Rho dependent termination

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The three main stages of transcription are known as initiation, elongation and termination, the latter of which can be assisted by a protein that associates with the RNA polymerase complex, in Escherichia coli this known as a Rho (ρ) factor. Rho dependent termination is one of two types of termination in prokaryotic transcription, the other being intrinsic (or Rho-independent).

RNA polymerase is able to terminate the transcription of some genes without assistance, for example at a series of several U residues following a hairpin; at other sites, however, it requires the participation of an additional factor. The method of termination observed in the presence of the Rho factor is similar to that found in intrinsic termination through the terminator sequences recognised by the protein are long and complex[1].

The Rho protein is an ATP dependent helicase composed of six identical subunits that bind to an exposed region of single-stranded RNA following the open reading frame. Each of the subunits has an ATP-hydrolysis domain and an RNA-binding domain allowing RNA to pass through the centre of the hexameric protein. Rho is initiated by sequences that are rich in cytosine but poor in guanine. After binding to the newly formed RNA chain, ρ factor moves along the molecule in a 5’-3’ direction and encourages dissociation from the DNA template and RNA polymerase[2]. It disrupts the transcriptional complex by acting as an RNA-DNA helicase when it reaches the transcriptional bubble. A polypeptide loop of the ρ protein is formed when it reaches a termination signal, this stops the elongation stage from being carried out as it is inserted into the main channel of the RNA polymerase complex, separating the RNA/DNA hybrid.

The discovery of this process was prompted by the observation that RNA molecules, when made in vivo, are shorter than those synthesised in vitro by RNA polymerase alone. This lead to the isolation of Rho as the protein that caused the appropriate termination of the chain and further information about the termination factor has been gained through the addition of ρ to an incubation mixture at varying times after the initiation of RNA synthesis[3].

References:

  1. Hartl, D.L. and Ruvolo, M. (2012) Genetics: Analysis of Genes and Genomes. 8th Edn. Burlington, MA: Jones and Bartlett Learning.
  2. Foster, J. W. and Slonczewski, J. (2013) Microbiology: An Evolving Science. 3rd Edn. New York: W.W. Norton and Co.
  3. Berg, J.M., Tymoczko, J.L. and Stryer, L. (2011) Biochemistry. 7th Edn. Basingstoke: Palgrave Macmillan.


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