Inducible l-Alanine Exporter Encoded by the Novel GeneygaW(alaE) in Escherichia coli
ABSTRACTWe previously isolated a mutant hypersensitive tol-alanyl-l-alanine from a non-l-alanine-metabolizingEscherichia colistrain and found that it lacked an induciblel-alanine export system. Consequently, this mutant showed a significant accumulation of intracellularl-alanine and a reduction in thel-alanine export rate compared to the parent strain. When the mutant was used as a host to clone a gene(s) that complements the dipeptide-hypersensitive phenotype, two uncharacterized genes,ygaWandytfF, and two characterized genes,yddGandyeaS, were identified. Overexpression of each gene in the mutant resulted in a decrease in the intracellularl-alanine level and enhancement of thel-alanine export rate in the presence of the dipeptide, suggesting that their products function as exporters ofl-alanine. SinceygaWexhibited the most striking impact on both the intra- and the extracellularl-alanine levels among the four genes identified, we disrupted theygaWgene in the non-l-alanine-metabolizing strain. The resulting isogenic mutant showed the same intra- and extracellularl-alanine levels as observed in the dipeptide-hypersensitive mutant obtained by chemical mutagenesis. When each gene was overexpressed in the wild-type strain, which does not intrinsically excrete alanine, only theygaWgene conferred on the cells the ability to excrete alanine. In addition, expression of theygaWgene was induced in the presence of the dipeptide. On the basis of these results, we concluded that YgaW is likely to be the physiologically most relevant exporter forl-alanine inE. coliand proposed that the gene be redesignatedalaEforalanineexport.