Characterization of SpoIIB, SpoVG, and SpoVS: Defining a relationship between the transition phase and engulfment during sporulation in Bacillus subtilis

During environmental stress, Bacillus subtilis assesses external and internal conditions to determine how best to cope with the ensuing stress. The bacterium has several options, ranging from becoming motile to initiating competence, biofilm formation, or sporulation. Research has aimed to understand both the specific details of differentiation and the cross-regulation involved in transitioning from one life cycle to another. Research presented in this dissertation has answered questions in each of these categories.; The first morphological event in sporulation is the positioning of an asymmetric septum that unevenly divides the cell into the small forespore and larger mother cell compartments. Immediately following thinning of the septal peptidoglycan, the mother cell membrane moves up and around the forespore, in a process known as engulfment. An important question that had yet to be answered at the onset of my studies was the mechanism of septal thinning. My studies on the sporulation protein, SpoIIB, provided a starting point to answer this question. Electron and fluorescence microscopy studies revealed that spoIIB mutants display a transient bulge phenotype during engulfment due to uneven septal thinning. Other studies in our lab confirmed that SpoIIB is involved in spatially regulating septal thinning by localizing the proteins necessary for the degradation of septal peptidoglycan and the migration of the mother cell membranes during engulfment.; A spoVG mutant enhances the transient spoIIB phenotype, resulting in a block at polar septation due to a complete failure to degrade the peptidoglycan. SpoVS, another sporulation protein, was identified in a genetic screen looking for suppressors of the spoIIB spoVG double mutant. The second part of my dissertation discusses the mechanism through which spoVS mediates suppression of the spoIIB spoVG mutant. Genetic analysis revealed that SpoVS negatively regulates SinR, which is a global transition state regulator involved in repressing sporulation an enhancing σD-directed transcription of motility and autolysin genes. Biochemical analysis revealed that the lack of SinR repression in the spoVS mutant results in increased σ D-directed expression of vegetative peptidoglycan hydrolases. Therefore, a spoVS mutation bypasses the spoIIB spoVG block by facilitating septal thinning and allowing the completion of engulfment.