SpoIIB localizes to active sites of septal biogenesis and spatially regulates septal thinning during engulfment in Bacillus subtilis

A key step in the Bacillus subtilis spore formation pathway is the engulfme nt of the forespore by the mother cell, a phagocytosis-like process normall y accompanied by the loss of peptidoglycan within the sporulation septum, W e hale reinvestigated the role of SpoIIB in engulfment by using the fluores cent membrane stain FM 4-64 and deconvolution microscopy, We have found tha t spoIIB mutant sporangia display a transient engulfment defect in which th e forespore pushes through the septum and bulges into the mother cell, simi lar to the situation in spoIID, spoIIM, and spoIIP mutants. However, unlike the sporangia of those three mutants, spoIIB mutant sporangia are able to complete engulfment; indeed, by time-lapse microscopy, sporangia with promi nent bulges were found to complete engulfment. Electron micrographs showed that in spoIIB mutant sporangia the dissolution of septal peptidoglycan is delayed and spatially unregulated and that the engulfing membranes migrate around the remaining septal peptidoglycan. These results demonstrate that m other cell membranes will move around septal peptidoglycan that has not bee n completely degraded and suggest that SpoIIB facilitates the rapid and spa tially regulated dissolution of septal peptidoglycan. In keeping with this proposal, a SpoIIB-myc fusion protein localized to the sporulation septum d uring its biogenesis, discriminating between the site of active septal biog enesis and the unused potential division site within the same cell.