Transcription in Prokaryotes

The bacterial core RNA polymerase binds a sigma factor to form a holoenzyme, which recognizes promoter sequences like the -35 and -10 regions. This positions the holoenzyme near the transcription start site. The holoenzyme unwinds DNA at the -10 site to expose the template strand, allowing transcription to begin.

The sigma factor interacts with promoter regions, specifically the -35 and -10 sequences, to position the RNA polymerase correctly for transcription initiation. After a few ribonucleotides are added to the nascent RNA, the sigma factor dissociates, leaving the core RNA polymerase to continue elongation independently.

Intrinsic termination occurs when the RNA transcript forms stem-loop structures followed by uracil residues, destabilizing the RNA-DNA hybrid and causing RNA polymerase to dissociate. Rho-dependent termination involves the Rho protein binding to specific RNA sequences and moving along the transcript to displace the RNA polymerase.

In archaea, the TATA-binding protein binds to the TATA box, while transcription factor B recognizes the B recognition element sequence. These factors recruit RNA polymerase to the initiation site, resembling eukaryotic transcription initiation rather than the sigma factor mechanism used in bacteria.

During elongation, RNA polymerase synthesizes RNA in the 5' to 3' direction by adding ribonucleotides complementary to the DNA template strand. The polymerase unwinds DNA ahead of the transcription bubble and rewinds it behind, ensuring continuous RNA synthesis as it moves along the template.

Archaeal termination is generally intrinsic, relying on stem-loop structures in the RNA transcript to release RNA polymerase. Unlike bacterial transcription, archaeal termination does not involve additional protein factors like Rho, making it a simpler mechanism dependent solely on RNA secondary structure.

Transcription converts genetic information into RNA molecules, serving as the first major step in coordination of gene expression processes in bacteria. The three stages—initiation, elongation, and termination—are each driven by specific molecular mechanisms that regulate when and how genes are expressed.