Ribosomal RNA methyltransferases contribute to Staphylococcus aureus virulence

Post-transcriptional modifications in rRNA, such as methylation, are observed in functionally important regions of the ribosome. The methyltransferases responsible for these modifications work as housekeeping enzymes to fine-tune ribosomal function, and the roles of some methyltransferases become more evident under stress conditions. Recently, the 16S rRNA methyltransferases RsmI and RsmH, which are responsible for cytidine dimethylation at the decoding center of the ribosome, were identified in Escherichia coli. The physiological relevance of the rRNA modification, however, remains obscure. We identified rsmI and rsmH as novel virulence genes in Staphylococcus aureus using a silkworm infection model. These genes induced 2'-O- and N(4)-methylations, respectively, of m(4) Cm1412 of S. aureus 16S rRNA. Deletion of either rsmI or rsmH in S. aureus attenuated the virulence of S. aureus in silkworms, and led to its sensitivity to oxidative stress. Dual luciferase assay revealed that the double-knockout strain exhibited decreased translational fidelity under oxidative stress conditions. In addition, administration of N-acetyl-L-cysteine, a free-radical scavenger, restored the killing ability of the double-knockout strain against silkworms. These findings suggest that the methyl-modifications of cytidine at position 1412 in 16S rRNA contribute to S. aureus animal killing by conferring resistance to oxidative stress in host animals.