Biological function of the pld gene product that degrades epsilon-poly-L-lysine in Streptomyces albulus

Epsilon-poly-L-lysine (epsilon-PL) is one of the few naturally occurring biopolymers and is characterized by a peptide bond between the alpha-carboxyl and epsilon-amino groups. Previously, we purified and characterized the epsilon-PL-degrading enzyme (Pld) from Streptomyces albulus, which is an epsilon-PL producer, and this enzyme was expected to confer self-resistance to the epsilon-PL produced by the organism itself. The gene encoding Pld was cloned based on the N-terminal amino acid sequence determined in this study, and a sequencing analysis revealed eight open reading frames (ORFs), i.e., ORF1 to ORF8 in the flanking region surrounding the pld gene (present in ORF5). To investigate the biological function of Pld, we constructed a knockout mutant in which the pld gene is inactivated. Studies on epsilon-PL susceptibility, epsilon-PL-degrading activity, and epsilon-PL productivity demonstrated that the pld gene does play a partial role in self-resistance and that S. albulus was found to produce other epsilon-PL-degrading enzyme(s) in addition to Pld. To the best of our knowledge, this is the first report on a self-resistance gene for a biopolymer possessing antibacterial activity.