Sporulation in the model endospore-forming organism Bacillus subtilis proceeds via the sequential and stage-specific activation of the sporulation-specific sigma factors, σH (early), σF, σE, σG, and σK (late). Here we show that the Clostridium acetobutylicum σK acts both early, prior to Spo0A expression, and late, past σG activation, thus departing from the B. subtilis model. The C. acetobutylicum sigK deletion (ΔsigK) mutant was unable to sporulate, and solventogenesis, the characteristic stationary-phase phenomenon for this organism, was severely diminished. Transmission electron microscopy demonstrated that the ΔsigK mutant does not develop an asymmetric septum and produces no granulose. Complementation of sigK restored sporulation and solventogenesis to wild-type levels. Spo0A and σG proteins were not detectable by Western analysis, while σF protein levels were significantly reduced in the ΔsigK mutant. spo0A, sigF, sigE, sigG, spoIIE, and adhE1 transcript levels were all downregulated in the ΔsigK mutant, while those of the sigH transcript were unaffected during the exponential and transitional phases of culture. These data show that σK is necessary for sporulation prior to spo0A expression. Plasmid-based expression of spo0A in the ΔsigK mutant from a nonnative promoter restored solventogenesis and the production of Spo0A, σF, σE, and σG, but not sporulation, which was blocked past the σG stage of development, thus demonstrating that σK is also necessary in late sporulation. sigK is expressed very early at low levels in exponential phase but is strongly upregulated during the middle to late stationary phase. This is the first sporulation-specific sigma factor shown to have two developmentally separated roles.
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