In response to the binding of tyrosine or phenylalanine, the TyrR protein (513 amino acids) activates certain promoters and represses others. In a previous study (J. Cui and R. L. Somerville, J. Bacteriol. 175:303-306, 1993), it was shown that promoter activation was selectively abolished in mutant proteins lacking amino acid residues 2 to 9. An additional series of constructs that encoded mutant TyrR proteins having deletions or point mutations near the N terminus were analyzed. Residues Arg-2 and Leu-3 were shown to be critical for the activation of the mtr promoter. In confirmation of previous findings, none of the activation-defective mutant TyrR proteins had lost significant repression function. The TyrR protein was shown by chemical cross-linking to be dimeric. The polypeptide segments critical for dimer formation in vivo were identified by evaluating the negative dominance phenotypes of a series of mutant proteins, all defective in DNA binding, lacking progressively greater numbers of amino acid residues from either the N terminus or the C terminus. Amino acid residues 194 to 438 were found to contain all of the essential dimerization determinants
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