Asymmetric cell division generates two cells that contain different regulatory proteins and express different fates. In an example of asymmetric cell division from B. subtilis, a site on the membrane of the dividing cell is chosen to establish the initial asymmetry. Recent show that a key regulatory protein, SpollE, is localized to one side of a sporulating B. subtilis cell, and subsequently functions in an asymmetric manner. SpollE is a phosphatase at the beginning of a regulatory cascade that leads to activation of a cell fate-determining transcription factor in only one daughter cell.
Asymmetric cell division during Baci//us subtifis sporulation
The spore of B. subfilis is a specialized cell that is resistant to extreme environmental conditions, but can germinate into a dividing cell under favorable circumstances. The spore derives from a single cell that divides asymmetrically to produce a small cell (the forespore) and a large mother cell, which together comprise the sporangium. The forespore is engulfed by the mother cell, which lays down the specialized spore coat over the forespore membrane. The DNA of the forespore is condensed and relatively few genes are expressed. The mother cell is a somatic cell -once it completes the task of coating the spore, it lyses(3).
In B. subtilis, the goal of the asymmetric cell division is twofold. First, a division septum must be produced on one side of the dividing cell, so that the forespore receives a smaller cytoplasm volume. Second, distinct proteins need to be expressed in the forespore and in the mother cell. Recent results from the laboratories of Losick and Stragier indicate that the SpollE protein plays a key role in connecting these two events(',*). SpollE has an N-terminal domain with eight to ten membrane-spanning segments, and a Cterminal region with a phosphatase activity.
The SpollE protein is localized, within the accuracy of the light microscope, to the site of asymmetric septum formation(') (Fig. ) . During vegetative growth, a septum is formed in the middle of a B. subfiliscell and it divides into two equalsized daughters. During sporulation, formation of the septum at the normal site is inhibited, and instead a septum is formed near one end of the cell. It appears that there are two potential sites for asymmetric septum formation, since in certain 'disporic' mutants (e.g. spollG) a forespore compartment is formed at each end of the sporangium and a central mother cell is left devoid of DNA. At an early stage of sporulation,