The Bacillus SpoIIGA protein is targeted to sites of spore septum formation in a SpoIIE-independent manner

The process of bacterial cell division involves the assembly of a complex of proteins at the site of septation that probably provides both the structural and the cytokinetic functions required for elaboration and closure of the septal annulus. During sporulation in Bacillus subtilis, this complex of proteins is modified by the inclusion of a sporulation-specific protein, SpollE, which plays a direct role in gene regulation and also has a genetically separable role in determining the gross structural properties of the specialized sporulation septum. We demonstrate by both green fluorescent protein (GFP) fusions and indirect immunofluorescence microscopy that SpollGA, a protein required for proteolytic cleavage of pro-sigma(E), is also targeted to the sporulation septum. Septal localization of SpollGA-GFP occurred even in the structurally abnormal septum formed by a SpoIIE null mutant. We also report the isolation of a spollGA homologue from Bacillus megaterium, a species in which the cells are significantly larger than those of B. subtilis. We have exploited the physical dimensions of the B. megaterium sporangium, in conjunction with wide-field deconvolution microscopy, to construct three-dimensional projections of sporulating cells. These projections indicate that SpollGA-GFP is initially localized in an annulus at the septal periphery and is only later localized uniformly throughout the septa. Localization was also detected in a B. subtilis spoOH null strain that fails to construct a spore septum. We propose that SpollGA is sequestered in the septum by an interaction with components of the septation machinery and that this interaction begins before the construction of the asymmetric septum.