Transcription

Only a few of an organism's genes are active or "expressed" at any particular time. Which genes are expressed in a particular cell depends on such factors as the nutrients available, the cell's state of differentiation, and the cell's age. There are intricate mechanisms that let the cell regulate the expression of many of its genes. Transcription, the first step in the expression of the genetic information, is an important point at which gene expression can be regulated.

There are two types of regulation: positive control, in which transcription is enhanced in response to a certain set of conditions; and negative control, in which transcription is repressed. Usually, positive control is used at promoters that are otherwise engaged in the initiation of few RNA molecules. Negative control is used at promoters where many molecules of RNA are initiated.

Activator proteins enable positive control by binding to the promoter to recruit RNA polymerase or other required initiation proteins. Such activator proteins usually bind upstream of the promoter (Figure 1). Increased recruitment then leads to an increased rate of synthesis of RNA for a particular gene. The more regulatory sites that are bound, the greater the increase in the rate of RNA synthesis. Repressor proteins can inhibit initiation of transcription by binding to the promoter and preventing RNA polymerase or a required initiation protein from binding.

In eukaryotes, DNA is "packaged" into nucleosomes by being wrapped around histone proteins. This can dramatically reduce the ability of genes to be transcribed, because the packaging may hide promoter sequences that are recognized by initiation factors.

Two mechanisms are used to alter the DNA packaging, to regulate transcription. First, enzymes called chromatin remodeling factors can move his-tone proteins around on the DNA, so that promoter sequences are more accessible or less accessible to the transcription initiation machinery. Second, enzymes can attach small chemical groups, including acetyl, phosphate, methyl or other groups, to the histone proteins. This modification of his-tone proteins may alter the interaction between the DNA and the histones, or between histones and other proteins, either facilitating or blocking the ability of initiation factors to bind DNA.

Transcription also is regulated by proteins that influence how quickly RNA polymerase moves along the DNA. These proteins, called regulatory elongation factors, may help the polymerase traverse pause sites, and they may facilitate elongation through packaged DNA. On the other hand, they may also facilitate the termination of transcription at specific sites.