Transcription is a highly regulated process that converts genetic information into RNA molecules. The transcription cycle is divided into three key stages: initiation, elongation, and termination, each driven by specific molecular mechanisms.
Initiation of Transcription
In bacteria, transcription begins when the RNA polymerase core enzyme associates with a sigma factor to form a holoenzyme. For example, the E. coli sigma factor called σ70 forms a holoenzyme, which recognizes the -10 (Pribnow box) and -35 regions upstream of the transcription start site. When binding to the promoter, the RNA polymerase holoenzyme can position on the DNA correctly, and localized unwinding at the -10 region exposes the template strand. Once a few nucleotides are incorporated into the nascent RNA, the sigma factor dissociates, leaving the core RNA polymerase to proceed with elongation.
In archaea, the process is mediated by a TATA-binding protein (TBP), which binds to the TATA box, and transcription factor B (TFB), which recognizes the B recognition element (BRE). These factors recruit RNA polymerase to the transcription initiation site in a manner reminiscent of eukaryotic transcription initiation.
Elongation of the RNA Strand
During elongation, the RNA polymerase synthesizes RNA in the 5' to 3' direction by adding ribonucleotides complementary to the DNA template strand. The polymerase moves along the DNA, unwinding it ahead of the transcription bubble and rewinding it behind, ensuring continuous RNA synthesis.
Termination Mechanisms
Transcription termination in bacteria involves two primary mechanisms. In intrinsic termination, the RNA transcript forms a stem-loop structure followed by a series of uracil residues, destabilizing the RNA-DNA hybrid and causing RNA polymerase to dissociate. Alternatively, Rho-dependent termination involves the Rho protein, which binds to specific RNA sequences and moves along the transcript to displace RNA polymerase.
In archaea, termination is generally intrinsic, relying on stem-loop structures in the RNA transcript to release the RNA polymerase. Unlike bacterial Rho-dependent termination, archaeal transcription does not involve additional protein factors.
Transcription includes initiation, elongation, and termination.
The bacterial core RNA polymerase binds a sigma factor, forming a holoenzyme for initiation.
This sigma factor interacts with promoter regions, like the -35 and -10 sequences, positioning the RNA polymerase near the transcription start site.
After binding, the holoenzyme unwinds the DNA at the -10 site, exposing the template strand.
The sigma factor dissociates after adding a few ribonucleotides, and the core RNA polymerase continues elongating the RNA until it encounters a terminator sequence.
Bacterial transcription termination occurs through intrinsic signals in the RNA transcript, which form stem-loop structures, or through proteins like Rho factors, which dissociate RNA polymerase and RNA.
In archaeal promotors, the TATA-binding protein binds to the TATA box, while transcription factor B recognizes the B recognition element sequence. These factors recruit the RNA polymerase to the initiation site.
Termination in archaea occurs via intrinsic signals in the RNA transcript, like the stem-loop structures, and is independent of protein factors.
繁體中文
