For example, hunchback in moderate concentrations is an activator of kruppel, but a suppressor of kruppel in large concentrations.

From what I've seen in literature, that's because the kruppel's enhancer has several binding sites for hunchback: some responsible for activation, while others for suppression. But why would hunchback in moderate concentrations bind only to activation sites, and why would higher concentration "change its preferences"? What chemical mechanism is responsible for this preference?

P.S. I provide hunchback and kruppel as an example, but this is a general question: how could the same transcription factor be both an activator and suppressor of the same gene?

  • $\begingroup$ It could be due to higher affinity for the activation sequence than the repression sequence and/or higher avidity for activation sequences mediated by cooperative binding. Consider that the weaker something the binds, the more of it you need to ensure that a significant portion is binding. This paper looks promising in addressing your specific example, though I haven’t read it. You may also be interested in this answer. $\endgroup$
    – canadianer
    Dec 26, 2018 at 22:20


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