Identification of a new σ-factor involved in compartmentalized gene expression during sporulation of Bacillus subtilis

During sporulation of Bacillus subtilis, two identical genomes segregate in two compartments, the forespore and mother cell. These genomes are expressed differentially, with some genes such as sspE turned on only in the forespore. In vitro transcription of sspE was obtained only with RNA polymerase extracted from sporulating cells. Fractionation of factors associated with this enzyme and reconstitution with core RNA polymerase from vegetative cells generated an enzyme accurately transcribing sspE in vitro and led to purification of a polypeptide with the amino-terminal sequence of the spoIIIG product. Inactivation of spoIIIG abolished expression of sspE and five other forespore-specific genes, whereas synthesis of the spoIIIG product in vegetative cells rapidly turned these genes on. Therefore, spoIIIG encodes a sigma-factor, sigma G, which controls the expression of multiple genes in the forespore compartment.