Mlc of Thermus thermophilus: a Glucose-Specific Regulator for a Glucose/Mannose ABC Transporter in the Absence of the Phosphotransferase System

ABSTRACT

We report the presence of Mlc in a thermophilic bacterium. Mlc is known as a global regulator of sugar metabolism in gram-negative enteric bacteria that is controlled by sequestration to a glucose-transporting EII^Glc of the phosphotransferase system (PTS). Since thermophilic bacteria do not possess PTS, Mlc in Thermus thermophilus must be differently controlled. DNA sequence alignments between Mlc from T. thermophilus (Mlc_Tth) and Mlc from E. coli (Mlc_Eco) revealed that Mlc_Tth conserved five residues of the glucose-binding motif of glucokinases. Here we show that Mlc_Tth is not a glucokinase but is indeed able to bind glucose (K_D = 20 μM), unlike Mlc_Eco. We found that mlc of T. thermophilus is the first gene within an operon encoding an ABC transporter for glucose and mannose, including a glucose/mannose-binding protein and two permeases. malK1, encoding the cognate ATP-hydrolyzing subunit, is located elsewhere on the chromosome. The system transports glucose at 70°C with a K_m of 0.15 μM and a V _max of 4.22 nmol per min per ml at an optical density (OD) of 1. Mlc_Tth negatively regulates itself and the entire glucose/mannose ABC transport system operon but not malK1, with glucose acting as an inducer. MalK1 is shared with the ABC transporter for trehalose, maltose, sucrose, and palatinose (TMSP). Mutants lacking malK1 do not transport either glucose or maltose. The TMSP transporter is also able to transport glucose with a K_m of 1.4 μM and a V _max of 7.6 nmol per min per ml at an OD of 1, but it does not transport mannose.