Non-coding RNAs (ncRNAs) are widespread in bacteria and possess critical regulatory roles in bacterial physiology, metabolism and virulence. The Listeria monocytogenes ncRNA rli60 is a ~190 bp element specific to Listeria, located upstream to the branch chained amino acids (BCAAs) biosynthesis operon (the ilv operon). rli60 was identified in several transcriptome studies, however its function has not been studied. Here we report that rli60 is transcribed alone under conditions of high concentrations of BCAAs, while it is co-transcribed with the ilv operon during low concentrations of BCAAs, suggesting it might functions as a riboswitch. We further demonstrate that under high concentrations of BCAAs rli60 represses the expression of the ilv operon, while during BCAAs depletion this repression is gradually relieved. Interestingly, CodY, the BCAAs responsive global regulator, functions together with rli60 to repress the ilv operon. This is the first demonstration that the ilv operon is regulated by a ncRNA/a riboswitch. Finally, since BCAAs and CodY were shown to regulate L. m. virulence genes expression, the role of rli60 in L. m. virulence is examined.
