RHO AND RNA - MODELS FOR RECOGNITION AND RESPONSE

Escherichia coil Rho factor is required for termination of transcripti on at certain sites by RNA polymerase. Binding to unstructured cytosin e-containing RNA target sites, subsequent RNA-dependent ATP hydrolysis , and an RNA-DNA helicase activity that presumably facilitates termina tion, are considered essential for Rho function. Yet the RNA recogniti on elements have remained elusive, the parameters relating RNA binding to ATPase activation have been obscure, and the mechanistic steps tha t integrate Rho's characteristics with its termination function in vit ro and in vivo have been largely undefined. Recent work offers new ins ights into these interactions with results that are both surprising an d satisfying in the context of Rho's emerging structure. These include the requirements for binding and ATPase activation by a variety of RN A substrates, dynamic analyses of Rho tracking, helicase and terminati on activity, and the participation of a new factor (NusG) that interac ts with Rho. Models for Rho function are considered in the light of th ese recent revelations.