sn-Glycerol-3-phosphate transport in Escherichia coli and Salmonella typhimurium

The gene necessary for the synthesis of the active transport system of sn-glycerol-3-phosphate (G3P) is located at 48 min on the Escherichia coli linkage map. Complementation analysis revealed that there is only one gene necessary for G3P transport. The gene was cloned into the multicopy plasmid pBR322. Strains harboring the hybrid plasmid synthesized large amounts of a protein of 33,000 molecular weight that was found in the cytoplasmic membrane. This protein was identified as the G3P permease. In addition, the periplasm of the hybrid plasmid carrying strain contained large amounts of a soluble protein, identical with the previously recognized GLPT-protein of 40,000 molecular weight. The analysis of amber mutants isolated on the hybrid plasmid showed that the gene for the G3P permease is the first gene in an operon that codes for two genes; the distal gene being the structural gene for the periplasmic GLPT-protein. The corresponding gene region from Salmonella typhimurium has been cloned from an EcoRI libary in lambda gt7. The EcoRI fragment containing the gene necessary for G3P transport was subcloned into the multicopy plasmid pACYC184. The hybrid plasmid directed the synthesis of the G3P permease that behaved identically to the protein from Escherichia coli. However, the gene for the GLPT-protein was not intact but truncated by the EcoRI restriction site. The synthesis of the remaining polypeptide of 30,000 prevented the proper assembly of other transport related binding proteins such as the ribose- and galactose-binding protein.