Function of the UVR marker in dark repair of DNA molecules.

It had been found earlier that the excision repair mechanism in E. coli B/R Hcr+ could be depressed by preirradiation, amino acid and thymine starvation; such an interference proved to have no appreciable influence on survival after ultraviolet irradiation. A comparison between Hcr+ and Hcr- cells had revealed that the former were capable of tolerating a greater amount of unexcised dimers than the latter. In this paper it is demonstrated that the above-mentioned pretreatment will depress excision activity also in cultures of E. coli K12 and E. coli 15T- both strains of the uvr+ rec+ genotype. A comparison of two E. coli K12 strains of the uvr+ and uvr- genotype shows that uvr+ cells also have a greater capacity to tolerate unexcised dimers. To throw light on the nature of that increased capacity to tolerate unexicsed dimers we have compared restoration of DNA daughter chains in cells of the uvr+ and uvr- genotype and found that integrity of uvr loci is a conditio sine qua non for an effective restoration of daughter chains, but that depression of excision activity by the mentioned pretreatment does not influence restoration of DNA daughter chains. This suggest that uvr loci are involved not only in excision but also in postreplication mechanism of DNA repair.