| In addition to knowing where to start transcription, RNA polymerase must have a defined site at which to stop RNA synthesis, so that the appropriate size of transcript is produced. The three eukaryotic RNA polymerases employ different mechanisms to terminate transcription. For example, RNA polymerase I uses a specific protein to terminate the transcription of rRNAs, whereas RNA polymerase III uses a specific termination sequence. In contrast, RNA polymerase II is more versatile, utilizing both sequence and protein factors to facilitate pausing of the polymerase and termination of transcription.
|
| page 441 |  | | page 442 |
| Figure 31.6 Transcription termination in prokaryotes. Two mechanisms of transcription termination in bacterial cells. Rho-independent termination relies on the formation of a secondary structure in the newly transcribed RNA to dislodge the RNA polymerase from the DNA template and stop transcription. Rho-dependent termination requires the action of the rho protein. This protein will move along the newly transcribed RNA, catching up with the RNA polymerase when it pauses at the termination site, and causing the polymerase to leave the DNA template. |
| Transcription termination in bacterial cells occurs by one of two well-characterized mechanisms (Fig. 31.6). The first mechanism, rho-independent termination, is similar to that described for eukaryotic transcription, except that the secondary structure and sequences involved are much better characterized in bacterial cells. In rho-independent termination, a hairpin loop is formed just before a sequence of six to
eight uridine (U) residues near the 3' end of the newly synthesized RNA. The formation of this secondary structure dislodges the RNA polymerase from the DNA template, resulting in termination of RNA synthesis in the U stretch. The second mechanism, rho-dependent termination, requires the action of a protein factor called rho, which has an ATP-dependent helicase activity that is required for transcription termination. The rho protein is believed to travel along the newly synthesized RNA, chasing the RNA polymerase. In this mechanism, the formation of a hairpin loop in the RNA structure causes the RNA polymerase to pause, allowing the rho protein to catch up with and displace the RNA polymerase from the template. Transcription termination in eukaryotes resembles the rho-dependent termination in prokaryotes.
|
|
