Mutational separation of transport systems for branched-chain amino acids in Pseudomonas aeruginosa

Cloning and nucleotide sequence of the gene braB coding for the sodium-coupled branched-chain amino acid carrier in Pseudomonas aeruginosa PAO

The gene braB, encoding the Na(+)-coupled carrier for branched-chain amino acids in Pseudomonas aeruginosa PAO, was cloned on cosmid pMMB34. The cosmid clones carrying the braB gene were identified as those that restored growth at low leucine concentration and Na(+)-dependent leucine transport activity to P. aeruginosa PAO3536 defective in the transport of branched-chain amino acids. Determination of the nucleotide sequence of the DNA fragment shows that the braB gene comprises 1311 bp and encodes a hydrophobic protein of 437 amino acids with a calculated Mr of 45,279. The hydropathy profile suggests that there exist in the carrier protein 12 hydrophobic segments long enough to traverse the membrane. The amino acid sequence shows a high degree of homology with the brnQ product, a branched-chain amino acid carrier of Salmonella typhimurium, while no homology in the nucleotide sequences is found in the braB and brnQ genes.

Na+(Li+)/branched-chain amino acid cotransport in Pseudomonas aeruginosa

A transport system for branched-chain amino acids (designated as LIV-II system) in Pseudomonas aeruginosa requires Na+ for its operation. Coupling cation for this system was identified by measuring cation movement during substrate entry using cation-selective electrodes. Uptakes of Na+ and Li+ were induced by the imposition of an inwardly-directed concentration gradient of leucine, isoleucine, or valine. No uptake of H+ was found, however, under the same conditions. In addition, effects of Na+ and Li+ on the kinetic property of the system were examined. At chloride salt concentration of 2.5 mM, values of apparent Km and Vmax for leucine uptake were larger in the presence of Na+ than Li+. These results indicate that the LIV-II transport system is a Na+(Li+)/substrate cotransport system, although effects of Na+ and Li+ on kinetics of the system are different.

Genetic mapping of bra genes affecting branched-chain amino acid transport in Pseudomonas aeruginosa

Pseudomonas aeruginosa PAO mutants defective in the transport systems for branched-chain amino acids were isolated and characterized. Two mutations in strains selected for trifluoroleucine resistance, braA300 and braB307, were mapped in the met-9020-dcu-9108 and the nar-9011-puuC10 region, respectively. The mutation loci in strains selected for azaleucine resistance, braC310 and bra-311 through bra-314, were all located near the fla genes, with an order of region I fla-bra-region II fla. Strains with braA300 showed a marked reduction in the high-affinity branched-chain amino acid transport system (LIV-I) and a considerable decrease in the lower-affinity system (LIV-II). Strains with braB307 were found to be defective in the LIV-II system. Strains selected for azaleucine resistance were all defective only in the LIV-I system and fell into three phenotypically distinct classes. Strains with braC310 produced a binding protein for leucine, isoleucine, valine, alanine, and threonine (LIVAT-BP) altered in binding ability, indicating that the braC gene is the structural one for the LIVAT-BP. Strains with bra-311 or bra-312 showed a complete loss of production of the LIVAT-BP. Strains with bra-313 or bra-314 produced normal levels of functional LIVAT-BP, suggesting that these mutations are located in a gene(s) other than braC.

Isolation and characterization of a Pseudomonas aeruginosa PAO mutant defective in the structural gene for the LIVAT-binding protein

A mutant of Pseudomonas aeruginosa PAO which has a defect in the structural gene for a binding protein for leucine, isoleucine, valine, alanine, and threonine (LIVAT-binding protein) was isolated and characterized. DL-4-azaleucine was taken up via the high-affinity branched-chain amino acid transport system (LIV-I), but not via the low affinity system (LIV-II), and then inhibited the growth of P. aeruginosa cells. This finding enabled us to select mutants defective in the LIV-I transport system alone. Among such mutants, strain PAO3530 was found to produce an altered LIVAT-binding protein. The shock fluid of this strain contained a normal level of the protein which corresponded to the wild-type LIVAT-binding protein as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and by an immunological test. However, the shock fluid showed almost no binding activity for branched-chain amino acids, suggesting that strain PAO3530 has a defect in the structural gene for the LIVAT-binding protein. The mutation locus (bra-310) was mapped in a region between cnu-9001 and oru-325 on the chromosome of P. aeruginosa PAO by conjugation mediated by plasmid FP5 or R68.45.