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 comp lex of proteins at the site of septation that probably provides both t he structural and the cytokinetic functions required for elaboration a nd closure of the septal annulus. During sporulation in Bacillus subti lis, this complex of proteins is modified by the inclusion of a sporul ation-specific protein, SpollE, which plays a direct role in gene regu lation and also has a genetically separable role in determining the gr oss structural properties of the specialized sporulation septum. We de monstrate by both green fluorescent protein (GFP) fusions and indirect immunofluorescence microscopy that SpollGA, a protein required for pr oteolytic cleavage of pro-sigma(E), is also targeted to the sporulatio n septum. Septal localization of SpollGA-GFP occurred even in the stru cturally abnormal septum formed by a SpoIIE null mutant. We also repor t the isolation of a spollGA homologue from Bacillus megaterium, a spe cies in which the cells are significantly larger than those of B. subt ilis. We have exploited the physical dimensions of the B. megaterium s porangium, in conjunction with wide-field deconvolution microscopy, to construct three-dimensional projections of sporulating cells. These p rojections indicate that SpollGA-GFP is initially localized in an annu lus at the septal periphery and is only later localized uniformly thro ughout the septa. Localization was also detected in a B. subtilis spoO H 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 th e construction of the asymmetric septum.