<P> Direct interaction with DNA is the simplest and the most direct method by which a protein changes transcription levels . Genes often have several protein binding sites around the coding region with the specific function of regulating transcription . There are many classes of regulatory DNA binding sites known as enhancers, insulators and silencers . The mechanisms for regulating transcription are very varied, from blocking key binding sites on the DNA for RNA polymerase to acting as an activator and promoting transcription by assisting RNA polymerase binding . </P> <P> The activity of transcription factors is further modulated by intracellular signals causing protein post-translational modification including phosphorylated, acetylated, or glycosylated . These changes influence a transcription factor's ability to bind, directly or indirectly, to promoter DNA, to recruit RNA polymerase, or to favor elongation of a newly synthesized RNA molecule . </P> <P> The nuclear membrane in eukaryotes allows further regulation of transcription factors by the duration of their presence in the nucleus, which is regulated by reversible changes in their structure and by binding of other proteins . Environmental stimuli or endocrine signals may cause modification of regulatory proteins eliciting cascades of intracellular signals, which result in regulation of gene expression . </P> <P> More recently it has become apparent that there is a significant influence of non-DNA - sequence specific effects on transcription . These effects are referred to as epigenetic and involve the higher order structure of DNA, non-sequence specific DNA binding proteins and chemical modification of DNA . In general epigenetic effects alter the accessibility of DNA to proteins and so modulate transcription . </P>

How does gene expression influence the behavior of cells that contain the same genome