Factor-dependent termination

Prokaryotes use two means for terminating transcription, factor-independent and factor-dependent. Certain DNA sequences function as signals that tell the RNA polymerase to terminate transcription. The DNA of a terminator sequence encoded an inverted repeat and an adjacent stretch of uracils. Factor-dependent termination involves a terminator sequence as well as a factor or protein called rho. The mechanisms by which eukaryotes terminate transcription are poorly understood. Most eukaryotic genes are transcribed for upto several thousand base pairs beyond the actual end of the gene. The excess RNA is then cleaved from the transcript when the RNA is processed into its mature form. 

See also Structure of RNA Polymerase, Interactions with Promoters, Initiation and Elongation, Factor-Independent Termination of Transcription, Factor-Dependent Termination of Transcription... 

See also Factor-dependent termination of transcription, eukaryotic termination of transcription... 

Factor-dependent termination - Factor-dependent termination sites are less frequent than factor-independent termination sites, and the mechanism of factor-dependent termination is complex. The P protein, a hexamer composed of identical subunits, has been characterized as an RNA—DNA helicase (see here for more about helicases) and contains a nucleoside triphosphatase activity that is activated by binding to polynucleotides. Apparently P acts by binding to the nascent transcript at a specific site near the 3 end, when RNA polymerase has paused (Figure 26.16). Then P moves along the transcript toward the 3 end, with the helicase activity unwinding the 3 end of the transcript from the template (and/or the RNA polymerase molecule), thus causing it to be released. 

Rho-dependent termination requires participation of rho factor. This protein binds to the newly formed RNA and moves toward the RNA polymerase that has paused at a termination site. Rho then displaces RNA polymerase from the 3 end of the RNA. 

Termination The process of elongation of the RNA chain contrv ues until a termination signal is reached. An additional protein, p (rho) factor, may be required for the release of the RNA product (p-dependent termination). Alternatively, the tetrameric RNA polymerase can, in some instances, recognize termination regions on the DNA template (p-independent termination). 

Hong J-X, Haun RS, Tsai S-C, et al. (1994) Effect of ADP-ribosylation factor amino-terminal deletions on its GTP-dependent stimulation of cholera toxin activity. In J. Biol. Chem. 269 9743-9745. 

Termination sequences contain palindromes. The RNA transcript of the DNA palindrome forms a stable hairpin turn. Apparently, this structure causes the RNA polymerase to slow or stop and partially disrupts the RNA-DNA hybrid structure. In some termination sequences, referred to as p (rho)-independent termination sites, several (about six) uridine residues follow the hairpin structure. Because U-A base pair interactions are weak, the short U-A sequence promotes the dissociation of the newly synthesized RNA from the DNA strand. In p-dependent termination the p factor (an enzyme that catalyzes the ATP-dependent unwinding of RNA-DNA helices) promotes the dissociation of the RNA polymerase complex from the RNA-DNA hybrid, p binds to RNA and not to RNA polymerase. 

Involvement of NusA protein - It is not clear what causes RNA polymerase to pause at P-dependent termination sites. However, the action of another protein, NusA, is somehow involved. The NusA protein evidently associates with RNA polymerase, and there is reason to believe that it binds at some point in transcription after the factor has dissociated, because the two purified proteins compete with each other for binding to core RNA polymerase. 

Automatic correction is always recommended to eliminate manual dependence and to achieve better accuracy. It also elimintites the risk of a leading power factor by a human error that may cause an excessive voltage at the motor and the control gear terminals. 

The ease with which the dipolarophile interacts with vinylacetylenes depends mainly on a spatial factor. The study of the reactions of alkylthiobuten-3-ynones-l and their selenic and telluric analogs with DPNT shows that, in this case, nitrilimine also acts as a nucleophilic agent with a nucleophilic center on the carbon atom of the 1,3-dipole and always adds to the terminal carbon of the enyne system to form l,3-diphenyl-5-/ -2-pyrazolenines. The oxidation of the latter with chloranil leads to alkynylpyrazoles (65ZOR51). 

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