Activators are modular in structure. How a repressor might repress. How tryptophan is used to toggle a repressor of tryptophan synthesis genes in bacteria. How a protein can act as a repressor or an activator at the same promoter. How activators and repressors work together to control sugar utilization in bacteria.
Combinatorial control allows for the integration of many signals.
How a protein binds to a particular DNA sequence. The outside of a DNA helix can be read by proteins. The DNA's major groove contains an array of precisely positioned hydrogen bond donors and acceptors, as well as hydrophobic patches. The DNA nucleotide sequence of A, T, G, and C determines the precise arrangement of these hydrogen bonds and hydrophobic patches. Since the major groove provides a more distinctive array of interactions than the minor groove, most proteins bind to the major groove.