Novel spore lytic enzyme from a Bacillus phage leading to spore killing.

Bacterial spores maintain metabolic dormancy and have high resistance to external pressure. Germination requires degradation of the spore cortex and the participation of germination-specific cortex-lytic enzymes (GSLEs). Previously reported GSLEs have been identified in bacteria and facilitate germination. In this study, we have characterized a novel spore lytic enzyme, Ply67, from Bacillus pumilus phage vB_BpuM_BpSp. Ply67 had a similar cortex-lytic activity to GSLEs but disrupted the inner membranes (IMs) of spores, leading to spore killing rather than germination. The amino acid sequence of the complete protein, Ply67FL, exhibited 40% homology to the GSLE SleB. Domain prediction showed that Ply67FL was composed of three domains: a signal peptide, N-terminal domain protein and C-terminal domain protein. Ply67FL rapidly caused E. coli cells lysis when it was expressed in E. coli. The protein containing the C-terminal domain protein, Ply67C, could kill B. pumilus spores. The protein containing the N-terminal domain protein, Ply67N, could combine with the decoated B. pumilus spores, indicating that N-terminal was the binding domain and C-terminal was the hydrolase domain. The protein lacking the signal peptide but containing the N-terminal and C-terminal domain proteins, Ply67, had activity against spores of various Bacillus species. The surface of spores treated with Ply67 shrank and the permeability barrier was disrupted, and the inner contents leaked out. Immunoelectron microscopic observation showed that Ply67 was mainly acted on the spore cortex. Overall, Ply67 is a novel spore lytic enzyme that differs from other GSLEs not only in amino acid sequence but also in activity against spores, and Ply67 might have the potential to kill spores of pathogenic Bacillus species, e.g., B. cereus and B. anthracis.