CHARACTERISTICS OF THE MELIBIOSE TRANSPORTER AND ITS PRIMARY STRUCTURE IN ENTEROBACTER-AEROGENES

Cells of Enterobacter aerogenes can grow on melibiose as a sole source of carbon. This suggests the presence of melibiose operon in this org anism. We found that E. aerogenes cells possess both cr-galactosidase activity and melibiose transport activity, which were induced by melib iose, Neither Na+ nor Li+ stimulated the melibiose transport, However, transport of methyl-beta-thiogalactoside (TMG) was stimulated by Libut not by Na+. These findings suggest that the major coupling cation for the melibiose transporter in E. aerogenes is H+. In fact, we obser ved H+ entry into cells caused by an influx of melibiose and some of i ts analogs. We cloned the melB gene which encodes the melibiose transp orter, and sequenced it. Deduced amino acid sequence of the transporte r revealed that the melibiose transporter consists of 471 amino acid r esidues and the molecular weight was calculated to be 52214 Da. The se quence showed high homology with the sequences of the melibiose transp orters of Escherichia coli, Salmonella typhimurium and Klebsiella pneu moniae. Higher homology was found with the melibiose transporter of K. pneumoniae than with that of E. coli and S. typhimurium.