Pyrimidine dimer excision in a Bacillus subtilis Uvr- mutant.

A technique which allows the measurement of small numbers of pyrimidine dimers in the deoxyribonucleic acid (DNA) of cells of Bacillus subtilis irradiated with ultraviolet light has been used to show that a strain mutant at the uvr-1 locus is able to excise pyrimidine dimers. Excision repair in this strain was slow, but incision may not be rate limiting because single-strand breaks in DNA accumulate under some conditions. Excision repair probably accounted for a liquid-holding recovery previously reported to occur in this strain. Recombinational exchange of pyrimidine dimers into newly replicated DNA was readily detected in uvr-1 cells, but this exchange did not account for more than a minor fraction of the dimers removed from parental DNA. Excision repair in the uvr-1 strain was inhibited by a drug which complexes DNA polymerase III with DNA gaps. This inhibition may be limited to a number of sites equal to the number of DNA polymerase III molecules, and it is inferred that large gaps are produced by excision of dimers. Because the uvr-1 mutation specifically interferes with excision of dimers at incision sites, it is concluded that the uvr-1 gene product may be an exonuclease which is essential for efficient dimer excision.