Sensitization of ultraviolet-irradiated Escherichia coli K-12 by different agars: inhibition of A rec and exr gene-dependent branch of the uvr gene-dependent excision-repair process

Abstract—E. coli K‐12 wild‐type cells plated immediately after UV irradiation had a much lower survival on minimal medium solidified with agar‐agar No. 3 (Oxoid), purified agar (Difco) or ionagar (Colab) than on plates solidified with laboratory‐washed Noble agar (Difco). An intermediate survival was obtained on plates solidified with unwashed Noble agar (Difco). When irradiated cells were incubated in liquid minimal medium for various times before subsequent plating on agar‐agar No. 3, their survival increased rapidly and became identical to the survival of cells plated on washed Noble agar. The same phenomenon was found with polA 1 cells, but no differences in survival on the different agars were observed with uvrB, exrA, recA or recB cells. These‐ data suggest that a repair process dependent on the uvrB, recA, recB and exrA (but not the polA) gene products is inhibited when UV irradiated E. coli K‐12 cells are plated on minimal medium solidified with agar‐agar No. 3, purified agar or ionagar. This implies that the uvr gene‐dependent excision‐repair process consists of at least two branches, one controlled by the polA gene and a second controlled by the recA, recB and exrA genes and inhibitable by a substance present in certain agars. For maximum sensitivity, experiments designed to study chemical inhibitors of repair should use agar plates that do not themselves inhibit repair.