Construction of Escherichia coli K12 phr deletion and insertion mutants by gene replacement

We replaced an Escherichia coli phr gene by a 1.4-kb fragment of DNA coding for resistance to chloramphenicol. Characterization of 2 deletions (phr-19 and phr-36) and 1 insertion (phr-34) in the phr gene revealed no photoreactivation. Photoreactivation-deficient strains of either recA56 or lexA1(ind-) were more sensitive to UV radiation in the dark than phr-proficient counterparts. The presence of the phr defect in uvrA6 strains increased by 1.5-2-fold his-4(Ochre) to His+ mutation induced by ultraviolet light compared to uvrA6 phr+ strains, although there was no difference in UV sensitivity between uvrA6 phr+ and uvrA6 phr- strains. 30-35% of the His+ mutations thus induced were suppressor mutations in uvrA6 phr+ and 49-55% in uvrA6 phr- strains. The UV mutagenesis results are consistent with the previous observations that suppressor mutations targeted by a thymine-cytosine pyrimidine dimer are reduced in the dark in cells with amplified DNA photolyase.