Novel SOS phenotypes caused by second-site mutations in the recA430 gene of Escherichia coli

E coli recA430 mutants are recombination-proficient, extremely UV sensitive, UV nonmutable and partially deficient in RecA-mediated proteolysis and in RecA-dependent 'induced replisome reactivation' (IRR), the ability to recover DNA replication activity after UV irradiation. To determine how this pleiotropic phenotype can be altered by mutation, we isolated 10 independent derivatives of a recA430 strain, selecting for increased UV resistance. Eight of the 10 owed their resistance to altered recA alleles. We here describe the phenotypes conferred by two of the new recA alleles (recA720 and recA727), each of which contains the original recA430 mutation (G₆₆₂ to A) and a second-site transition: T₁₆₇ to C in recA720, and G₁₀₃ to A in recA727. The second-site change in recA720 suppresses all the defects caused by recA430, and causes RecA720 to exhibit greater activity than RecA⁺ in some respects. Some, but not all, of the recA430 defects are partially corrected by the second-site mutation in recA727.