Effects of a recA operator mutation on mutant phenotypes conferred by lexA and recF mutations.

Derepression of recA by an operator mutation (recAo281) produces effects opposite to those obtained from its derepression following DNA damage. Inducible reactivation of lambda vir and S13 phages is decreased and inducible UV mutagenesis of a phi X174 amber mutant is lessened in a recAo281 strain compared to a recAo+ strain. The decreases could not be accounted for by increases in constitutive levels of these processes. Consistent with these results the UV resistance of a recAo281 strain is less than that of a recAo+ strain. This may indicate that too much recA protein immediately after irradiation interferes with derepression of the lexA regulon or functioning of its products. Effects of increasing the recAo+ and recA+ copy number on a Co1E1 plasmid are compared with the effects of recAo281. recAo281 partially suppresses UV sensitivity due to lexA102 and lexA3 in E. coli K-12. This increase in resistance is not correlated with an increase in constitutive or inducible reactivation of UV-irradiated lambda vir or S13. This is consistent with the previous suggestion that the UV resistance stems from a decrease in DNA degradation allowing an increase in DNA repair. lexA3 blocks UV mutagenesis of phi X174 as measured by reversion of amber mutations and this was not suppressed by recAo281. recF143 blocks UV mutagenesis of phi X174. recAo281 suppresses neither this effect nor the decrease in bacterial UV resistance caused by recF143.