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Shio Makino, Ryuichi Moriyama
Med Sci Monit 2002; 8(6): RA119-127
Despite the most extreme dormancy and resistance properties among livingsystems, bacterial endospores retain an alert sensory mechanism to respond to the germinants and initiategermination. Although the molecular mechanism of the germination process is not completely described,current progress in the studies on the enzymes involved in the process gave us a somewhat clearer pictureof the process of spore peptidoglycan (cortex) hydrolysis, a major biochemical event in germination.Germination-specific cortex-lytic enzymes require muramic acid d-lactam in their substrates. At leasttwo types of enzymes are involved in the germination process: a spore cortex-lytic enzyme (SCLE) anda cortical fragment-lytic enzyme (CFLE). Except for their peptidoglycan-binding regions, the primarystructures of SCLE and CFLE vary according species. Both enzymes differ in their hydrolytic bond-specificitiesand recognition of the substrates morphology. SCLE appears to initiate germination by uncrosslinkingthe intract cortex, and the CFLE further degrades the polysaccharide moiety of the SCLE-modified cortex.In vivo CFLE activity is likely regulated by its requirement for partially un-crosslinked cortex, whileSCLE requires activation process. Clostridium perfringens SCLE is activated by a germination-specificserine protease during germination, but the activation mechanism of SCLE in Bacillus species is unknown.Cortex-lytic enzymes are expressed at the early stage of sporulation but the compartment of expressiondepends on proteins. However, all enzymes are located outside the cortex layer in dormant spores, suggestingthat the hydrolysis process initiates at the exterior side of the cortex. The assembly of the germinationapparatus is also discussed.