Protein X is the product of the recA gene of Escherichia coli.

The inducible protein X of Escherichia coli has been compared to the recA+ protein made by specialized recA transducing phages. The molecular weights and isoelectric points of these proteins are identical. Two mutations located in the recA gene that alter the electrophoretic mobility or the isoelectric point of protein X have been studied. A recA12 mutant strain, deficient in homologous recombination and repair, produces a smaller-than-normal protein X. A transducing phage carrying the recA12 allele directs the synthesis of a smaller recA protein after infection of irradiated cells. A transducing phage carrying the recA region of a tif-1 mutant strain codes for a recA protein with an isoelectric point more basic than that of the lambdaprecA+ product. The protein X of a tif-1 mutant strain shows an identical shift in its isoelectric properties. Examination of several tsl- recA- strains indicates that protein X can be induced in several missense recA mutants but is not detected in tsl- strains carrying amber or deletion mutations of the recA gene. These results demonstrate that protein X is the product of the recA gene and that the tif-1 mutation alters the properties of the recA protein. A model is suggested for autoregulation of the recA protein in the induction of functions expressed in response to DNA damage (SOS functions).