The lactose transport protein (LacS) of Streptococcus thermophilus is a chimeric protein consisting of an amino-terminal carrier domain and a carboxyl-terminal phosphoenolpyruvate:sugar phosphotransferase system (PTS) IIA protein domain. The histidine residues of LacS were changed individually into glutamine or arginine residues. Of the 11 histidine residues present in LacS, only the His-376 su...