Participation of the SOS system in producing deletions in E. coli plasmids

The participation of the SOS response in the deletion of palindromic and non-palindromic inserts of about 66 and 100 bp cloned within the EcoR1 site of the chloramphenicol acetyl transferase (cat) gene of plasmid pBR325 was tested after introducing the derived plasmids into strains containing different combinations of lexA, recA and umuC alleles and the auxotrophic mutation trpE65. This allowed for a comparison of deletion frequency in the plasmids, measured as the reversion of chloramphenicol sensitivity to resistance (Cms-->Cmr), to point-mutation frequency measured from the reversion of trpE65 to tryptophan independence (Trp(-)-->Trp+). We found that the spontaneous deletion frequency of palindromic inserts was increased by the overproduction of activated RecA* and UmuC+ in lexA (Def) backgrounds but the deletion of the non-palindromic inserts was unaltered. Overproduction of RecA+ had no significant effect on deletion incidence but it did increase Trp(-)-->Trp+ reversions. The SOS stimulation of palindrome deletions paralleled the SOS mutator effect of certain recA and umuC alleles on Trp(-)-->Trp+ reversions, suggesting that some form of SOS processing was responsible for the observed increases. The results further suggest that the SOS effect on deletions depends on the distinction between palindromy vs. non-palindromy, rather than on the sizes or sequences of the inserts or those of the terminal homologies bracketing them.