X-ray structure of the E. coli met repressor–SAM–operator complex as initially viewed along its twofold axis of symmetry. The self-complementary 18-bp DNA is drawn in stick form and SAM, which must be bound to the repressor for it to also bind DNA, is shown in space-filling form with DNA C white, SAM C green, N blue, O red, P orange, and S yellow. The DNA binds four identical 104-residue repressor subunits. Pairs of subunits (light cyan and lavender) form symmetric dimers in which each subunit donates one strand of a 2-stranded antiparallel β ribbon that is inserted into the DNA's major groove. Two dimers pair across the complex's twofold axis via their antiparallel N-terminal helices, which contact one another over the DNA's minor groove. Click on the "Identify DNA-Binding Regions" button to highlight the β sheets in cyan and hide the SAM molecules. Click on the "Closeup of DNA Binding Region" button for a detailed view of the lower portion in the initial view showing the 2-stranded antiparallel β ribbon (residues 21–29) inserted into the DNA's major groove, as viewed along its local twofold axis (rotated relative to the initial view by ~50° about the vertical axis). The DNA is shown in space-filling form and the polypeptide chains are drawn in stick form with C green. [Based on an X-ray structure by Simon Phillips, University of Leeds, U.K. PDBid 1CMA.]