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In exponentially growing and dividing cells of Streptococcus faecalis, it is proposed that the leading edge of the annularly closing cross wall is the point of extension for both cross wall and peripheral wall. Peripheral wall extension is thought to be produced by the separation or splitting of the cross wall at its junction with peripheral wall. This results in the pushing of the equatorial wall bands, found on S. faecalis walls, to subequatorial positions. These bands therefore mark the separation of old wall from new wall. Mesosomal formation was observed usually to precede cross wall initiation.

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A hallmark of sporulation of Bacillus subtilis is the formation of two distinct cells by an asymmetric division. The developmental programs in these two cells involve the compartmentalized activities of E in the larger mother cell and of F in the smaller prespore. Activation of E requires expression of the F -directed gene spoIIR. By immunofluorescence microscopy of a strain containing a spoIIR-lacZ fusion, we have shown that spoIIR is transcribed exclusively in the prespore. By placing spoIIR under the control of P spoIIE , it was possible to express spoIIR before the spore septum was formed. Strains containing the P spoIIE -spoIIR construct activated E only in the mother cell in organisms that underwent the asymmetric sporulation division. During an early stage of spore formation in bacilli and clostridia, an asymmetric division occurs, yielding two distinct cell types, the mother cell and the prespore (also called the forespore). Upon the formation of these two cell types, two different genetic programs are initiated. These two programs are transcriptionally controlled by the compartmentalized activities of a set of sigma factors (4, 21). Early transcription occurs in the mother cell through the action of RNA polymerase containing E (E-E ) and in the prespore through the action of E-F . The structural genes for the two sigma factors are transcribed and translated before the asymmetric (sporulation) division (6, 26) occurs, but the sigma factors do not become active until the formation of the asymmetric septum (1, 17). Both the activation and compartmentalization of F in the prespore can occur in the absence of E (2, 22). In contrast, the activation of E requires the activity of F (12, 34) through the activity of the F -directed gene spoIIR (15,20). It has been suggested by Losick and Stragier (21) that spoIIR, which was originally called X, would be transcribed by E-F in the prespore and act as a vectorial cell-cell signal that would activate E only in the mother cell. Thus, compartmentalization of spoIIR expression in the prespore could in itself lead to the compartmentalization of E activity in the mother cell. Consistent with SpoIIR acting as a cell-cell signal, characterization of the spoIIR gene indicated that it encoded a protein with a secretion signal (15), and recent biochemical analysis has shown that SpoIIR is secreted (10). Once secreted, SpoIIR could then interact with SpoIIGA, a multispanning membrane protein that has been proposed to be the protease that cleaves the pro sequence from pro-E to activate it (14, 23, 34). However, because SpoIIR is secreted, as opposed to it being membrane bound, it may not act vectorially across the sporulation septum from the prespore membrane to the mother cell membrane. Rather, SpoIIR may be free to diffuse in the space between the membranes and to interact with SpoIIGA or other factors that might be in both the prespore and mother cell membranes. Thus, although SpoIIR clearly acts as the prespore signal required to activate transcription in the mother cell, an...