Characterization of the Bacillus anthracis SleL Protein and its Role in Spore Germination

Bacillus anthracis is a spore-forming bacterium that is included on the list of select agents compiled by the Centers for Disease Control. When a B. anthracis spore germinates, a protective layer of peptidoglycan known as the cortex must be depolymerized by germination-specific lytic enzymes (GSLEs) before the bacterium can become a metabolically active vegetative cell. By exploiting cortex lytic enzymes it may be possible to control germination. This could be beneficial in elucidating ways to enhance current decontamination methods.

In this work we created in-frame deletion mutants to study not only the role of one GSLE, SleL, but by creating multi-deletion mutants, we were able to analyze how the protein cooperates with other lytic enzymes to efficiently hydrolyze the cortical PG. We determined that SleL plays an auxiliary role in complete peptidoglycan hydrolysis, secondary to cortex lytic enzymes CwlJ1, CwlJ2, and SleB. The loss of sleL results in a delay in the loss of optical density during germination. However, spores are capable of completing germination as long as CwlJ1 or SleB remains active. HPLC analysis of muropeptides collected from B. anthracis sleL strains indicates that SleL is an N-acetylglucosamidase that acts on cortical PG to produce small muropeptides which are quickly released from the germinating spore.

By analyzing the in vitro and in vivo activities of SleL we confirmed the enzymatic activity of the protein, characterized its substrates, and studied the roles of its putative LysM domains in substrate binding and spore-protein association. We were able to show that purified SleL is capable of depolymerizing partially digested spore PG resulting in the production of N-acetylglucosaminidase products that are readily released as small muropeptides. In vitro, loss of the LysM domain(s) decreases hydrolysis effectiveness. The reduction in hydrolysis is likely due to LysM domains being involved in substrate recognition and PG binding. When the SleL derivatives are expressed in vivo those proteins lacking one or both LysM domains do not associate with the spore, suggesting that LysM is involved in directing protein localization.