Four “y” genes in B. subtilis genome encode γ-PGA hydrolases

Poly-γ-glutamic acid (γ-PGA) is a high molecular weight homo-polyamide composed of glutamic acid monomers connected by amide linkages between the α-amino and γ-carboxylic groups which is synthesized by several microorganisms, mainly belonging to the Bacillales. The polymer is endowed with a number of favourable characteristics, being nontoxic, biodegradable, highly anionic, water soluble, superabsorbent and rather stable. For this reason it represents a promising biomaterial in a plurality of biotechnological applications. Several hypotheses have been advanced to explain the biological function of γ-PGA in bacteria: to provide protection against dehydration and heavy metals toxicity; to enhance virulence by shielding the bacterium from the host immune surveillance (e.g. in pathogens, such as B. anthracis); to act as an anti-phage shield; to serve as nourishing reserve once secreted in the environment. Four “y” genes present in B. subtilis genome, buried in genetic elements of prophage origin, have been identified as coding for γ-PGA hydrolases. Indeed, homologues are present in several B. subtilis infecting phages, reinforcing the notion that γ-PGA constitutes an effective anti-viral defense in Bacteria. However, highly homologous genes are also found in a large number of non-Bacillales species (126) that neither carry the γ-PGA biosynthetic genes nor have species-specific hydrolases-encoding phages. Thus, the wide distribution of γ-PGA hydrolases genes in those bacterial species is unexpected. Our hypothesis is that such genes spread to non-Bacillales species through horizontal gene transfer, rather than de-novo phage infections, for the reason that they confer an evolutionary advantage during growth in the natural habitat. The relevance of γ-PGA hydrolases in microbial fitness might reside in the possibility of endowed organisms to feed on short glutamate oligomers released from γ-PGA secreted in the common environment by other producing species. In B. subtilis the hypothesis of an evolutionary advantage conferred by γ-PGA hydrolases is strengthened by the fact these genes are expressed (Nicolas et al., 2012; Rasmussen et al., 2009) and that four different paralogues are maintained in the genome of several strains. Karamata and coworkers (Lazarevic et al., 1999) had postulated that the maintenance of B. subtilis prophages was under positive selective pressure; our work suggests that γ-PGA hydrolases genes might be partly involved in driving such positive selection. References Nicolas et al., 2012. Science 335:1103-6. Rasmussen et al., 2009. Mol Microbiol 73:1043-57. Lazarevic et al., 1999. Microbiology 145:1055-67.