User:Chandler.c.ho/sandbox/Terminator (genetics)


 * This article is about genetics. For other uses, see Terminator (disambiguation)

In genetics, a transcription terminator is a section of genetic sequence that marks the end of a gene or operon on genomic DNA during transcription. This sequence mediates transcriptional termination by providing signals in the newly synthesized mRNA that trigger processes that release the mRNA from the transcription complex. The release of the nascent mRNA can be the result of the direct activity of the mRNA secondary structure or the indirect activities of recruited termination factors which varies depending upon the specific gene and organism.

In prokaryotes
In prokaryotes, two classes of transcription terminators are known to be distributed throughout the genome, functioning to end transcription following complete transcription of the gene/operon and to mediate early regulatory termination of transcription.

Rho-independent terminators
Intrinsic transcription terminators or Rho-independent terminators require the formation of a hairpin structure on the nascent elongating transcript which results in the disruption of the mRNA-DNA-RNA polymerase ternary complex. The terminator sequence contains a GC-rich region of dyad symmetry followed by a short poly-T track or "T stretch" which are transcribed to RNA to form the terminating hairpin and a 7-9 nucleotide "U track" respectively. The mechanism of termination is hypothesized to occur through a combination of direct promotion of dissociation through allosteric effects of hairpin-binding of RNA polymerase and "competitive kinetics" where hairpin formation causes RNA polymerase stalling indirectly promoting dissociation of the complex.

Rho-dependent terminators
Rho-dependent transcription terminators require a protein called Rho factor, which exhibits RNA helicase activity, to disrupt the nascent mRNA-DNA-RNA polymerase ternary complex. Rho-dependent terminators are found in bacteria and phage. The Rho-dependent terminator occurs downstream of translational stop codons and is comprised of a Rho utilization site (rut) which consists of an unstructured cytosine-rich sequence on the mRNA and a downstream transcription stop point (tsp). The rut serves as a mRNA loading site for Rho from which it can translocate down the mRNA to the RNA polymerase which stalls at the downstream tsp site. Contact between Rho and the RNA-mRNA-DNA complex causes dissociation of the transcriptional complex through an unclear mechanism which involves Rho's helicase activity.

In eukaryotes
In eukaryotes, terminators are recognized by protein factors and termination is followed by polyadenylation.