|MoBio||Transcriptional Termination in Prokaryotes||Chapter 4|
In prokaryotes, the transcription is terminated by two major mechanisms: Rho-independent (intrinsic) and Rho-dependent.
The Rho-independent termination signal is a stretch of 30-40 bp sequence, consisting of many GC residues followed by a series of T ( "U" in the transcribed RNA). The resulting RNA transcript will form a stem-loop structure to terminate transcription.
Rho is a ~ 50 kD protein, involved in bout half of E. coli transcriptional terminations. It has been well established that six Rho proteins form a hexamer to terminate transcription, but the precise mechanism is not clear. Experiments suggest that two components are essential: (i) the upstream Rho loading site and (ii) the downstream termination site. The Rho hexamer first binds to the RNA transcript at the upstream site which is 70-80 nucleotides long and rich in C residues. Upon binding, the Rho hexamer moves along the RNA in the 3' direction. If movement of the polymerase is slow, the Rho hexamer will catch up and terminate the transcription at the downstream termination site. Rho has ATPase activity which can induce release of the polymerase from DNA.