Sporulation Phenotype of a Bacillus subtilis Mutant Expressing an Unprocessable but Active σE Transcription Factor

σE, a sporulation-specific sigma factor of Bacillus subtilis, is formed from an inactive precursor (pro-σE) by a developmentally regulated processing reaction that removes 27 amino acids from the proprotein's amino terminus. A sigE variant (sigE335) lacking 15 amino acids of the prosequence is not processed into mature σE but is active without processing. In the present work, we investigated the sporulation defect in sigE335-expressing B. subtilis, asking whether it is the bypass of proprotein processing or a residual inhibition of σE activity that is responsible. Fluorescence microscopy demonstrated that sigE335-expressing B. subtilis progresses further into sporulation (stage III) than do strains lacking σE activity (stage II). Consistent with its stage III phenotype, and a defect in σE activity rather than its timing, the sigE335 allele did not disturb early sporulation gene expression but did inhibit the expression of late sporulation genes (gerE and sspE). The Spo− phenotype of sigE335 was found to be recessive to wild-type sigE. In vivo assays of σE activity in sigE, sigE335, and merodiploid strains indicate that the residual prosequence on σE335, still impairs its activity to function as a transcription factor. The data suggest that the 11-amino-acid extension on σE335 allows it to bind RNA polymerase and direct the resulting holoenzyme to σE-dependent promoters but reduces the enzyme's ability to initiate transcription initiation and/or exit from the promoter.