Abstract
We isolated mutant strains of the facultative anaerobe Salmonella typhimurium that grow either aerobically or anaerobically. Strict anaerobic mutants contained a defective DNA topoisomerase I gene (topI), while strict aerobic mutants contained a defective DNA gyrase subunit A gene (gyrA, also nalA). Topoisomerase I activity was detected in cell-free extracts of strict aerobic mutants but not of strict anaerobic mutant strains, whereas gyrase activity was detected in extracts of strict anaerobic mutants but not of strict aerobic mutants. Furthermore, extracts of wild-type cells, cultured under vigorous aerobic condition, contain topoisomerase I activity but no significant gyrase activity. In contrast, the extracts of anaerobically cultured wild-type cells contain gyrase activity but no significant topoisomerase I activity. Sucrose gradient centrifugation with ethidium bromide showed that chromosomal DNA in strict aerobic mutants and aerobically grown wild-type cells was relaxed, while the chromosomal DNA of strict anaerobic mutants and anaerobically grown wild-type cells was more supercoiled. Aerobic cultures of wild type and strict aerobic mutants produced both superoxide dismutase and catalase, whereas anaerobic cultures of wild type and strict anaerobic mutants did not. These results lead us to conclude that activity of topoisomerase I, associated with relaxation of chromosomal DNA, is necessary for expression of genes required for aerobic growth, whereas activity of gyrase, associated with supercoiling of chromosomal DNA, is necessary for expression of genes required for anaerobic growth.