DNA REPAIR IN ESCHERICHIA COLI STRAINS DEFICIENT IN SINGLE-STRAND DNA BINDING PROTEIN

Abstract

E. coli strains deficient in single-strand DNA binding protein (SSB) were examined for their ability to carry out dark repair pathways which act on DNA damaged by ultraviolet (UV) light. The ssb-1 and ssb-113 mutations were found to confer UV sensitivity which was additive with that conferred by the uvrB5 but not the recA1 mutation. Furthermore host-cell reactivation of UV irradiated phage (lamda) proceeded normally in ssb('-) strains. These results suggest that SSB facilitates recA-dependent repair, but is not required for constitutive excision repair. Also discussed are the result of repair synthesis and strand rejoining measurements made in ssb-1 strains.;SSB deficiency impaired Weigle reactivation and mutagenesis, defects which parallel those previously found in other recA-lexA dependent processes such as (lamda) prophage induction and amplification of RecA protein synthesis. These latter two processes could be restored in ssb-1 strains by increased synthesis of the mutant protein. On the other hand, overproduction of RecA protein did not alleviate but only exacerbated the defect in (lamda) prophage induction. Together these results demonstrate a positive role for SSB in recA-lexA regulon induction and suggest that it facilitates RecA-mediated repressor cleavage more subtly than merely by sequestering excess ssDNA.;In order to study SSB's role in DNA repair apart from its capacity in recA-lexA regulation, I constructed recAo('+) and recAo281 derivatives of lexA('-) ssb('-) strains. The ssb-1 mutation blocked the ability of recAo281 to alleviate UV sensitivity and postirradiation DNA degradation in a lexA('-) strain while only the former effect was blocked by ssb-113. These results suggest that in addition to its role in facilitating induction of the recA-lexA operon, SSB participates directly in recombinational repair or some other as yet uncharacterized repair pathway.