Prokaryotic Transcription: RNA Polymerase and Transcription Process

Prokaryotic Transcription

Overview

Prokaryotic transcription involves the conversion of genetic information stored in DNA into functional RNA molecules. This process consists of initiation, elongation, termination, and the involvement of regulatory proteins known as transcription factors.

RNA Polymerase

RNA polymerase is the key enzyme responsible for catalyzing the formation of RNA molecules from DNA templates. It recognizes and binds specifically to certain sequences within the DNA, called promoters, and uses them as starting points for transcription.

Initiation

Initiation of transcription involves the recruitment and assembly of the RNA polymerase at the promoter site. The recognition of promoter sequences relies on the presence of consensus sequences such as the Pribnow box (-35) and the promoter -10 site, both located upstream of the actual start of transcription (+1).

Consensus Sequences

These consensus sequences serve as targets for the binding of RNA polymerase and play crucial roles in determining which genes are transcribed and how frequently. They are highly conserved across various bacterial species, ensuring a uniform mechanism for gene expression.

DNA Unwinding

Once bound to the promoter site, RNA polymerase is capable of unwinding the DNA without the need for additional helicases. This process, known as DNA melting, allows the enzyme to access the underlying DNA template.

Elongation

After initiation, RNA polymerase starts to move along the DNA template, synthesizing mRNA in the 5' to 3' direction. The DNA is continually unwound ahead of the polymerase and refolded behind it to accommodate the ongoing transcription process.

Base Complementarity

As the nascent RNA strand grows, it matches the template DNA in a complementary manner, forming base pairs with its opposite strand. However, since RNA contains uracil instead of thymine, these base pairs are weak compared to those found in DNA. Despite this weakness, the base complementarity ensures the accuracy of the RNA copy.

Stable Linker

To maintain the integrity of the transcription machinery, RNA polymerase acts as a stable linker between the DNA template and the nascent RNA strands, preventing the premature dissociation of these components.

Termination

Termination of transcription marks the completion of an individual transcription event. This process occurs when RNA polymerase dissociates from the DNA template, releasing the newly synthesized mRNA.

Rho-Dependent Termination

In rho-dependent termination, the rho protein is involved in the process. When the RNA polymerase encounters a run of guanine nucleotides on the DNA template, it stalls, causing the rho protein to collide with the polymerase. This collision leads to the release of the mRNA from the transcription bubble, effectively halting further transcription.

Rho-Independent Termination

Alternatively, rho-independent termination occurs when the RNA polymerase stalls upon encountering a stable hairpin structure formed by a region of complementary cytosine-guanine (C-G) nucleotides within the transcribed mRNA. The resulting intramolecular interactions lead to the stalling of the RNA polymerase and the subsequent release of the mRNA.

Transcription Factors

Regulatory proteins known as transcription factors play a critical role in modulating gene expression by either enhancing or inhibiting transcription. They can interact with the DNA at specific locations to either stabilize or destabilize the transcription complex, thereby influencing the overall transcription process.