8.20 : Bacterial Transcription

To initiate transcription, the RNA polymerase holoenzyme binds non-specifically to the DNA with low affinity. It then slides along the DNA to locate the promoter sequence. 

When it slides into the promoter, specifically the -10 and the -35 regions upstream of the initiation site, the sigma subunit of the RNA polymerase binds to the promoter tightly and unwinds the DNA to form the ‘open-promoter’ complex. 

The RNA polymerase holoenzyme, now positioned at the transcription initiation site, synthesizes the RNA transcript by adding the complementary nucleotides to the DNA template. Once a 10-nucleotide long RNA chain has been synthesized, the sigma factor gets released. 

The RNA polymerase core enzyme continues to add nucleotides to the growing RNA transcript. As the polymerase moves forward, DNA is continuously unwound ahead of the enzyme and rewound behind it.

The process continues until the gene is transcribed and the core polymerase encounters the termination signal. The most common termination signal is a symmetrical inverted repeat of a GC-rich sequence followed by a poly-A tail.

When this sequence is transcribed into RNA, the self-complementary sequence base-pairs to form a stable hairpin loop structure. 

The hairpin destabilizes the association of the mRNA and the DNA template and causes the polymerase to stall and dissociate. Thus, the transcription bubble collapses and the DNA rewinds to a double helix, and the newly formed pre-mRNA transcript is released.