Transcription factor

Transcription factors are proteins that mediate the binding of RNA polymerase and the initiation of transcription. Transcription only starts when the appropriate transcription factors are attached and the RNA polymerase is bound in the correct orientation. The transcription factors and RNA polymerase on the promoter form the transcription initiation complex.

The rate of transcription can be strongly increased or decreased by the binding of specific transcription factors (activator to the enhancer, and repressor to the silencer). Enhancers and silencers can affect the expression of genes located hundreds, or even thousands of nucleotides away. This is made possible by DNA bends that bring the enhancers/silencers enhancers silencers and promoters in close proximity.