Evidence for repair-replication of ultraviolet damaged DNA in bacteria

These findings are consistent with the view that in ultraviolet-resistant organisms a mechanism for repair replication exists in which damaged single-strand regions of the chromosome can be excised and replaced, using the undamaged DNA strand as template.

Abstract

Density-labeling with the thymine analogue, 5-bromouracil, was used to follow DNA replication in ultraviolet irradiated bacteria. The partial degradation of the damaged DNA and simultaneous synthesis at random positions in the genome was demonstrated. This non-conservative mode of replication eventually resulted in density heterogeneity (due to differences in the 5-bromouracil-thymine ratio) among the isolated DNA fragments. Normal semi-conservative replication was observed if photoreactivating conditions followed the ultraviolet irradiation prior to density labeling. Molecular fragments containing these regions of random replication were thermally denatured and/or fragmented by sonication. Density-distribution analysis in the CsCl density-gradient indicated that the density label was incorporated into very short segments along single DNA strands. Our findings are consistent with the view that in ultraviolet-resistant organisms a mechanism for repair replication exists in which damaged single-strand regions of the chromosome can be excised and replaced, using the undamaged DNA strand as template.