Structural organization of transcription termination factor Rho

Rho-dependent termination of transcription is governed primarily by the upstream Rho utilization (rut) sequences of a terminator

A Rho-dependent transcription terminator in Escherichia coli DNA consists of an upstream part for Rho utilization (rut) and the transcription stop point (tsp) region. To test the role of the tsp region variants of the coliphage lambda cro gene terminator, tR1, containing inserts of non-terminator sequences between its rut and tsp regions were tested for termination function. The results showed that termination occurred with high efficiency at multiple sites in each of the new sequences with the positions of the sites coinciding with transcriptional pause points in the insert sequence and that the efficiency of termination was not directly proportional to the extent of pausing at those points. Thus, in contrast to the rut sequences, which are relatively rare in DNA, many different sequence segments can function as a tsp region. Studies with isolated transcripts showed that a rut element and sequences 3' of the rut element were both needed to activate ATP hydrolysis by Rho factor with the degree of activation depending on the length and the specific sequence of the 3' segment. These results support models for Rho action in which ATP hydrolysis is coupled to interactions of Rho protein with RNA 3' of the rut region.

Homomorphous hexameric helicases: tales from the ring cycle

Low-resolution structures have now been determined for several hexameric ring proteins, members of a large superfamily that includes helicases and, probably, a range of DNA-binding motors. A common symmetry and mode of DNA-binding may emerge.