The process of formation of RNA (Ribonucleic Acid) from DNA (Deoxyribonucleic Acid) is called transcription.
A transcriptional unit is a sequence of specific nucleotides in DNA which codes for RNA molecule, it consists of
> a promoter region (or sequence), which initiates the process of transcription of RNA,
> a structural gene, which codes for RNA molecule, and
> a terminator region (or sequence), which stops (or terminates) the process of transcription of RNA.
Mechanism of transcription in prokaryotes
The mechanism of transcription in prokaryotes involves three phases, these are:
1. 1. Initiation
2. 2. Elongation
3. 3. Termination
> Transcription in prokaryotes is initiated by RNA polymerase holoenzyme at the promotor region of a transcriptional unit.
> RNA polymerase enzyme of bacteria (or prokaryotes) is a core enzyme that consists of five subunits: two α subunits, one β subunit, one β’ subunit and one ω subunit.
> The core RNA polymerase enzyme associates with an initiator factor called sigma factor (σ factor) which helps in finding suitable 5’- TTGACA -3’ and 5’- TATAAT -3’ base pairs at -35 and -10 region respectively.
> These sequences are present in the upstream of the site where the transcription process starts i.e., at the promoter region of transcriptional unit, and binding of this holoenzyme to the promoter sequence is called closed complex.
> After the formation of a closed complex, the RNA polymerase holoenzyme separates 10-14 bases extending from -11 to +3 region and this is called melting. This forms an open complex and this changing from closed complex to open complex is called isomerization.
> RNA polymerase enzyme then starts synthesizing a short non-productive RNA molecule of less than 10 bp, this is called abortive initiation.
> When the RNA polymerase enzyme synthesizes RNA more than 10 bp long, the σ-factor gets released and the RNA polymerase enzyme move along 5’-3’ direction in the structural gene of transcriptional unit continuously synthesizing (or elongating) the RNA molecule.
> The synthesized RNA then gets separated from transcriptional unit through RNA exit channel.
> The synthesized RNA is then proof read by Hydrolytic editing and the Gre factor further enhances this proof-reading process.
There are two different mechanisms of termination, they are:
i. Rho independent
ii. Rho dependent
i. Rho independent termination:
> In this mechanism of termination, the transcription process gets terminated due to specific sequence in terminator DNA.
> The terminator DNA contains an invert repeat which performs complimentary base pairing and forms hair pin like structure.
> This invert repeat is extended by large number of TTTTTTTT base pairs (8 bp) on template DNA. The uracil appears in RNA instead of thymine. Load of this hair pin structure is not tolerated by A=U base pair and thus the RNA get separated from RNA-DNA heteroduplex.
ii. Rho dependent termination:
> In this mechanism of termination, the transcription process is terminated by ring shaped single strand binding ATPase protein called rho protein (ρ protein).
> After the completion of elongation process, the elongated single stranded RNA molecule then gets bind to this rho protein and exit from polymerase enzyme complex and hydrolyse the RNA from RNA polymerase enzyme complex.
> Rho protein does not bind to that RNA whose protein has been translated, instead it binds to RNA after translation.
> But in prokaryotes, since both transcription and translation occurs simultaneously so in this case rho protein gets bind to the RNA after the completion of translation but the process of transcription still remains ON.
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