA, the regulators of the global SOS stress response (SKALITER et al. 1992 ).

We have recently developed a new mutagenesis assay system that monitors the inactivation of the phage 434 cI gene carried on a low-copy plasmid (EICHENBAUM and LIVNEH 1995 ). Using this system we found that intermolecular transposition of IS10 caused coupled homologous recombination at the insertion site, leading to the formation of cointegrate structures (replicon fusion) (EICHENBAUM and LIVNEH 1995 ). Here we report that UV irradiation of Escherichia coli cells stimulates intermolecular transposition of IS10, and that this process is under the control of the SOS stress response.

	MATERIALS AND METHODS

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Materials:
Sources were as follows: restriction endonucleases, New England Biolabs (Beverly, MA); radiolabeled materials, multiprime labeling kit, and Hybond-N nylon membranes, The Radiochemical Center, Amersham (Arlington Heights, IL); antibiotics, Sigma (St. Louis); bacterial media, Difco (Detroit).

Media:
The medium used in this study was LB (Luria-Bertani), containing Bacto-trypton 10 g/liter; Bacto-yeast extract 5 g/liter; and NaCl 5 g/liter. Kanamycin (70 mg/liter), ampicillin (100 mg/liter), tetracycline (5 mg/liter), and chloramphenicol (30 mg/liter) were supplemented as required. Dilution and irradiation of bacteria were done in buffer PS (10 mM NaH₂PO₄ and 150 mM NaCl, pH 7.0).

Bacterial strains and plasmids:
The strains used in this study are listed in Table 1. Plasmid pZF42 carries the origin of replication of the F episome, the cat gene, the cI(434) gene, and the tet gene fused to the O_RP_R operator-promoter of phage 434 (EICHENBAUM and LIVNEH 1995 ). Plasmids pMVIS10 and pMV05 were taken from a collection of spontaneous Cro^- mutants that we isolated and characterized previously (SKALITER et al. 1992 ). Both plasmids are derivatives of plasmid pMV2. They carry the phage cro gene, the bla and kan genes, and the origin of replication of plasmid pBR322. pMVIS10 contains an IS10 insertion in cro, whereas pMV05 carries a point mutation in cro.

Preparation, fractionation, and hybridization of DNA:
Rapid preparation of chromosomal DNA was done according to KEMPTER and GROSSBADERN 1992 , or by further purification on a CsCl gradient according to WEEKS et al. 1986 . Rapid preparation of plasmid DNA from small-volume cultures was done using the boiling method (HOLMES and QUIGLEY 1981 ), and large-scale preparation was done as described by DAVIS et al. 1980 . DNA samples were fractionated by agarose gel electrophoresis, after which the DNA was transferred bidirectionally onto Hybond-N nylon membranes and hybridized as described by SMITH and SUMMERS 1980 . The IS10 specific DNA was the 0.94-kb StuI-NdeI fragment obtained from plasmid pNK290 (SIMONS and KLECKNER 1983 ). Plasmid pMV2, digested with PstI, XhoI and BglII, and pZF42, digested with BglII, were radiolabeled and used as probes for the detection of plasmid sequences.

UV-induced survival and mutagenesis:
Cells containing plasmid pZF42 were grown to early log at 37° on LB supplemented with chloramphenicol. The cells were concentrated fivefold in buffer PS, after which 4-ml portions were UV irradiated on ice using a low pressure mercury germicidal lamp (254 nm). The dose rate was 0.1 J m^-2 s^-1, as determined by a UV products radiometer equipped with a UVX-25 sensor. UV survival was determined by plating the appropriately diluted cultures on LB plates containing chloramphenicol. In order to determine mutation frequency, UV-irradiated cells were diluted 1:26 in LB, incubated for 90 min at 37°, and then harvested and resuspended in buffer PS. Determination of the total number of cells was done on LB plates containing chloramphenicol and the selection for Tet^R mutant colonies was done on LB plates containing chloramphenicol and tetracycline. Experiments for UV-induced interplasmid transposition of IS10 were performed in the same way, except that the cells harbored both plasmids pZF42 and pMVIS10, and the growth medium contained both chloramphenicol and kanamycin. Determination of the total number of cells was done on LB plates containing chloramphenicol and kanamycin, and the selection for Tet^R mutant colonies was done on LB plates containing chloramphenicol, kanamycin, and tetracycline. Mutation frequency is defined as the number of Tet^R colonies divided by the total number of colonies.

	RESULTS

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

The experimental system:
Our mutagenesis assay system monitors inactivation of the cI(434) gene carried on a low-copy number plasmid, containing the F episome origin of replication. This plasmid, termed pZF42, carries the cat gene as a selective marker, the cI(434) gene, and the tet gene fused to the O_RP_R operator-promoter of phage 434 (EICHENBAUM and LIVNEH 1995 ). Inactivation of the cI(434) gene is monitored via loss of the repressor function, which is detected by the activity of the tet gene expressed from the phage 434 P_R promoter. In order to study interplasmid transposition we used cells carrying plasmid pZF42 as the acceptor plasmid, and plasmid pMVIS10, a pBR322-derivative carrying an IS10R element, as the donor plasmid. Transposition of IS10 from the donor plasmid into the cI(434) reporter gene confers on the cell the ability to grow in the presence of tetracycline, and the type of mutational event was determined by Southern blot hybridization of the plasmids isolated from Tet^R colonies, using both IS10 and pZF42 DNA probes (EICHENBAUM and LIVNEH 1995 ).

As the first step we tested the response to UV radiation in the absence of the donor plasmid, of E. coli cells harboring the target plasmid. Exponentially growing cells harboring plasmid pZF42 were UV irradiated, after which they were grown up to 180 min without selection in order to enable cell recovery and expression of UV-inducible functions. As expected, UV irradiation caused inhibition of cell division (reviewed in LIVNEH et al. 1993 ), and no increase in colony count was observed up to 90 min. Subsequently, the cells continued to divide, exhibiting normal exponential growth. UV-irradiated cells were assayed for the formation of Tet^R mutants by plating on LB plates containing tetracycline. The frequency of UV-induced Tet^R mutants depended on the recovery period following irradiation, peaking at 90 min (data not shown). Thus, this period was used as the standard expression/recovery time in subsequent experiments.

UV-induced mutations in cI(434) were examined in E. coli MC4100, an E. coli K-12 derivative (Figure 1). As can be seen, UV irradiation caused an increase of mutation frequency of up to 60-fold over the spontaneous mutation frequency at a dose of 40 J m^-2, where survival was still relatively high (60%; Figure 1). UV mutagenesis in E. coli depends on the regulators of the SOS stress response, RecA and LexA, and on the UmuD and UmuC gene products (WALKER 1985 ; FRIEDBERG et al. 1995 ). This SOS dependence was examined in the cI(434) system using two mutations which render the SOS response noninducible: the recA13 mutation, encoding a nonfunctional RecA protein, or the lexA3 mutation, encoding a noncleavable LexA repressor. The UV sensitivity of an isogenic series of cells carrying plasmid pZF42 was found to be as expected, with AB2463(recA13) being more sensitive than DM49(lexA3), and the parent AB1157(wild type) being the least sensitive (Figure 1B). When examined for the production of UV-induced Tet^R mutations, the lexA3 and recA13 strains were completely nonmutable (Figure 1C). We then examined the effect of the umuC36 mutation on UV mutagenesis in cI(434). As can be seen in Figure 1D, the UmuC⁺ strain TK701 showed a 60-fold increase in mutagenesis at 40 J m^-2, whereas in the isogenic umuC36 strain TK702 mutagenesis was drastically reduced, and only a marginal twofold increase of Tet^R mutations was observed (Figure 1D). Thus, UV mutagenesis in cI(434) carried on plasmid pZF42 is dependent on RecA, LexA, and UmuC, similarly to chromosomal UV mutagenesis.

View larger version (71K): In this window In a new window Download PPT slide   Figure 1. UV survival and mutagenesis in the cI(434) plasmidic system. (A) Strain MC4100(pZF42) was UV irradiated and assayed for cell survival () and TetR mutagenesis (). (B and C) The isogenic strains AB1157 (), DM49(lexA3; ) and AB2463 (recA13; ) were UV irradiated and assayed for cell survival (B) and for TetR mutagenesis (C). (D) Strains TK701 () and TK702 (umuC36 ), each harboring plasmid pZF42, were UV irradiated and assayed for TetR mutagenesis.

UV radiation increases interplasmid IS10 transposition in a umuC strain:
The effect of UV radiation on interplasmid transposition of IS10 was examined using cells harboring the acceptor plasmid pZF42 and the donor plasmid pMVIS10. In order to reduce the "background" of regular umuC-dependent UV mutagenesis we utilized a strain carrying the umuC36 mutation (Figure 1D). UV irradiation of TK702(pZF42; pMVIS10) cells led to an increase of up to 22-fold in the frequency of Tet^R mutants (Figure 2). Control experiments conducted with plasmid pMV05, which contained no IS elements, led to a small increase of only threefold in the mutation frequency, as expected for a umuC strain (Figure 2). This suggested that the UV-induced increase of Tet^R mutants in TK702(pZF42; pMVIS10) is related to IS10.

View larger version (15K): In this window In a new window Download PPT slide   Figure 2. UV-induced mutagenesis in the cI(434) bi-plasmid system. E. coli strain TK702 (umuC36) harboring plasmids pZF42 and pMVIS10 (), or pZF42 and pMV05 () were UV irradiated and assayed for TetR mutagenesis.

In order to examine the type of events that caused the Tet^R mutations, the plasmids were extracted from Tet^R colonies and analyzed by agarose gel electrophoresis followed by Southern blot hybridization. We have previously shown that every IS10 transposition from pMVIS10 to pZF42 led to the formation of a cointegrate structure, composed of the fused acceptor and donor plasmids, and two copies of IS10 (EICHENBAUM and LIVNEH 1995 ; Figure 3). These structures were formed by a two-stage process involving transposition of IS10 followed by coupled homologous recombination at the transposition site. The cointegrate is easily detected as a high molecular weight DNA maintained at a high copy number, due to the activity of the pBR322 origin of replication (EICHENBAUM and LIVNEH 1995 ). It is further identified by digestion with restriction nucleases BamHI and XhoI. These enzymes, which do not cleave IS10, produce two characteristic bands of 6.65 and 3.62 kbp that cohybridize with both pZF42 and IS10 probes (EICHENBAUM and LIVNEH 1995 ; Figure 3).

View larger version (22K): In this window In a new window Download PPT slide   Figure 3. Schematic structure of a pZF42-pMVIS10 cointegrate. The cointegrate consists of plasmid pZF42 fused to plasmid pMVIS10 via an IS10-promoted transposition event. The IS10 element is duplicated in the process. Restriction sites used for the analysis of cointegrates are indicated. The determination of the structure of cointegrates was described in EICHENBAUM and LIVNEH 1995 .

Figure 4 shows the plasmid content of Tet^R colonies obtained after UV irradiation of TK702(pZF42; pMVIS10). Approximately half of the Tet^R mutants contained high molecular weight and high copy number DNA species, typical of cointegrates formed by interplasmid transposition, e.g., lanes 6–9 in Figure 4. Such structures were not observed when UV-induced Tet^R colonies of cells harboring pZF42 and the control plasmid pMV05 were analyzed. Further analysis was performed by digesting the plasmids with BamHI and XhoI, and subjecting them to Southern blot hybridization with radiolabeled IS10 or pZF42 probes. As can be seen in Figure 5, two bands of 6.65 and 3.62 kbp, characteristic of cointegrate structure (EICHENBAUM and LIVNEH 1995 ; Figure 3), were generated, each containing both IS10 and pZF42 sequences. Analysis of plasmids from spontaneous and UV-induced Tet^R mutants revealed that IS10 transposition increased ninefold upon UV irradiation at 30 J m^-2 (Table 2). In addition to the increase in IS10 transposition, an elevation in small mutations was observed as well. This UV induction of small mutations seems to be an IS10-promoted event, since it was not observed when plasmid pMV05 was used instead of pMVIS10. One possible interpretation of this result is that IS10 suppressed the umuC36 mutation. Alternatively, IS10 activation by UV irradiation may have induced a umuC-independent mutagenesis pathway.

View larger version (102K): In this window In a new window Download PPT slide   Figure 4. Plasmid DNA from UV-induced TetR colonies of strain TK702(pZF42; pMVIS10). Plasmid DNA from the mutant colonies was extracted and analyzed by gel electrophoresis in ethidium-stained agarose gels. Lanes 1 and 2 contain purified plasmids pMVIS10 and pZF42, respectively, as markers. Lanes 3–23 contain plasmids from 21 independent TetR mutant colonies.

View larger version (38K): In this window In a new window Download PPT slide   Figure 5. Hybridization pattern of pZF42 and IS10 probes to plasmid DNA from UV-induced TetR mutants. Plasmid DNA isolated from TetR mutants was digested with restriction nucleases BamHI and XhoI and fractionated by agarose gel electrophoresis. The DNA was then transferred bidirectionally onto nylon membranes, and probed in parallel with a pZF42-radiolabeled probe (A), or with an IS10-radiolabeled probe (B). U, uncut DNA; C, BamHI- and XhoI-cleaved plasmid DNA.

UV radiation increases interplasmid IS10 transposition in wild-type and uvrA strains, but not in recA or lexA3 strains:
Does UV radiation induce IS10 transposition also in a wild-type strain? We irradiated AB1157 cells harboring both the target plasmid pZF42 and the donor plasmid pMVIS10 and analyzed the Tet^R mutants as before. UV irradiation caused a sharp increase Tet^R mutants (Table 2). Analysis of the plasmids in these mutants by agarose gel electrophoresis and Southern blot hybridization revealed that most of the mutations were point mutations, as expected in a UmuC⁺ strain. However, along with the increase in point mutations, a 27-fold increase in IS10 transposition was observed (Table 2). The extent of the increase in IS10 transposition was threefold higher in the wild-type strain AB1157 (27-fold) as compared to the umuC36 mutant (ninefold); This may result from strain variation, since TK702 and AB1157 are not isogenic.

We examined the effect of UV radiation on IS10 transposition in recA and lexA3 strains, in which the SOS stress response cannot be induced. We found no induction of IS10 transposition in these strains (Table 3), suggesting that UV induction of IS10 transposition depends on the SOS response. Due to the extreme UV sensitivity of the recA and lexA mutants (FRIEDBERG et al. 1995 ), the UV dose used for their irradiation was an order of magnitude lower than for the wild-type or umuC cells. This raises the possibility that the lack of IS10 transposition in the recA and lexA strains was due to insufficient UV damage in DNA, rather than inactivation of SOS regulation. We addressed this possibility by examining UV-induced IS10 transposition in a uvrA umuC strain, which is sensitive to UV radiation due to a defect in nucleotide excision repair, but its SOS regulation is active (FRIEDBERG et al. 1995 ). As can be seen in Table 3, UV irradiation at 3 J m^-2 of the uvrA6 umuC36 strain TK610 harboring plasmids pZF42 and pMVIS10 caused a 28-fold increase in IS10 transposition. Thus, in this strain, IS10 transposition is induced at the same UV doses that did not induce transposition in lexA3 and recA strains. This strongly suggests that UV-induced transposition of IS10 is regulated by the SOS stress response.

View this table: In this window In a new window   Table 3. IS10 transposition in recA, lexA3, and uvrA6 strains

UV radiation increases chromosome-to-plasmid transposition of IS10:
We examined whether transposition of chromosomal IS10 elements is also induced by UV radiation. This was done with E. coli RKZ2, which carries two chromosomal IS10 elements (but not the tet gene). UV irradiation of RKZ2 cells harboring pZF42 led to a pronounced increase in the Tet^R mutation frequency. Analysis of Tet^R mutants revealed that transposition of IS10 from the chromosome to plasmid pZF42 was induced by UV up to 28-fold at a UV dose of 30 J m^-2 (Table 2). Thus, the induction of intermolecular IS10 transposition is not limited to plasmids only, and occurs also with chromosomal IS10 elements. As in the spontaneous mutagenesis experiments (EICHENBAUM and LIVNEH 1995 ), most (95%) of the transposition events of IS10 from the chromosome to pZF42 resulted in integration of the mutant pZF42 into the bacterial chromosome, as shown by the hybridization of chromosomal DNA to a pZF42 probe. Figure 6 shows an example of Southern blot hybridization of chromosomal DNA extracted from Tet^R colonies. The results were similar to those obtained with spontaneous mutants (EICHENBAUM and LIVNEH 1995 ). In every one of the mutants the production of a band that cohybridized with both IS10 and pZF42 specific probes was observed, e.g., arrows A, B. In addition to this band, which gave a strong hybridization signal, pZF42 integration was accompanied by the production of extra IS10 copies which gave fainter signals, e.g., arrows a, b.

View larger version (40K): In this window In a new window Download PPT slide   Figure 6. Southern blot analysis of chromosomal DNA from TetR mutants in which pZF42 had integrated into the bacterial chromosome. Chromosomal DNA was isolated from the TetR mutants on CsCl gradients, digested with EcoRI, and fractionated by agarose gel electrophoresis. The DNA was then bidirectionally transferred onto nylon membranes, and probed with a pZF42-radiolabeled probe (A), or with an IS10-radiolabeled probe (B). Arrows A and B mark DNA bands that hybridize to both pZF42 and IS10 probes. Arrows a–c mark additional copies of IS10.

	DISCUSSION

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

A major feature of transposons is their activity as mutagenic agents which are activated under conditions in which fast genetic changes and adjustment to the changing environment are needed (MCCLINTOCK 1984 ; KIDWELL and LISCH 1997 ). Indeed, DNA-damaging agents were found to induce transposon-related functions in yeast (ROLFE et al. 1986 ; BRADSHAW and MCENTEE 1989 ), flies (STRAND and MACDONALD 1985 ) and plants (MCCLINTOCK 1984 ; WALBOT 1992 ). Moreover, the presence of either Tn10 (CHAO et al. 1983 ) or IS50 (HARTL et al. 1983 ) was found to increase the fitness of E. coli in a chemostat. It was reported that whenever a Tn10 strain took over, IS10 transposition was observed. No transposition was detected when the Tn10 population was overcome by the competitor. It was therefore concluded that the mutagenic properties of this transposon confer an advantage in the same manner as mutator genes, i.e., by increasing the mutation rate of the host bacterium (CHAO et al. 1983 ).

In E. coli the response to DNA-damaging agents is controlled primarily by the SOS regulatory network, which functions to increase survival under adverse environmental conditions (LITTLE and MOUNT 1982 ; WALKER 1985 ). Part of this response is an increase in mutagenesis, which is dependent on the umuD and umuC genes and represents a DNA-damage dependent inducible mutator (KATO and SHINOURA 1977 ; ECHOLS 1981 ; WALKER 1985 ). Somewhat surprisingly, experiments designed to examine whether DNA-damaging agents induce transposition in bacteria gave negative results, and thus most of them remain unpublished. In the case of Tn5, a complex relationship between the SOS response and transposition was reported. Thus, Tn5 excision and transposition were found to be enhanced in a recA mutant which had constitutive activity of the SOS coprotease. In addition, a sequence homologous to the binding site of LexA, the global repressor of the SOS response, was identified in the region of the transposase gene of IS50. However, Tn5 transposition was not induced by UV light (KUAN et al. 1991 ; KUAN and TESSMAN 1991 ). Moreover, according to another report, induction of the SOS response reduced transposition by Tn5 and IS50 (WEINREICH et al. 1991 ). Tn10 excision was found to be activated by UV light (LEVY et al. 1993 ). However, unlike transposition, excision is a host-mediated function rather than a transposon-mediated event (KLECKNER 1989 ). Transposition of Tn10 (ROBERTS and KLECKNER 1988 ) and IS1 (LANE et al. 1994 ) causes induction of the SOS response. This is most likely due to the DNA cleavage associated with transposition, which provides an inducing signal for the SOS system.

The results presented in this study are, to our knowledge, the first report on the induction of transposition in E. coli under the control of the SOS stress response, by a DNA-damaging agent. It should be noted that the UV induction of IS10 transposition was observed with IS10 residing either on a plasmid or in the chromosome, suggesting that it is not limited to a particular donor. The insertion sites were not analyzed yet, and from the Southern hybridization data it is difficult to estimate whether there are many insertion sites. However, in our previous study with this system we have shown that there were at least two insertion sites in the acceptor plasmid (EICHENBAUM and LIVNEH 1995 ). In a related study, it was found that the induction of the SOS response by nalidixic acid did not elevate Tn10 transposition or the transposition of an artificial Kan^R construct of IS10 (ROBERTS and KLECKNER 1988 ). This may be due to a difference between IS10 and Tn10. Another possibility is that DNA damage is needed along with SOS induction to enhance IS10 transposition. UV lesions are known to inhibit DNA replication (reviewed in LIVNEH et al. 1993 ). This may delay the methylation at dam sites, and thus extend the time window during which the DNA is hemi-methylated, a state in which IS10 transposition occurs (ROBERTS et al. 1985 ). Alternatively, transposition of IS10, which occurs by a "cut and paste" mechanism, may be facilitated by ssDNA gaps formed in the UV-irradiated DNA due to the arrest of replication at DNA lesions, or due to processing of the DNA by repair enzymes (FRIEDBERG et al. 1995 ).

The fact that UV induction of IS10 was not observed in recA and lexA(Ind^-) strains suggests that DNA damage alone is not sufficient, and that one or more SOS-regulated proteins are required. Since there is no LexA-binding site in the coding sequence of IS10, this dependence on SOS must be indirect. The integration host factor (IHF), known to be involved in a multiplicity of processes in E. coli including regulation of gene expression, integration of phage , and transposition of IS1 (FRIEDMAN 1988 ; FREUNDLICH et al. 1992 ; OBERTO et al. 1994 ), influences IS10 transposition also (KLECKNER 1989 ). The expression of himA, encoding one of the subunits of IHF, was reported to be under the SOS control (MILLER et al. 1981 ), and thus, IHF might be one of the host factors involved in UV induction of IS10 transposition.

As mentioned above, the SOS stress response induces a DNA-dependent mutator activity as part of a multi-system rescue operation for E. coli populations challenged by environmental stress (LITTLE and MOUNT 1982 ; WALKER 1985 ). Tn10 is so far the only transposon shown to confer an advantage on its host, in a transposition-dependent manner (CHAO et al. 1983 ). This provides a possible rational basis for the recruitment of IS10 transposition to the UV-induced SOS network. Further studies are required to establish whether other bacterial transposable elements are also inducible by DNA-damaging agents in an SOS-dependent pathway.

	ACKNOWLEDGMENTS

This work was supported by the Forchheimer Center for Molecular Genetics. We thank TAMAR PAZ-ELIZUR, YOAV BARAK and ARIE SEGAL for their help.

Manuscript received September 22, 1997; Accepted for publication March 25, 1998.

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