MtlR negatively regulates mannitol utilization by .

The phosphoenopyruvate:carbohydrate phosphotransferase system (PTS) enables - and other bacteria - to recognize and transport exogenous carbon sources for energy, including the six-carbon sugar alcohol, mannitol. The mannitol-specific PTS transporter is encoded by and its expression is expected to be regulated by the putative repressor encoded by the gene. Here, we show that overexpression inhibits growth in medium supplied with mannitol as the sole carbon source and represses MtlA-mediated biofilm formation. We demonstrate that when is grown in non-mannitol medium, knocking out leads to both increased MtlA protein and mRNA levels, with these increases being especially pronounced in non-glucose sugars. We propose that in non-mannitol, non-glucose growth conditions, MtlR is a major regulator of transcription. Surprisingly, with regard to expression, transcript and protein levels are highest in mannitol medium, conditions where expression should not be repressed. We further show that MtlR levels increase during growth of the bacteria and linger in cells switched from mannitol to non-mannitol medium. Our data suggests an expression paradigm for where MtlR acts as a transcriptional repressor responsible for calibrating MtlA levels during environmental transitions.