(RNAP). Transcription to yield an
. The DNA between promoter and terminator is copied to mRNA, which is then
Promoters can differ in strength, that is, how attractive they are for RNAP. The more similar they are to a consensus sequence, the stronger they are. The "ideal" promoter in E. coli looks like this:
, made of four subunits (2α, β, β'), ω factor and the σ factor. The followings steps occur upon initiation:
The RNAP runs along the DNA, synthesizing mRNA in the process. In bacteria, the nascending mRNA is processed right away by ribosomes.
The elongation stops if:
- The terminator is reached. The terminator is usually a palindromic DNA sequence that forms a hairpin.
- A ρ factor (a protein) binds and runs along the mRNA towards the RNAP. When ρ-factor reaches the RNAP, it causes RNAP to dissociate from the DNA, terminating transcription.
- The RNAP comes across a region with repetitious base pairs (for example, TTTTTT). This will terminate transcription.
Gene expression in eukaryotes is largely controlled by transcription via transcription factors. As eukaryotes are much more complex than prokaryotes, and have their genetic material stored in the nucleus, the transcription mechanisms are more complicated here. For example, eukaryotes have three RNA polymerases, in contrast to prokaryotes, which only have one.
Also, eukrayotic RNAPs need specific accessory proteins to become active. The C-terminus of all RNAPs is highly conserved and contains the actual transctiptional mechanism.
- RNA Polymerase I is located in the nucleolus and transcribes only rRNAs.
- RNA Polymerase II is the "standard" RNAP.
- RNA Polymerase III transcribes tRNAs and other small RNAs.
The core promoter of eukaryotic genes stretches from position -45 to 0. Additionally, there can be an upstream control element present at the -180 to -107 region, which can amplify the RNAP binding by a factor of up to 100. This UCE usually contains a TATA box, a highly conserved DNA sequence that reads
A similar sequence, thus not that highly conserved, is found in the INR element (initiator element, part of the complex core promoter).
- T A T A T/A A
A major difference between prokaryotic and eukaryotic transcription is that the latter have splicing of the primary transcript, modifying the mRNA created during transcription.