Structures of bacterial and phage repressors and activators bound to DNA.A, phage 434 repressor dimer (2OR1) (28). The helix–turn–helix is highlighted in red. B, E. coli CAP dimer with cAMP bound to each monomer (1CGP). C, Drosophila engrailed homeodomain with helix–turn–helix is highlighted in red (1HDD). D, hydrogen bonding between Asn and Adenine (1HDD). E, E. coli Trp repressor showing waters (spheres) and water-mediated hydrogen bonds (1TRO). F, Met repressor dimer with two chains colored differently. There is one molecule of S-adenosine methionine (SAM) bound to each monomer (1CMA).