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RNA Polymerase from Prokaryotes and Organelles |
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RNA Polymerase from Prokaryotes and Organelles
Polymerase enzyme from E. coli is one of the largest soluble proteins yet studied. It measures about 100 AO in diameter and has a molecular weight of about 490,000 daltons. RNA polymerase of E. coli is a holoenzyme and is composed of two main subunits, a smaller subunit called sigma factor (σ) and a large unit called core polymerase which is composed of two alpha (α) units, two beta (β) units and a omega (ω) unit. The core enzyme alone is capable of making an RNA copy of DNA, but it begins transcription at random along the template and copies both DNA strands. When sigma factor is added back to core enzyme, transcription begins at the true promoter sites and on the correct strand. The precise role of sigma is by no means clear, but at present core enzymes and sigma factors are believed to become dissociated in the cell.
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| A. Core enzyme |
B. Sigma factor |
C.Complete enzyme |
D.Binding RNA |
| E.DNA |
F.RNA polymerase-DNA complex |
G.RNA |
H.RNA polymerase-DNA-RNA complex |
| I.Core enzyme synthesizing RNA |
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A sigma factor is thought to initiate the transcription process by (1) recognizing a promoter region possibly by its base sequence or possibly by its tertiary structure, and (2) mediating the binding of a core enzyme to the Promoter.
After a core enzyme is bound and begins to transcribe the continuous sequence into RNA the sigma is thought to be released so that it can associate with another core enzyme.
Out of 5 subunits of core enzyme, only β’ subunit is directly involved in the binding of RNA polymerase to DNA. The others are thought to play still undefined roles in transcription, perhaps in its regulation. Lastly, the RNA polymerase of E. coli and of certain chloroplasts and mitochondria is sensitive to the antibiotic rifampicin which probably combines with the P subunit of the E. coli enzyme. However, RNA polymerases from higher plant chloroplasts and T, bacteriophages are found to be rifampicin-resistant.
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