Methionine repressor MetJ domain Binding of a specific DNA fragment and S-adenosyl methionine (SAM) co-repressor molecules to the <taxon tax_id="562">Escherichia coli</taxon> methionine repressor (MetJ) leads to a significant reduction in dynamic flexibility of the ternary complex, with considerable entropy-enthalpy compensation, not necessarily involving any overall conformational change [<cite idref="PUB00006686"/>]. MetJ is a regulatory protein which when combined with S-adenosylmethionine (SAM) represses the expression of the methionine regulon and of enzymes involved in SAM synthesis. It is also autoregulated.<p>The crystal structure of the met repressor-operator complex shows two dimeric repressor molecules bound to adjacent sites 8 base pairs apart on an 18-base-pair DNA fragment. Sequence specificity is achieved by insertion of double-stranded antiparallel protein beta-ribbons into the major groove of B-form DNA, with direct hydrogen-bonding between amino-acid side chains and the base pairs. The repressor also recognises sequence-dependent distortion or flexibility of the operator phosphate backbone, conferring specificity even for inaccessible base pairs [<cite idref="PUB00008039"/>].</p>