Characterization of Bacillus subtilis γ-glutamyltransferase and its involvement in the degradation of capsule poly-γ-glutamate

During early stationary phase, Bacillus subtilis NAFM5 produces capsular poly(gamma-glutamic acid) (gammaPGA, 2 x 10(6) Da), which contains D- and L-glutamate, and then degrades it during late stationary phase. The gamma-glutamyltransferase (EC 2.3.2.2; GGT) of this strain successively hydrolysed gammaPGA from the amino-terminal end, to yield both D- and L-glutamate. This enzyme was specifically synthesized during the stationary phase through transcriptional activation of the corresponding ggt gene by the ComQXPA quorum-sensing system. A ggt knockout mutant degraded gammaPGA into 1 X 10(5) Da fragments, but not any further, indicating that the capsule gammaPGA is first internally degraded by an endo-type of gammaPGA hydrolase into 1 X 10(5) Da intermediates, then externally into glutamates via GGT. Due to its inability to generate the glutamates from the capsule, the ggt mutant sporulated more frequently than the wild-type strain. The results show that B. subtilis GGT has a powerful exo-gamma-glutamyl hydrolase activity that participates in capsule,PGA degradation to supply stationary-phase cells with constituent glutamates.