Base-change mutations induced by various treatments of Bacillus subtilis spores with and without DNA protective small, acid-soluble spore proteins

Previous work has shown that spores of wild-type Bacillus subtilis are more resistant to killing by dry and wet heat, low vacuum lyophilization and hydrogen peroxide than are spores lacking the majority of their DNA protective α/β-type small, acid-soluble spore proteins (SASP) (termed α −β − spores). These four treatments kill α −β − spores in large part by DNA damage with accompanying mutagenesis, but only dry heat kills wild-type spores by DNA damage and mutagenesis. DNA sequence analysis of nalidixic acid-resistant (nal r) mutants generated by these treatments has now shown that the nal r mutations are base changes in the gyrA gene that encodes one subunit of DNA gyrase. Analysis of the DNA sequence of the gyrA gene in a large number of nal r mutants also indicates that: (1) base changes induced by hydrogen peroxide and wet heat in α −β − spores are similar to those in spontaneous nal r mutants with only a few notable differences; (2) base changes induced by dry heat in wild-type spores and low vacuum lyophilization of α −β − spores are similar, and include a high level of a tandem base change seen previously only in spores treated with very high vacuum and (3) base changes induced by lyophilization and dry heat are very different from those in spontaneous mutants in wild-type and α −β − spores, which exhibit only one significant difference. While the initial DNA damage generated in spores by dry heat, lyophilization or high vacuum is almost certainly different than that generated by hydrogen peroxide or wet heat, the precise nature of the DNA damage remains to be determined.