FIS ACTIVATES SEQUENTIAL STEPS DURING TRANSCRIPTION INITIATION AT A STABLE RNA PROMOTER

FIS (factor for inversion stimulation) is a small dimeric DNA-bending protein which both stimulates DNA inversion and activates transcriptio n at stable RNA promoters in Escherichia coli. Both these processes in volve the initial formation of a complex nucleoprotein assembly follow ed by local DNA untwisting at a specific site. We have demonstrated pr eviously that at the tyrT promoter three FIS dimers are required to fo rm a nucleoprotein complex with RNA polymerase. We now show that this complex is structurally dynamic and that FIS, uniquely for a prokaryot ic transcriptional activator, facilitates sequential steps in the init iation process, enabling efficient polymerase recruitment, untwisting of DNA at the transcription startpoint and finally the escape of polym erase from the promoter. Activation of all these steps requires that t he three FIS dimers bind in helical register. We suggest that FIS acts by stabilizing a DNA microloop whose topology is coupled to the local topological transitions generated during the initiation of transcript ion.