Overexpression of MazFsa in Staphylococcus aureus induces bacteriostasis by selectively targeting mRNAs for cleavage

The role of chromosomally encoded toxin-antitoxin (TA) loci in bacterial physiology has been under debate, with the toxin proposed as either an inducer of bacteriostasis or a mediator of programmed cell death (PCD). We report here that ectopic expression of MazF(Sa), a toxin of the TA module from Staphylococcus aureus, led to a rapid decrease in CFU counts but most cells remained viable as determined by differential Syto 9 and propidium iodide staining after MazF(Sa) induction. This finding suggested that the toxin MazF(Sa) induced cell stasis rather than cell death. We also showed that MazF(Sa) selectively cleaves cellular mRNAs in vivo, avoiding "important" transcripts such as recA, gyrB, and sarA mRNAs in MazF(Sa)-induced cells, while these three mRNAs can be cleaved in vitro. The results of Northwestern blotting showed that both sarA and recA mRNAs bind strongly to a putative RNA-binding protein. These data suggest that S. aureus likely undergoes stasis by protecting selective mRNA with RNA-binding proteins upon the expression of MazF(Sa) in vivo.