MUTATIONAL ANALYSIS OF THE CITA CITRATE TRANSPORTER FROM SALMONELLA-TYPHIMURIUM - ALTERED SUBSTRATE-SPECIFICITY

The CitA citrate transporter in Salmonella typhimurium is encoded by t he citA gene and consists of 434 amino acid residues that probably inc lude 12 membrane-spanning segments [Shimamoto, T., et al. (1991) J. Bi ochem 110, 22-28]. CitA mutants with altered substrate specificities w ere isolated by in vitro mutagenesis using nitrous acid. The mutants c ould grow on isocitrate as a sole carbon source which normally cannot be transported well by the CitA transporter of S. typhimurium. The mut ation sites in the citA gene of the nine mutants were determined to in volve single residues at seven sites (one mutation per mutant). The or iginal amino acid residues at these sites (Arg-19, Ala-38, Glu-51, Gly -132, Ala-169, Pro-262 and Leu-271) were identified to be responsible for the altered substrate specificity. All these amino acid residues w ere conserved in four other homologous citrate transporters from Esche richia coli, Citrobacter amalonaticus and Klebsiella pneumoniae and ar e suggested to be involved in substrate recognition by the CitA transp orter. (C) 1996 Academic Press, Inc.