Directed evolution of bacterial alanine racemases with higher expression level

Bacterial alanine racemase (EC 5.1.1.1) is a pyridoxal 5'-phosphate-dependent enzyme that catalyzes the interconversion of L-alanine and D-alanine. It can be classified into two groups: biosynthetic enzymes with low catalytic activity and catabolic enzymes with high catalytic activity. It can react with serine to a limited extent. Two biosynthetic alanine racemase genes in Escherichia coli and Salmonella typhimurium were DNA shuffled, and a very diverse chimeric gene library was constructed. An E. coli serine auxotroph was transformed with the shuffled genes, and the recombinant clones were screened on selective media supplemented with 0.5-5 mM D-serine as an L-serine supplier. Selected clones were expected to contain racemases exhibiting higher catalytic activities toward alanine as well as serine. Three independent clones that grew on selective media were isolated. The specific activities of crude extracts prepared from cells expressing the chimeric racemases were increased up to approximately three times more than those expressing the parental enzymes. The best chimera Ser15 racemase was expressed at a level approximately twofold higher than the parental alanine racemases. This high protein expression was demonstrated to be posttranscriptionally achieved.