We design a new biomolecular circuit with the potential for bistable behavior. We study this candidate toggle switch and find sufficient conditions on key parameters that guarantee bistability. The circuit structure is based on a positive feedback loop created by the mutual repression of two synthetic genes. Repression is generated not by direct inactivation of the promoter region, but by inactivation of the enzymes performing transcription. Inactivation is experimentally possible by designing the RNA outputs of the two genes to be inhibitory aptamers for the enzymes.