RNA Chain Termination

Termination of RNA chain synthesis appears to be brought about by two types of mechanisms. In the first type the termination signal appears to be recognized by DNA itself. RNA polymerase reads an extended poly (A) sequence on DNA. This results in an RNA transcript with a terminal poly(U) sequence

The second type of termination signal involves an additional protein called the rho (P) factor. This is a tetramer of molecular weight 200,000. The (rho) factor probably binds to RNA polymerase. It is however, not certain whether it also, or exclusively, binds to DNA.
The function of termination factor rho. Richardson (1978) has proposed a model for rho function. The rho factor catalyses the reaction. Because of the fact that nucleoside triphosphates act as substrates, rho has been described as a nucleoside triphosphate phosphohydrolase (NTPase). NTPase activity of rho is essential for termination.

NTPase activity is in turn dependent on RNA. Thus the requirement for RNA in termination is absolute. It has been suggested that rho recognizes in RNA a site rich in pyrimidines and containing at least a few cytidylate residues. The requirement for magnesium ions is not absolute.
The following model has been proposed for rho termination: (A) RNA polymerase initiates the RNA chain and moves along the DNA template. (B) Nascent RNA elongates and the rho binding site becomes exposed. (C) RNA polymerase pauses for some time in certain regions of DNA ('pause region') where transcription is slower. This site defines termination. Rho moves along RNA from its binding site to RNA polymerase.

This movement may be driven by hydrolysis of ATP. (D) Rho now comes into contact with RNA polymerase. (E) RNA and rho dissociate from RNA polymerase. Rho thus primarily acts as a release factor. (F) RNA polymerase is finally released. If it were left bound to DNA at the pause site then it would rapidly dissociate as long as the sigma factor is available.
According to this model rho is important for correct transcription termination. E. coli mutants with defective rho factors have less effective terminationThe mRNA synthesized has its bases complementary to the region of the DNA where it is formed.

The process of transcription is essentially similar to replication except for the following differences:

(1) ribose sugar is used instead of deoxyribose,

(2) adenine is complemented by uracil instead of thymine, and

(3) only one DNA strand forms the RNA strand.
Not all RNA is copied along DNA templates. Certain viruses, e.g. the tobacco mosaic virus (TMV) contain RNA as the genetic material instead of DNA. This RNA can serve directly as mRNA. A complementary RNA strand can also be synthesized by an RNA primer

Messenger RNA functions in transcribing information from the DNA molecule and carrying it to the ribosomes where in is translated into protein. It is, therefore, the carrier of information for protein synthesis from the genes to the sites of protein synthesis. Each mRNA strand becomes associated with the smaller subunits of a group of ribosomes to form a polyribosome or polysome .

These may be found free in the cytoplasm, as in bacteria, or may be associated with the endoplasmic reticulum, as in higher cells. Bacterial ribosomes contain two subunits, the 30S and the 50S subunits. The 30S subunit alone can bind mRNA. In animal cells the mRNA binds to the 40S subunit.


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