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

Abstract

During early stationary phase, Bacillus subtilis NAFM5 produces capsular poly(gamma-glutamic acid) (gammaPGA, 2x10(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 1x10(5) Da fragments, but not any further, indicating that the capsule gammaPGA is first internally degraded by an endo-type of gammaPGA hydrolase into 1x10(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 gammaPGA degradation to supply stationary-phase cells with constituent glutamates.