- Prokaryotic transcription
Prokaryotic transcription is the process in which
messenger RNAtranscripts of genetic material in prokaryotes are produced, to be translated for the production of proteins. Prokaryotic transcription occurs in the cytoplasmalongside translation. Unlike in eukaryotes, prokaryotic transcription and translation can occur simultaneously. This is impossible in eukaryotes, where transcription occurs in a membrane-bound nucleus while translation occurs outside the nucleus in the cytoplasm. In prokaryotes genetic material is not enclosed in a membrane-enclosed nucleus and has access to ribosomes in the cytoplasm. [Benjamin Lewin. "Essential genes"] ["Oxford dictionary of biology."]
The following steps occur, in order, for transcription initiation:
* RNA polymerase (
RNAP) binds to one of several specificity factors, σ, to form a holoenzyme. In this form, it can recognize and bind to specific promoterregions in the DNA. At this stage, the DNA is double-stranded ("closed"). This holoenzyme/wound-DNA structure is referred to as the "closed complex".
* The DNA is unwound and becomes single-stranded ("open") in the vicinity of the initiation site (defined as +1). This holoenzyme/unwound-DNA structure is called the "open complex".
* The RNA polymerase transcribes the DNA, but produces about 10 abortive (short, non-productive) transcripts which are unable to leave the RNA polymerase because the exit channel is blocked by the σ-factor.
* The σ-factor eventually dissociates from the holoenzyme, and elongation proceeds.
Promoters can differ in "strength"; that is, how actively they promote transcription of their adjacent DNA sequence. Promoter strength is in many (but not all) cases, a matter of how tightly RNA polymerase and its associated accessory proteins bind to their respective DNA sequences. The more similar the sequences are to a
consensus sequence, the stronger the binding is. Additional transcription regulation comes from transcription factorsthat can affect the stability of the holoenzyme structure at initiation.
Most transcripts originate using adenosine-5'-triphosphate (ATP) and, to a lesser extent, guanosine-5'-triphosphate (GTP) (
purinenucleoside triphosphates) at the +1 site. Uridine-5'-triphosphate (UTP) and cytidine-5'-triphosphate (CTP) ( pyrimidinenucleoside triphosphates) are disfavoured at the initiation site.
Two termination mechanisms are well known:
*Intrinsic termination (also called
Rho-independent transcription termination) involves terminator sequences within the RNA that signal the RNA polymerase to stop. The terminator sequence is usually a palindromic sequence that forms a stem-loop hairpin structure that leads to the dissociation of the RNAP from the DNA template.
*Rho-dependent termination uses a
termination factorcalled ρ factor(rho factor) which is a protein to stop RNA synthesis at specific sites. This protein binds at a rho ultilisation site on the nascent RNA strand and runs along the mRNA towards the RNAP. A stem loop structure upstream of the terminator region pauses the RNAP, when ρ-factor reaches the RNAP, it causes RNAP to dissociate from the DNA, terminating transcription.
Other termination mechanisms include where RNAP comes across a region with repetitious thymidine residues in the DNA template, or where a GC-rich inverted repeat followed by 4 A residues. The inverted repeat forms a stable stem loop structure in the RNA, which causes the RNA to dissociate from the DNA template.
The -35 region and the -10 ("
Pribnow box") region comprise the basic prokaryotic " promoter", and |T| stands for the "terminator". The DNA on the template strand between the +1 site and the terminator is transcribed into RNA, which is then translated into protein.."'
* [http://highered.mcgraw-hill.com/sites/dl/free/0072835125/126997/animation21.html Prokaryotic Transcription] - animation
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