Cell cycle proteins and asymmetric division during Bacillus subtilis sporulation

The generation of asymmetry is a fundamental process in developmental biology, allowing one cell to differentiate into daughter cells with independent cell fates. The process of sporulation in the Gram-positive bacterium Bacillus subtilis involves a switch in the site of cytokinesis from the midcell position to an extreme polar position. This asymmetric division creates two unequally sized compartments and initiates a developmental pathway that culminates in the formation of a dormant spore from the smaller compartment, while the larger compartment lyses and dies upon spore maturation. Thus, B. subtilis is an ideal model organism with which to investigate asymmetric division and differing cell fates.; The cell division proteins FtsA and FtsZ are required for both medial and polar division in B. subtilis. This thesis reports the construction and execution of a genetic screen to identify mutants of FtsA and FtsZ that are defective in sporulation but unimpaired in vegetative growth. One such mutant, FtsAD265G exhibited little or no growth defect but was impaired in sporulation at the stage of polar division. This mutant produced normal medial septa but immature polar septa that were defective in undergoing engulfment, the next morphological stage of sporulation. FtsA D265G was defective in gene transcription under the control of sigma F, a developmental transcription factor whose activation depends on polar septation. However, FtsAD265G exhibited no defect in the subcellular localization of SpoIIE, a septum-bound protein phosphatase that is required for sigma F activation, or in the accumulation of the dephosphorylated form of SpoIIAA, an effector of sigma F that is generated by the SpoIIE phosphatase. Additionally, we found that an amino acid substitution at Y264, the residue adjacent to D265, caused a defect in sporulation. Since both of these residues are conserved among other spore-forming species, we postulate that the residues constitute a sporulation-specific domain of FtsA that could be a contact site for a protein required for polar septation and sigma F activation. Our results suggest that FtsA plays a distinctive role in polar septation and implicate FtsA in sigma F regulation. These and other FtsA and FtsZ mutants generated from the screen allowed investigation into the nature of asymmetric cell division during sporulation in B. subtilis.