Some Bacillus subtilis strains, including natto (fermented soybeans) starter strains, produce a capsular
polypeptide of
glutamate with a gamma-linkage, called
poly-gamma-glutamate (
gamma-PGA). We identified and purified a monomeric 25-kDa degradation
enzyme for
gamma-PGA (designated
gamma-PGA hydrolase, PghP) from bacteriophage PhiNIT1 in B. subtilis host cells. The monomeric PghP internally hydrolyzed
gamma-PGA to
oligopeptides, which were then specifically converted to tri-, tetra-, and
penta-gamma-
glutamates. Monoiodoacetate and
EDTA both inhibited the PghP activity, but Zn(2+) or Mn(2+)
ions fully restored the
enzyme activity inhibited by the
chelator, suggesting that a
cysteine residue(s) and these
metal ions participate in the catalytic mechanism of the
enzyme. The corresponding pghP gene was cloned and sequenced from the phage genome. The deduced PghP sequence (208
amino acids) with a calculated M(r) of 22,939 was not significantly similar to any known
enzyme. Thus, PghP is a novel
gamma-glutamyl hydrolase. Whereas phage PhiNIT1 proliferated in B. subtilis cells encapsulated with
gamma-PGA, phage BS5 lacking PghP did not survive well on such cells. Moreover, all nine phages that contaminated natto during fermentation produced PghP, supporting the notion that PghP is important in the
infection of natto starters that produce
gamma-PGA. Analogous to
polysaccharide capsules,
gamma-PGA appears to serve as a physical barrier to phage absorption. Phages break down the
gamma-PGA barrier via PghP so that phage progenies can easily establish
infection in encapsulated cells.