In this article we will discuss about the non-genetic RNA.
Non-genetic RNAs (tRNA, rRNA, mRNA) are synthesized on DNA templates through the process of transcription. The enzyme involved in transcription is “DNA dependent RNA polymerase”, or simply “RNA polymerase”. In E. coli, a single RNA polymerase catalyzes the synthesis of all kinds of RNAs.
In eukaryotes, there are at least three distinct RNA polymerases:
(1) RNA polymerase I (class A) transcribes genes for large rRNAs and is located in the nucleolus,
(2) RNA polymerase II (class B) transcribes unique genes to produce hnRNAs (Heterogeneous nuclear RNA) and is located in the nucleoplasm (chromatin), and
(3) RNA polymerase III (class C) transcribes genes for tRNAs and 5SrRNA and is found in nucleoplasm (chromatin).
Bacterial RNA polymerase:
This enzyme is composed of a “core enzyme” and the “sigma (σ) factor”; both together constitute the complete enzyme or the holoenzyme. This sigma factor contains one polypeptide chain while the core enzyme has five polypeptides, viz., 2 alpha (α), 1 beta prime (β), 1 beta (β) and 1 omega (ω) (Table 3.10). Therefore, the holoenzyme can be written as β’βα2ωσ, whereas the core enzyme can be represented as β’βα2ω.
At the time of transcription initiation, sigma factor joins the core enzyme. Sigma factor is responsible for selecting the correct strand of DNA to be transcribed and for reading the correct point at which transcription should begin.
The core enzyme, on the other hand, synthesizes RNA complementary to the selected DNA strand. There are different active sites on the RNA polymerase core complex for transcription (Fig. 3.22).
These sites are:
(i) DNA unwinding site,
(ii) DNA coding strand site,
(iii) DNA rewinding site,
(iv) transcribing strand site and
(v) RNA binding site. 