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Sigma (σ) factors direct programs of gene expression by binding to and determining the promoter recognition specificity of RNA polymerase (RNAP). Genes transcribed under the control of alternative σs allow cells to respond to stress and undergo developmental processes such as sporulation in the bacterium Bacillus subtilis where gene expression is controlled by a cascade of alternative σs. Much interest in RNAP function has therefore focused on σ regulation, and the mechanism of how one σ replaces another remains mysterious. While classic σ antagonists bind directly to σ to inhibit interaction with RNAP, we have identified a small protein, Fin, that binds to the RNAP β’ coiled-coil, a region that is critical for functional σ interaction with RNAP. During sporulation, fin is expressed under the control of σF, a σ factor active at early times during sporulation that is later replaced by σG. Cells deleted for fin are defective for spore formation and exhibit increased σF-directed gene transcription. We propose that Fin functions as part of a negative feedback loop to inhibit σF by competing for the β’ coiled-coil, and that this antagonism may contribute to the transition from σF to σG during sporulation., Medical Sciences