The spoIIG operon of Bacillus subtilis codes for a sporulation-specific sigma factor, sigma E. In vivo expression of the spoIIG promoter is activated shortly after the onset of sporulation and is dependent on kinA, spo0F, spo0B and spo0A genes. The products of these genes have been shown to participate in a phosphorelay reaction in vitro, culminating in phosphorylation of the transcription factor, Spo0A. The effect of Spo0A phosphorylation on in vitro transcription from the spoIIG promoter was determined. Aliquots from phosphorelay reactions enhanced spoIIG promoter activity 10-fold in transcription assays and stimulation of transcription was dependent on Spo0A phosphorylation. Our results provide biochemical evidence that Spo0A and the phosphorelay form a signal transduction pathway which activates spoII gene expression in development.