Effects of DNA-polymerase-defective and recombination-deficient mutations on the ultraviolet sensitivity of Bacillus subtilis spores.

The DNA of UV-irradiated Bacillus subtilis spores, which contains 5-thyminyl-5,6-dihydrothymine (TDHT) as the major thymine photoproduct, is known to be repaired during germination by two complementary mechanisms: (I) the well-known excision repair, and (2) a special process, "spore repair", which destroys TDHT in situ without rendering it acid-soluble. In the absence of both mechanisms TDHT is not removed, and spores are highly UV-sensitive. When either of two mutations (pol-59 and pol-151) giving defective DNA polymerase, or one (rec-A1) giving a recombination deficiency are introduced into strains defective in one of these known TDHT removal processes, the chemically measured elimination of TDHT from spore DNA is unaltered, but spore UV-sensitivity is increased. The pol mutations produce their greatest sensitivity increase in spores of strains already deficient for the in situ destruction of TDHT, while the rec mutation gives its maximum sensitivity increase to spores of strains lacking excision. These facts argue that the pol mutations interfere mostly with excision repair (presumably its later resynthesis step), shile the rec mutation impairs "spore repair" in some step occurring subsequent to the TDHT destruction in situ. With either of these impairments of the later repair steps, DNA of UV-irradiated and germinated spores is considerably degraded, unless germination is carried out in the presence of chloramphenicol.