Identification and characterization of an endolysin encoded by the Streptomyces aureofaciens phage μ1/6

An open reading frame homologous to the genes encoding several cell-wall hydrolyzing enzymes was identified on the genome of actinophage mu1/6. This open reading frame encoding the putative endolysin was amplified by polymerase chain reaction and cloned into the expression vector pET-21a. This gene consisted of 1182 bp encoding a 393 amino acid polypeptide with a molar mass of 42.1 kDa. The gene product was overexpressed in Escherichia coli, and then the lytic enzyme was purified by a two-step chromatographic procedure. When applied exogenously, the endolysin of phage mu1/6 was active against all tested Streptomyces strains but did not affect other bacteria. The amino acid sequence showed a high homology with a putative amidase of the Streptomyces phage PhiC31. Downstream of the endolysin gene, an open reading frame encoding an 88 amino acid protein was identified. Structural analysis of its sequence revealed features characteristic for holin.