From bacteria on up, all the way to the cells of our bodies, most cells divide by binary fission. That is, the process initiates at midcell, usually by constriction, and proceeds until the two resultant daughter cells become separated. Most often, these two sister cells are identical in size and shape. Boring. Enter Bacillus subtilis (and other Gram positive sporeformers) and things liven up. Although these organisms follow the conventional path when dividing vegetatively, that is, growing unhindered, they go a separate way when making spores. Exciting.
Spore formation in bacteria is an elaborate matter. It involves making a division septum at a precise spot away from the cell center, near one of the poles. Don't ask how B. subtilis figures out where this spot is – nobody knows. But the result is an asymmetric division, resulting in a larger cell called the mother cell, and a smaller one, the forespore, which will eventually become a spore. All this orchestration is masterminded by a series of regulatory genes that kick in at precise times. The question arises, are the two septa (in vegetative growth and during spore formation) the same? If you guessed that they are different, you would be right. The subsequent events in sporulation are just as fascinating. In time, the mother cell engulfs the forespore, a phagocytosis-like process that is otherwise nearly unknown in the prokaryotic world. This allows the spore to mature (and drop the "fore" moniker) and be released when the mother cell lyses.
A study from the labs of Elizabeth Villa and Kit Pogliano at the University of California at San Diego sheds new light on details of the formation of the two septa. But first, some details about bacterial cell division in general. In most bacteria, it starts when a somewhat magical protein called FtsZ polymerizes into a peripheral ring, the FtsZ-ring, at the division site. The ring constricts progressively until the cell divides. FtsZ is a tubulin homolog, the first such to have been found in bacteria. It serves as a scaffold for about a dozen other proteins that form a division machinery called the "divisome." In B. subtilis, one such protein, an actin homolog called FtsA, sticks FtsZ to the cell membrane, making the so called FtsAZ filaments.
The septa in vegetative and sporulating cells differ in various ways besides their cellular position, the former being about four times thicker than the latter. For convenience, we will refer to the two as the "vegetative septum" and the "sporulation septum." Involved in making the sporulation septum is a sporulation-specific protein called SpoIIE *) that is localized on the forespore side of the septum. "Treadmilling" along this septum, that is, growing at the head and losing material at the tail of the FtsZ filament, allows the circumferential motion of enzymes to carry out the synthesis of the main cell wall polymer, peptidoglycan (PG). The work in this preprint reveals that the FtsAZ filaments are only present on the mother side of the sporulation septum, which, for all we know, may well explain why it is thinner. SpoIIE regulates both the location of the divisome and the thickness of the sporulation septum.
This work relied on cryo-electron tomography carried out with a special twist called FIB, which Kanika Khanna, first author of the present study, had explained in a recent STC post. Briefly, because B. subtilis is too thick for cryo-ET, it must be first scraped into thin slices using a process called cryo-focused ion beam milling (FIB). Such "milled" sections can now be conveniently studied via cryotomography.
Using these techniques to visualize both the vegetative and the sporulating septa, the authors added detailed knowledge to our understanding of these processes. Thus, they found that in vegetative cells the FtsA filaments are located near the cell membrane, the FtsZ ones, away from it. Also, they found interesting differences in the architecture of Gram negative bacteria such as E. coli and Gram positive ones, such as B. subtilis, some of which are explainable by the thicker peptidoglycan layer in the cell walls of the latter.
Why is the sporulation septum thinner than the vegetative one? The authors propose that, among other things, this may facilitate the engulfment process of the forespore by the mother cell. The article carries an impressive amount of data dealing with other details of the formation of both vegetative and sporulation septa.
The authors end their paper by stating: "Our data provide a significant advancement in the understanding of the organization of the divisome, its regulators and its role in septal PG synthesis in Bacillus subtilis, providing, to the best of our knowledge, the first evidence for the asymmetric assembly of the divisome in bacteria, and showing that this depends on the SpoIIE protein that is required for the onset of cell specific gene expression. Future efforts to identify the molecular arrangement of FtsZ and its regulators inside the cell such as SpoIIE, will aid in development of new antimicrobials targeting the cell division machinery in important pathogens."
*) SpoIIE is a phophatase active in the pre-spore compartment of B. subtilis during septum formation, and should not to be confused with the DNA translocase SpoIIIE.