sn-Glycerol-3-phosphate transport in Salmonella typhimurium

Abstract

Salmonella typhimurium contains a transport system for sn-glycerol-3-phosphate that is inducible by growth on glycerol and sn-glycerol-3-phosphate. In fully induced cells, the system exhibited an apparent K(m) of 50 μM and a V(max) of 2.2 nmol/min.108 cells. The corresponding system in Escherichia coli exhibits, under comparable conditions, a K(m) of 14 μM and a V(max) of 2.2 nmol/min.108 cells. Transport-defective mutants were isolated by selecting for resistance against the antibiotic fosfomycin. They mapped in glpT at 47 min in the S. typhimurium linkage map, 37% cotransducible with gyrA. In addition to the glpT-dependent system, S. typhimurium LT2 contains, like E. coli, a second, ugp-dependent transport system for sn-glycerol-3-phosphate that was derepressed by phosphate starvation. A S. typhimurium DNA bank containing EcoRI restriction fragments in phage λgt7 was used to clone the glpT gene in E. coli. Lysogens that were fully active in the transport of sn-glycerol-3-phosphate with a K(m) of 33 μM and a V(max) of 2.0 nmol/min.108 cells were isolated in a ΔglpT mutant of E. coli. The EcoRI fragment harboring glpT was 3.5 kilobases long and carried only part of glpQ, a gene distal to glpT but on the same operon. The fragment was subcloned in multicopy plasmid pACYC184. Strains carrying this hybrid plasmid produced large amounts of cytoplasmic membrane protein with an apparent molecular weight of 33,000, which was identified as the sn-glycerol-3-phosphate permease. Its properties were similar to the corresponding E. coli permease. The presence of the multicopy glpT hybrid plasmid had a strong influence on the synthesis or assembly of other cell envelope proteins of E. coli. For instance, the periplasmic ribose-binding protein was nearly absent. On the other hand, the quantity of an unidentified E. coli outer membrane protein usually present only in small amounts increased.