Activators generally bind upstream of the promoter and help RNA polymerase to bind. Conversely, repressors bind downstream of the promoter and either block the binding of RNA polymerase or prevent it from moving forward and transcribing the gene. Not surprisingly, the same DNA-binding protein can act as an activator for one gene and a repressor for another if it binds at different locations in the two cases (Fig. 9.14).
Regulatory proteins can alternate between two different forms, both of which bind DNA. This is somewhat different than the binding already discussed, which alternates between an active, DNA-binding form and an inactive, nonbinding form. Here, the two forms of the protein act as an activator and a repressor that both bind DNA, but at gratuitous inducer A molecule (usually artificial) that induces a gene but is not metabolized like the natural substrate;the best known example is the induction of the lac operon by IPTG IPTG (iso-propyl-thiogalactoside) A gratuitous inducer of the lac operon operator The site on DNA where a repressor binds
Some Proteins May Act as Both Repressors and Activators 247
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