Difference in Sodium Requirement of Branched Chain Amino Acid Carrier between Pseudomonas aeruginosaPAO and PML Strains Is Due to Substitution of an Amino Acid at Position 292*

Sodium dependence of leucine transport, a measure of the Na+-coupled leucine-isoleucine-valine II (LIV-II) transport system in Pseudomonas aeruginosa, was compared between two wild-type strains, PAO and PML. The leucine transport activity was saturated at 0.1 mMNaCl for PAO and at 5.0 mMfor PML. From kinetics experiments, the apparent Kmvalue for Na+with respect to leucine transport was estimated to be 3 µMfor PAO and 95 µMfor PML. The Kmvalue for leucine was 6 µMfor PAO and 13 µMfor PML. The LIV-II carrier gene (braB) of PML was isolated for comparison of its amino acid sequence with that of the PAO carrier cloned previously. The Kmvalues for Na+and leucine of the cloned LIV-II carriers of PAO and PML expressed in LIV-II defective mutants were similar to those in wild-type strains. Determination of the nucleotide and deduced amino acid sequences of the LIV-II carrier gene of PML showed an amino acid difference at position 292 between the PAO and PML carriers. The amino acid was threonine for PAO and alanine for PML. These results indicate that the substitution of the amino acid at position 292 of the LIV-II carrier causes a difference in the sodium requirement of the carriers of the PAO and PML strains.