Coordination between Chromosome Translocation and Peptidoglycan Remodeling during Spore Development

University of Technology Sydney. Faculty of Science.In all cells, including bacteria, coordination between different molecular processes is fundamental for successful growth, division and differentiation. In sporulating bacteria, two fundamental processes, peptidoglycan remodeling and chromosome segregation, occur at the same time and are essential for the early stages of spore development. However, it remains unclear if and how they are coordinated. This thesis addresses this question using the model organism Bacillus subtilis. Upon starvation, some bacteria enter a developmental process called sporulation to produce highly-resistant and dormant cells known as spores. Initially, the starving cell divides asymmetrically in two compartments of different size: the larger one is called the mother cell and the smaller one is called the forespore. Asymmetric division triggers compartment-specific transcription controlled by sigma factors, with σF in the forespore and σE in the mother cell. Interestingly, the asymmetric septum also traps ⁓30% of the forespore chromosome in the forespore, while the remaining ~70% resides in the mother cell. Through the septum, A DNA transporter called SpoIIIE translocates the remaining ~70% of the chromosome into the forespore. Concurrently with chromosome translocation, the peptidoglycan within the asymmetric septum undergoes remodeling by hydrolytic and synthetic enzymes, which drive the internalization of the forespore into the mother cell, through a process called engulfment. During engulfment, two forespore enzymes that function to synthesize a new layer of peptidoglycan are suggested to be functionally redundant, PbpG and PbpF. However, previous observations suggest that PbpG and PbpF could function in separate pathways and thus have specialized roles during sporulation. To investigate this hypothesis, stemming from a genetic screen, this thesis identified SpoIIIM (formerly YqfZ) as being required for efficient sporulation in cells lacking PbpG. Through the phenotypic characterization of cells lacking SpoIIIM and PbpG, multiple lines of evidence led to the conclusion that SpoIIIM, PbpG and SpoIIIE coordinate peptidoglycan remodeling and chromosome translocation at a septal pore. This coordination is required to ensure septal pore stability and its closure upon complete chromosome translocation. Interestingly, other data revealed an important role for the SpoIIIAH-SpoIIQ interaction in the stabilization of the septal pore. Furthermore, the coordination between peptidoglycan remodeling and chromosome translocation was shown to happen through direct interactions between SpoIIIM, PbpG and SpoIIIE. Collectively, this thesis reveals that peptidoglycan remodeling and chromosome translocation are coordinated at a septal pore, to ensure septal pore stability, successful chromosome translocation and transcriptional compartmentalization during spore development