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

Department of Microbiology and Immunology, University of Texas Health Science Center, San Antonio, Texas 78229-3900

Received 6 October 2003/ Accepted 22 December 2003

^E, a sporulation-specific sigma factor of Bacillus subtilis, is formed from an inactive precursor (pro-^E) 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 ^E 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 ^E activity that is responsible. Fluorescence microscopy demonstrated that sigE335-expressing B. subtilis progresses further into sporulation (stage III) than do strains lacking ^E activity (stage II). Consistent with its stage III phenotype, and a defect in ^E 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 ^E activity in sigE, sigE335, and merodiploid strains indicate that the residual prosequence on ^E335, still impairs its activity to function as a transcription factor. The data suggest that the 11-amino-acid extension on ^E335 allows it to bind RNA polymerase and direct the resulting holoenzyme to ^E-dependent promoters but reduces the enzyme's ability to initiate transcription initiation and/or exit from the promoter.