NH2-terminal arm of phage lambda repressor contributes energy and specificity to repressor binding and determines the effects of operator mutations.

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Abstract
Several lines of evidence indicate that the phage .lambda. repressor recognizes its operator by using, in part, an .alpha. helix (the recognition helix), which it inserts into the major groove of DNA. In addition to its recognition helix, .lambda. repressor has an arm, consisting of the first 6 amino acids, that wraps around the DNA helix. Plasmids were constructed that, in Escherichia coli, direct the expression of derivatives of .lambda. repressor that lack the NH2-terminal 1, 3, 6 or 7 amino acids. These modified proteins were studied in vivo and in vitro, and from these results it is argued that the arm: contributes a large portion of the binding energy; helps to determine sequence specificity of binding and, in particular, the relative affinities for 2 wild-type binding sites; determines entirely repressor''s response to 1 operator mutation (a back-side mutation); magnifies repressor''s response to other operator mutations (front-side mutations); and increases the sensitivity of repressor binding to salt concentration and temperature.