The regulation of transcription of the gerA spore germination operon of Bacillus subtilis

The gerA operon of Bacillus subtilis 168 comprises three genes concerned with the triggering of spore germination by l‐alanine and its analogues. The expression of this operon has been characterized using chromosomal lacZ fusions to the gerA promoter. The gerA promoter is switched on 2.5–3 hours after the initiation of sporulation, in parallel with glucose dehydrogenase. A high proportion of the gerA‐driven β‐galactosidase detected in sporulating cells is found in the mature spore; the gerA promoter is therefore active in the forespore compartment of the sporulating cell. The gerA promoter is not expressed in spo0, spoII or spoIIIA, B, E and G mutant backgrounds, but is expressed in spoIIICand D and in spoIV and V mutants. The in vivo transcriptional startpoint of the operon has been mapped by primer extension experiments; sequences upstream from this startpoint show significant homology with recognition sequences for RNA polymerase containing σG(EσG). The gerA operon was transcribed in vitro by EσG with a startpoint identical to that used in vivo, and expression of the gerA operon was rapidly induced in vegetative cells by induction of σG synthesis. These data indicate that the gerA operon is an additional member of the σG regulon, which includes a number of genes expressed in parallel only in the forespore compartment of sporulating B. subtilis cells.