PHOTOREACTIVATION IN A phrB MUTANT OF Escherichia coli K-12: EVIDENCE FOR THE ROLE OF A SECOND PROTEIN IN PHOTOREPAIR

Abstract In Escherichia coli , the light-dependent repair of pyrimidine dimers in UV-irradiated DNA is now accepted as being due to enzymatic photoreactivation (PR) by a 50 kDa enzyme, photolyase (EC 4.1.99.3). The gene for this enzyme has been mapped at 16.2 min and designated phr . This gene was earlier described as phr B, another locus phr A having been proposed in association with PR. The relevance of the putative phr A gene has now been placed in doubt. The recent report of the discovery of a photoreactivating enzyme in Drosphila melanogaster . which specifically repairs pyrimidine (6–4) pyrimidone photoproducts (6–4] photoproducts), and that E. coli does possess a protein with specific affinity for the (6–4) photoproduct, has cast new light on the prospective role of phr A in PR. We have determined the nucleotide sequence of the putative phr A gene, which suggests it codes for a protein of 38 kDa. When the putative phr A gene was cloned into an expression vector and transformed into a phr A phr B mutant of E. coli , a level of photorepair was observed, which could correspond to repair of (6–4) photoproducts.