γ-PGA hydrolases of phage origin in Bacillus subtilis and other microbial genomes

Poly-gamma-glutamate (gamma-PGA) is an industrially interesting polymer secreted mainly by members of the class Bacilli which forms a shield able to protect bacteria from phagocytosis and phages. Few enzymes are known to degrade gamma-PGA; among them is a phage-encoded gamma-PGA hydrolase, PghP. The supposed role of PghP in phages is to ensure access to the surface of bacterial cells by dismantling the gamma-PGA barrier. We identified four unannotated B. subtilis genes through similarity of their encoded products to PghP; in fact these genes reside in prophage elements of B. subtilis genome. The recombinant products of two of them demonstrate efficient polymer degradation, confirming that sequence similarity reflects functional homology. Genes encoding similar gamma-PGA hydrolases were identified in phages specific for the order Bacillales and in numerous microbial genomes, not only belonging to that order. The distribution of the gamma-PGA biosynthesis operon was also investigated with a bioinformatics approach; it was found that the list of organisms endowed with gamma-PGA biosynthetic functions is larger than expected and includes several pathogenic species. Moreover in non-Bacillales bacteria the predicted gamma-PGA hydrolase genes are preferentially found in species that do not have the genetic asset for polymer production. Our findings suggest that gamma-PGA hydrolase genes might have spread across microbial genomes via horizontal exchanges rather than via phage infection. We hypothesize that, in natural habitats rich in gamma-PGA supplied by producer organisms, the availability of hydrolases that release glutamate oligomers from gamma-PGA might be a beneficial trait under positive selection.