Some strains of Bacillus species, such as B. subtilis, B. licheniformis and B. anthracis, become encapsulated with polypeptides of D-glutamate or of D- and L-glutamates linked through γ-carboxyl-α-amide bonds (poly-γ-glutamate; γ-PGA). We recently discovered two functions of capsule γ-PGA in B. subtilis. Like the polysaccharide capsules of other bacteria, the polypeptide capsule can protect B. subtilis cells from infection with bacteriophages (phages) by hampering the access of phages to their receptors on the host cell surface. In addition to the protective function, capsule γ -PGA serves B. subtilis as a nutrient reserve for starvation during the stationary phase. The bacterium degrades the g-PGA via a two-step process late in the stationary phase. Endo- γPGA hydrolase internally fragments 2 MDa capsule γ-PGA into 100-kDa intermediates that are then externally decomposed by γ-glutamyltransferase (GGT, the ggt product) into constituent D- and L-glutamates. The B. subtilis cells uptake and utilize the resultant glutamates as nitrogen sources. The D-isomer is converted into the L-isomer via glutamate racemases (racE and yrpC products) before being catabolized. The ComQXPA quorum-sensing regulatory system controls the synthesis of both γ-PGA synthetic proteins and the degradation enzyme GGT so that they can be specifically expressed during the dense-cell stationary phase when nutrients become limited. Such cell-density dependent expression and genetic evidence acquired from a ggt knockout mutant support the notion that γPGA is the glutamate storage that allows B. subtilis to survive during the stationary phase and provides a novel bacterial adaptation strategy under conditions of nutrient deficiency.
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