Positively regulates the expression of the mtlAFD operon involved in the uptake and catabolism of mannitol. D-mannitol(out) + N(pros)-phospho-L-histidyl-[protein] = D-mannitol 1-phosphate(in) + L-histidyl-[protein] The regulatory activity of MtlR is modulated by phosphorylation and dephosphorylation of the various MtlR domains. It becomes activated via phosphoryl group transfer from PEP, EI and HPr on the two conserved histidine residues in the PRD 2 domain, whereas phosphorylation of the EIIA-like domain on His-599 by the PTS EIIB-Mtl domain of MtlA inactivates MtlR (By similarity). Up-regulated by mannitol or glucitol. Cells lacking this gene cannot grow on either mannitol or glucitol as sole carbon source. Belongs to the transcriptional antiterminator BglG family.