Sin is a Bacillus subtilis DNA-binding protein which is essential for competence, motility, and autolysin production but also, if expressed on a multicopy plasmid, is inhibitory to sporulation and alkaline protease synthesis. We have now examined the physiological role of Sin in sporulation and found that this protein specifically represses three stage II sporulation genes (spoIL4, spoIIE, and spoIIG) but not the earlier-acting stage 0 sporulation genes. sin loss-of-function mutations cause higher expression of stage II genes and result in a higher frequency of sporulation, in general. Sin binds to the upstream promoter region of spoILU in vitro and may thus gate entry into sporulation by directly repressing the transcription of stage II genes. In vivo levels of Sin increase rather than decrease at the time of stage II gene induction, suggesting that posttranslational modification may play a role in downregulation of negative Sin function.Bacillus subtilis escapes adverse environmental conditions, such as lack of nutrients, by differentiating into a dormant cell known as the endospore. This differentiation process can be viewed as a cellular decision to stop simple vegetative growth and to begin a series of physiological and morphological changes which result in a new cell type. Two major requirements must be satisfied for a successful completion of any procaryotic differentiation process: (i) the correct processing of multiple environmental signals and (ii) the successful coordination of subsequent morphological changes. Many sporulation genes which control the latter have been identified, and the network through which they communicate is just starting to reveal a sophisticated set of mechanisms which coordinate their temporal and spatial regulation (reviewed in references 17 and 34). In the case of the first requirement, which deals with whether the cell should enter a dormant way of life, it was shown recently that a Bacillus cell communicates environmental signals (low nutrients in the environment) to the sporulation genetic machinery by means of a multicomponent phosphorelay system (5). Although it is not clear as yet how the phosphorelay cascade is set into motion, i.e., how and through which component(s) the intercellular sensor KinA (also known as SpoIIJ) (2, 21) recognizes environmental change, the genetic evidence suggests that phosphorylation of the last phosphorelay protein, SpoOA, is a sporulation-triggering signal (20). The earliest known response to phosphorylation of SpoOA is downregulation of the negative regulator AbrB and the resultant increase in expression of late growthregulated genes (35), including spoOH (42), which codes for transcription factor &e (9,45). This in turn causes increased expression of kinA, spoOF, spoOA, and spoVG, which are transcribed by 26). We have recently suggested that this double-negative control may serve as one of the first checkpoints for whether a stress-subjected cell will enter sporulation (33). The second major function of SpoOA phosphate is to increase the ...
