The functioning of the alanine/D-alanyl-D-alanine pathway of was investigated by determining prefnsor pool levels and specific enzyme activities under various growth conditions. Cells grown on D- or L-alanine showed several remarkable features compared with cells grown on other carbon sources: 10-fold higher values of the D-alanyl-D-alanine and the UDP-MurNAc-pentapeptide pools, a 240-fold increase of the alanine racemase activity, and the absence of bacteriolysis after treatment with D-cycloserine at high concentrations (50 μg ml). In cells grown on glucose, D-cycloserine (1 μg ml) led to depletion of the D-alanyl-D-alanine pool and to lysis, which was efficiently antagonized by chloramphenicol. A threefold increase of the dipeptide pool was observed when cells were treated with chloramphenicol alone. The alanine racemase activity was lowest in glucose-grown cells and the D-alanine: D-alanine ligase and D-alanyl-D-alanine-adding activities were the same whatever the carbon source. Molecular masses of 53–56 kDa and 56–60 kDa were estimated for the partially purified inducible alanine racemase and D-alanine: D-alanine ligase respectively.


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