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Conjugational crosses trigger SOS induction in Escherichia coli F− cells mated with Salmonella enterica serovar Typhimurium Hfr donors. Using an epigenetic indicator of SOS induction, we showed that a strong SOS response occurring in a subpopulation of mated mismatch repair-deficient cells totally abolishes genetic barriers between these two genera.
The SOS response is a set of cellular responses induced by the exposure of bacterial cells to a variety of genotoxic and metabolic stresses which generally interfere with DNA replication (3, 10, 16). Regulation of the SOS system is mediated by the LexA and RecA proteins. LexA acts as a repressor of about 30 genes, including recA and lexA. The SOS-inducing signal is single-strand DNA, to which RecA binds and becomes activated as a coprotease (RecA*). RecA* promotes proteolytic self-cleavage of the LexA repressor and of some phage repressors, such as λ CI, thus derepressing the SOS regulon (3, 4, 13).
Strong SOS induction occurs in recipient cells during interspecies conjugation, whereas only a weak induction takes place during intraspecies matings (7). This SOS induction was measured using a recA::lacZ operon fusion, which gives a mean value of the global recA expression in a population of recipient cells (7). In the present study, we wanted to detect SOS induction in individual recipient cells. To that end, we used an epigenetic switch as a reporter system that responds to inactivation of the λ CI repressor (18) to assess the following: (i) any potential heterogeneity of SOS induction among individual recipient cells, (ii) the viability of the recipient cells that have undergone strong SOS induction, and (iii) possible correlations between the levels of SOS induction and interspecies recombination in individual cells.
The Hfr strains used in this study were Salmonella enterica serovar Typhimurium SA977 and Escherichia coli PK3 (7). SOS induction in E. coli F− cells was monitored in MT1-derived strains bearing a cI-cro-gal fusion whereby the expression of the gal operon was under negative control by the CI repressor of phage lambda (18). Whereas the MT1 cells displayed a Gal− phenotype (white colonies on MacConkey agar-galactose plates) when the λ cI gene was expressed, temporary RecA*-assisted inactivation of the CI repressor during SOS induction resulted in a heritable epigenetic Gal+ phenotype (red colonies on MacConkey agar-galactose plates) (16, 18). An isogenic strain called MT5 has a noninducible cI (Ind−) mutation that detects, by color change, only the rare mutations in the cI gene. MT1 strain derivatives were constructed by P1-mediated transduction of the following alleles: mutS201::Tn5, lexA1 (Ind−) (coding for a noncleavable LexA repressor) malB::Tn9, and recAo98 (an operator constitutive and RecA-overproducing mutant) srl::Tn10 (7). Conjugations were performed as described previously (7). Ilv+ recombinants were selected on M63 minimal medium plates and scored after 48 h at 37°C. All recipient strains were resistant to nalidixic acid, which was used to counterselect donor cells.