Prokaryotic transcription is a fundamental biological process that differs significantly from eukaryotic transcription, particularly in its initiation, elongation, and termination phases. The process begins with initiation, which requires specific factors, notably a promoter. The promoter is a DNA sequence located upstream of the transcription start site, signaling the start of transcription by recruiting proteins to the area. In prokaryotes, these promoter sequences are known as consensus sequences, indicating that while they are not identical across species, they share significant similarities.
One of the most recognized prokaryotic promoter sequences is the Pribnow box, which is located approximately 10 base pairs upstream of the transcription start site. Additionally, there is another consensus sequence found 35 base pairs upstream, and occasionally, a sequence located 40 to 60 base pairs upstream. The notation of negative numbers (e.g., -10, -35) indicates that these sequences are upstream of the transcription start site.
During initiation, the RNA polymerase holoenzyme binds to the promoter. This holoenzyme consists of the core enzyme, responsible for catalyzing the transcription reaction, and a sigma factor, which is crucial for determining the specificity of the RNA being transcribed. Different sigma factors are utilized depending on the type of RNA required, such as ribosomal RNA or messenger RNA for protein synthesis.
Once transcription is initiated, the process moves into elongation, where the RNA polymerase synthesizes RNA. The region actively being transcribed is referred to as the transcription bubble, which is about 18 nucleotides long. Within this bubble, approximately 8 nucleotides are being synthesized at any given moment, while the remaining nucleotides accommodate the RNA polymerase and associated proteins.
Transcription continues until the RNA polymerase encounters a termination sequence, which signals the end of transcription. This termination sequence can be classified into two main types: rho-dependent and rho-independent terminators. Rho-dependent termination requires the presence of a rho protein, while rho-independent termination occurs in its absence. Additionally, intrinsic termination, which is less common, involves the presence of uracil-rich sequences in the RNA transcript that lead to the disassociation of the RNA polymerase from the DNA template due to weak bonding.
A significant distinction between prokaryotic and eukaryotic transcription is the production of polycistronic mRNA in prokaryotes. This type of mRNA can contain multiple genes transcribed into a single RNA strand, as opposed to eukaryotic mRNA, which typically represents a single gene. Consequently, prokaryotic mRNA must undergo further processing to separate the individual genes into distinct mRNA molecules, each coding for separate proteins.
In summary, prokaryotic transcription involves a series of well-coordinated steps, beginning with the binding of RNA polymerase to the promoter, followed by the elongation of the RNA transcript within the transcription bubble, and concluding with termination at specific sequences. Understanding these processes is crucial for grasping the fundamental differences between prokaryotic and eukaryotic gene expression.
