Roles and Regulation of the Glutamate Racemase Isogenes, RacE and YrpC, in Bacillus Subtilis

Many bacteria, including Escherichia coli, have a unique gene that encodes glutamate racemase. This enzyme catalyses the formation of d-glutamate, which is necessary for cell wall peptidoglycan synthesis. However, Bacillus subtilis has two glutamate racemase genes, named racE and yrpC. Since racE appears to be indispensable for growth in rich medium, the role of yrpC in d-amino acid synthesis is vague. Experiments with racE- and yrpC-knockout mutants confirmed that racE is essential for growth in rich medium but showed that this gene was dispensable for growth in minimal medium, where yrpC executes the anaplerotic role of racE. LacZ fusion assays demonstrated that racE was expressed in both types of media but yrpC was expressed only in minimal medium, which accounted for the absence of yrpC function in rich medium. Neither racE nor yrpC was required for B. subtilis cells to synthesize poly-gamma-dl-glutamate (gamma-PGA), a capsule polypeptide of d- and l-glutamate linked through a gamma-carboxylamide bond. Wild-type cells degraded the capsule during the late stationary phase without accumulating the degradation products, d-glutamate and l-glutamate, in the medium. In contrast, racE or yrpC mutant cells accumulated significant amounts of d- but not l-glutamate. Exogenous d-glutamate utilization was somewhat defective in the mutants and the double mutation of race and yrpc severely impaired d-amino acid utilization. Thus, both racemase genes appear necessary to complete the catabolism of exogenous d-glutamate generated from gamma-PGA.