Analysis of transcription of the Staphylococcus aureus aerobic class Ib and anaerobic class III ribonucleotide reductase genes in response to oxygen

Staphylococcus aureus is a gram-positive facultative aerobe that can grow i n the absence of oxygen by fermentation or by using an alternative electron acceptor. To investigate the mechanism by which S. aureus is able to adapt to changes in oxygen concentration, we analyzed the transcriptional regula tion of genes that encode the aerobic class lb and anaerobic class III ribo nucleotide reductase (RNR) systems that are responsible for the synthesis o f deoxyribonucleotides needed for DNA synthesis. The S. aureus class Ib RNR nrdIEF and class Ell RNR nrdDG genes and their regulatory regions were clo ned and sequenced. Inactivation of the nrdDG genes showed that the class M RNR is essential for anaerobic growth. Inhibition of aerobic growth by hydr oxyurea showed that the class lb RNR is an oxygen-dependent enzyme. Norther n blot analysis and primer extension analysis demonstrated that transcripti on of class III nrdDG genes is regulated by oxygen concentration and was at least 10-fold higher under anaerobic than under aerobic conditions. In con trast, no significant effect of oxygen concentration was found on the trans cription of class lb nrdIEF genes. Disruption or deletion of S. aureus nrdD G genes caused up to a fivefold increase in nrdDG and nrdIEF transcription under anaerobic conditions but not under aerobic conditions. Similarly, hyd roxyurea, an inhibitor of the class I RNRs, resulted in increased transcrip tion of class lb and class III RNR genes under aerobic conditions. These fi ndings establish that transcription of class Ib and class III RNR genes is upregulated under conditions that cause the depletion of: deoxyribonucleoti de. Promoter analysis of class Ib and class III RNR operons identified seve ral inverted-repeat elements that may account for the transcriptional respo nse of the nrdIEF and nrdDG genes to oxygen.