Cell Cycle Machinery in Bacillus subtilis

Bacillus subtilis is the best described member of the Gram positive bacteria. It is a typical rod shaped bacterium and grows by elongation in its long axis, before dividing at mid cell to generate two similar daughter cells. B. subtilis is a particularly interesting model for cell cycle studies because it also carries out a modified, asymmetrical division during endospore formation, which can be simply induced by starvation. Cell growth occurs strictly by elongation of the rod, which maintains a constant diameter at all growth rates. This process involves expansion of the cell wall, requiring intercalation of new peptidoglycan and teichoic acid material, as well as controlled hydrolysis of existing wall material. Actin-like MreB proteins are the key spatial regulators that orchestrate the plethora of enzymes needed for cell elongation, many of which are thought to assemble into functional complexes called elongasomes. Cell division requires a switch in the orientation of cell wall synthesis and is organised by a tubulin-like protein FtsZ. FtsZ forms a ring-like structure at the site of impending division, which is specified by a range of mainly negative regulators. There it recruits a set of dedicated division proteins to form a structure called the divisome, which brings about the process of division. During sporulation, both the positioning and fine structure of the division septum are altered, and again, several dedicated proteins that contribute specifically to this process have been identified. This chapter summarises our current understanding of elongation and division in B. subtilis, with particular emphasis on the cytoskeletal proteins MreB and FtsZ, and highlights where the major gaps in our understanding remain.