An Epistasis Analysis of and in K-12.

RecA is essential for double-strand-break repair (DSBR) and the SOS response in K-12. RecN is an SOS protein and a member of the Structural Maintenance of Chromosomes family of proteins thought to play a role in sister chromatid cohesion/interactions during DSBR. Previous studies have shown that a plasmid-encoded (Q300R) mutant had a phenotype similar to ∆ (mitomycin C sensitive and UV resistant). It was hypothesized that RecN and RecA physically interact, and that specifically eliminated this interaction. To test this model, an epistasis analysis between and ∆ was performed in wild-type and cells. To do this, was first transferred to the chromosome. As single mutants, and ∆ were Rec as measured by transductional recombination, but were 3-fold and 10-fold decreased in their ability to do I-SceI-induced DSBR, respectively. In both cases, the double mutant had an additive phenotype relative to either single mutant. In the background, and ∆ cells were very UV (sensitive), Rec, had high basal levels of SOS expression and an altered distribution of RecA-GFP structures. In all cases, the double mutant had additive phenotypes. These data suggest that (Q300R) and ∆ remove functions in genetically distinct pathways important for DNA repair, and that RecA Q300 was not important for an interaction between RecN and RecA (Q300R) revealed modest phenotypes in a wild-type background and dramatic phenotypes in a strain, reflecting greater stringency of RecA's role in that background.