Knowing the gene expression regulation in eucharyotic organisms,is actualy very difficult since its great
complexity and the high number of factors that play some role in it. Essentially, it's thought that the
most important gene expression checkpoint is the DNA to RNA transcription, but it seems quite probable
that there are other controls: pre-transcriptional (as chromosomal structure, CpG islands, methylation...)
and post-traductionals (transport, mRNA degradation, protein synthesis regulation...).
The eucharyotic gene transcriptionThe eucharyothic
gene transcription is made by RNA polymerase II. The regulation of the beginning of this
process lies in two stages:
- Atraction of the RNA polymerase II to the transcriptional starting point by General Transcriptional Factors
(FT) as TATA box, ... tha allow the starting point recognisement by the RNApol II through binding the DNA
strand and the RNA pol II at the same time.
- Transcriptional starting according to given productivity and need paramethers of the encoded protein. These paramethers depend on a combination of gene specific Transcriptional Factors that act together closely binded to the former to the starting point region. They can be positive (activators) or negative (inhibitors) regulators.
How is gene expression regulated?
There are several methods used by eukaryotes.- Altering the rate of transcription of the gene. This is the most important and widely-used strategy and the one we shall examine here.
- However, eukaryotes supplement transcriptional
regulation with several other methods:
- Altering the rate at which RNA transcripts are processed while still within the nucleus. [Discussion of RNA processing]
- Altering the stability of mRNA molecules; that is, the rate at which they are degraded.
- Altering the efficiency at which the ribosomes translate the mRNA into a polypeptide.
Given that multiple FTs and their binding sites are known and that the publishing of several genomes has reported us several regulator sequences, the necessity of bioinformatic tools to predict which combinations of FTs regulate each gene is made clear. A simple way of dealing with the problem is to use Positional Weight Matrices (PWM) previously derived from real binding sites collections, to build a list of candidate regions along a promoting sequence. In TRANSFAC data base, there is the widest collection of PWM for protein/DNA binding sites
Fig. 1. This picture shows the basic mechanism of transcriptional control, with the RNA Pol II and the FT (TATA binding proteins...) as basic elements.
