Two alanine racemase genes in Salmonella typhimurium that differ in structure and function

Abstract

Mutations were isolated in a previously undescribed S. typhimurium gene encoding an alanine racemase essential for utilization of L-alanine as a source of C, energy and N. This new locus, designated dadB, lies within one kilobase of the D-alanine dehydrogenase locus (dadA), which is also required for alanine catabolism. The dadA and dadB genes are coregulated. Mutants (including insertions) lacking the dadB alanine racemase do not require D-alanine for growth unless a mutation is introduced at a 2nd locus, designated dal. Two genes specifying alanine racemase activity were cloned from S. typhimurium. The 2 cloned DNA sequences do not cross-hybridize with each other; one contains the dadB gene.