Other significant regions to transcription need not be at fixed locations relative to transcription and are termed distance independent cis-acting elements . They include enhancers and silencers . Remember that RNA polymerase II is the polymerase that transcribes mRNA in eukaryotes. Repressors binding to silencers may also bind to the promoter making it less attractive for transcription. Enhancers may act in a tissue specific fashion if activators are present only in those tissues, or tissue specific repressors by to silencers near enhancers thereby interfering with the attachment of activators.
Transcription factors are generally proteins with multiple domains.
One domain interacts with DNA and characteristically has a motif that will bind in the major groove of the DNA helix. The other domain represents an allosteric site that will interact with either a steroid hormone or a secondary signal that is produced by a cell signaling system thereby changing the configuration of the DNA binding domain.