In all organisms, genetic and transcriptional compartmentalization during cytokinesis requires coordination between chromosome segregation and remodeling of the cell envelope. In bacteria that produce spores, such as the model bacterium Bacillus subtilis and pathogen Clostridioides difficile, how these processes are coordinated during sporulation remains a mystery.
A hallmark of sporulation is the formation of an asymmetric septum that divides the sporulating cell into two cells: a smaller forespore that develops into a dormant spore and a larger mother cell that contributes to spore maturation but then dies. Interestingly, the asymmetric septum forms over the forespore chromosome, trapping ⁓25% of the chromosome in the forespore, with the remaining 75% being translocated across the septum by the highly conserved DNA translocase SpoIIIE. Asymmetric division also triggers cell-specific transcription that initiates septal peptidoglycan remodeling by hydrolytic and synthetic enzymes required for spore envelope formation, during a process called engulfment. How the chromosome is translocated across the septum has remained controversial. In one model SpoIIIE translocates DNA across an unfused septal membrane (Aqueous Pore Model). In a second model, SpoIIIE forms channels that translocate DNA across fused septal membranes (Channel Model). Neither model consider the fact that DNA translocation occurs concurrently with peptidoglycan remodeling of the asymmetric septum.
Using B. subtilis, and various genetic, biochemical and microscopy approaches, we demonstrate that peptidoglycan remodeling and chromosome segregation are coordinated at a highly stabilized septal pore. We reveal that septal pore stability is maintained by multiple factors including SpoIIIE, a poorly-characterized protein called SpoIIIM, the peptidoglycan synthase PbpG and the highly conserved SpoIIIAH-SpoIIQ zipper-like interaction across the septal membrane. In the absence of these factors, septal peptidoglycan hydrolysis, and chromosome-induced turgor pressure on the septal peptidoglycan, lead to septal pore expansion, loss of cytoplasmic and chromosomal compartmentalization, and a block to sporulation.
Collectively, our work reveals the coordination between peptidoglycan remodeling and chromosome segregation that maintains genetic and cytoplasmic compartmentalization during bacterial sporulation.