The release of the transcription factor σ F from its inhibitory complex with SpoIIAB is a key regulatory step in the control of sporulation in Bacillus subtilis as it initiates a pattern of differential gene expression in the mother cell and prespore compartments. The σ F .SpoIIAB complex is dissociated by the unphosphorylated form of the protein SpoIIAA, the alternative binding partner of SpoIIAB. Here, we employ fluorescence spectroscopy to examine the mechanism by which SpoIIAA acts on the σ F .SpoIIAB complex. We constructed a mutant of σ F , σ F -W46L, which displayed a reproducible fluorescence response on binding to SpoIIAB. Using this mutant we were able to quantify the amount of σ F bound to SpoIIAB in real time. The results provide physical evidence for the ''induced release'' mechanism of σ F activation. We demonstrate that SpoIIAA interacts directly with the σ F .SpoIIAB complex, greatly decreasing the affinity of SpoIIAB for σ F and thus causing the release of the latter. We also demonstrate that σ F is released before SpoIIAA is phosphorylated and that release occurs on a similar time scale to the binding of SpoIIAA to SpoIIAB.