Defective cation-coupling mutants of Escherichia coli Na+/proline symport carrier. Characterization and localization of mutations

A major proline carrier in Escherichia coli encoded by the putP gene mediates proline/Na+ or Li+ symport. Proline carrier mutants with altered cation specificity were obtained by mutagenesis with nitrous acid in vitro of a plasmid carrying the wild-type putP gene. Two mutant strains harboring plasmid pMOP4135 and pMOP4141 could transport proline efficiently only in the presence of an increased concentration of sodium ion. Mutations of these plasmids, putP4135 and putP4141, caused reduction of affinity for Na+ of proline transport and binding, without remarkable change in the affinity for proline or in production of the carriers. Consistent with the lower affinity of the putP4141 carrier for Na+, the mutant carrier was supersensitive to N-ethylmaleimide inhibition. The pH dependence of proline binding was also changed in these mutant carriers. The lesions of putP4135 and putP4141 were located in the N-terminal part of the putP gene (ClaI-PvuII fragment) by in vitro recombination and subsequent examination of the phenotype of the transformants. DNA sequencing of these fragments revealed one base alteration of G to A at nucleotides 299 and 656 in pMOP4141 and pMOP4135, respectively, which corresponded to amino acid changes from Gly22 to glutamic acid and Cys141 to tyrosine, respectively.