Specific amino acid residues in both the PstB and PstC proteins are required for phosphate transport by the Escherichia coli Pst system.

Three mutant alleles of the pstC gene and one mutant allele of the pstB gene were produced by site-directed mutagenesis. The pstC gene encodes an integral membrane protein of the phosphate-specific transport (Pst) system of Escherichia coli. The amino acid substitutions resulting from the pstC gene mutations, Arg-237----Gln, Glu-240----Gln, or a combination of both, caused the loss of phosphate transport through the Pst system, but the alkaline phosphatase activity remained repressed. The pstB gene encodes a peripheral membrane protein of the Pst system which carries a putative nucleotide-binding site. The amino acid substitutions Gly-48----Ile and Lys-49----Gln, resulting from the pstB mutations, caused the loss of phosphate transport through the Pst system and the derepression of alkaline phosphatase activity. The residues Gly-48 and Lys-49 are key residues in the putative nucleotide-binding site.