However, there can be many control sequences, called enhancers and silencers, responsive to many different signals. L ong regions of the DNA can loop over to enable the regulatory connections. Activators bind to enhancer sites, controlled by hormones or other signals. They increase transcription of the regulated gene.
Repressors bind to silencer sites, controlled by hormones or other signals.
They decrease transcription of the regulated gene, possibly by interfering with activators. Basal factors act similarly to bacterial sigma factors. They enable RNA polymerase to initiate transcription. However, they require interaction with coactivators. To Tjian's surprise, however, when Factor D was better purified, SP1 failed to increase transcription.