|MoBio||Transcription Mechanisms in Prokaryotes||Chapter 4|
In prokaryotes, binding of the polymerase's σ factor to promoter can catalyze unwinding of the DNA double helix. The most important σ factor is Sigma 70, whose structure has been determined by x-ray crystallography.
Note that the promoter is rich in A and T. The AT pair involves two hydrogen bonds whereas the CG pair involves three hydrogen bonds. Therefore, AT pairs are easier to separate. The DNA replication origin is also rich in A and T.
After the DNA strands are separated at the promoter region, the core polymerase (ααββ') can then start to synthesize RNA based on the sequence of the DNA template strand (see Figure 4-B-1). Since the role of the σ factor is mainly to initiate transcription, it will be released after about 10 ribonucleotides have been polymerized.
Elongation of the RNA strand continues until the core polymerase reaches the termination site (more info).
Regulation by Transcription Factors