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In recA718 lexA+ strains of Escherichia coli, induction of the SOS response requires DNA damage. This implies that RecA718 protein, like RecA+ protein, must be converted, by a process initiated by the damage, to an activated form (RecA) to promote cleavage of LexA, the cellular repressor of SOS genes. However, when LexA repressor activity was abolished by a lexA-defective mutation [lexA(Def)], strains carrying the recA718 gene (but not recA+) showed strong SOS mutator activity and were able to undergo stable DNA replication in the absence of DNA damage (two SOS functions known to require RecA activity even when cleavage of LexA is not necessary). lambda lysogens of recA718 lexA(Def) strains exhibited mass induction of prophage, indicative of constitutive ability to cleave lambda repressor. When the cloned recA718 allele was present in a lexA+ strain on a plasmid, SOS mutator activity and beta-galactosidase synthesis under LexA control were expressed in proportion to the plasmid copy number. We conclude that RecA718 is capable of becoming activated without DNA damage for cleavage of LexA and lambda repressor, but only if it is amplified above its base-line level in lexA+ strains. At amplified levels, RecA718 was also constitutively activated for its roles in SOS mutagenesis and stable DNA replication. The nucleotide sequence of recA718 reveals two base substitutions relative to the recA+ sequence. We propose that the first allows the protein to become activated constitutively, whereas the second partially suppresses this capability.

Constitutive expression of the SOS response in recA718 mutants of Escherichia coli requires amplification of RecA718 protein