Amelioration of the ultraviolet sensitivity of an Escherichia coli recA mutant in the dark by photoreactivating enzyme.

When a recA strain of Escherichia coli is transformed with a multicopy plasmid, pKY1, carrying the phr gene of E. coli, its extreme ultraviolet sensitivity is decreased. Derivatives of pKY1 were prepared in which the phr gene was inactivated by inserting transposon Tn1000. None of the 20 phr- derivatives decreased the UV sensitivity of the recA strain. In an analogous experiment, we obtained 11 derivatives which failed to decrease the UV sensitivity of the recA strain. None of them complemented phr strain. Furthermore, TN1000 insertion sites in both types of derivatives were mapped in the same region of the plasmid. From these observations, we propose that the E. coli phr gene product has repair activity in the dark.