Overproducing the Bacillus subtilis mother cell sigma factor precursor, pro-σ(K), uncouples σ(K)-dependent gene expression from dependence on intercompartmental communication

During sporulation of Bacillus subtilis, proteolytic activation of pro- σ(K) and ensuing σ(K)-dependent gene expression normally require the activity of many sporulation gene products. We report here that overproducing pro-σ(K) at the onset of sporulation substantially uncouples σ(K)-dependent gene expression from its normal dependency. Overproducing pro-σ(K) in strains with a mutation in spoIIIG, spoIIIA, spoIIIE, or spoIVB partially restored σ(K)-dependent gene expression in the mother cell and resulted in accumulation of a small amount of polypeptide that comigrated with σ(K), but these mutants still failed to form spores. In contrast, sporulation of spoIVF mutants was greatly enhanced by pro-σ(K) overproduction. The products of the spoIVF operon are made in the mother cell and normally govern pro-σ(K) processing, but overproduction of pro-σ(K) appears to allow accumulation of a small amount of pro-σ(K), which is sufficient to partially restore mother cell gene expression and spore formation. This spoIVF-independent mechanism for processing pro-σ(K) depends on σ(E), an earlier-acting mother cell- specific sigma factor. The spoIIID gene, which encodes a mother cell- specific DNA-binding protein that is normally required for pro-σ(K) production, was shown to be required for efficient pro-σ(K) processing as well, bof (bypass of forespore) mutations bypassed this requirement for spoIIID, suggesting that SpoIIID is less directly involved in pro-σ(K) processing than are spoIVF gene products. However, bof spoIIID double mutants overproducing pro-σ(K) still failed to sporulate, indicating that SpoIIID serves another essential role(s) in sporulation in addition to its multiple roles in the production of σ(K).