Traditionally, prokaryotic transcriptional regulators were thought to be controlled by the binding of low-molecular-weight effector molecules--inducers and co-repressors. Here, we describe two examples of a novel mode of regulator control. In this mode, transcription factors "shuttle" between their operator sites on the chromosome and the inner face of the cytoplasmic membrane, where they are sequestered by specific transport systems. This change in the subcellular address corresponds to the on/off state of the target genes; thus, release or binding of the transcription regulators is controlled by the activity of these transporters.