MUTATIONS IN THE TYRR GENE OF ESCHERICHIA-COLI WHICH AFFECT TYRR-MEDIATED ACTIVATION BUT NOT TYRR-MEDIATED REPRESSION

Site-directed mutagenesis has been used to further characterize amino acid residues necessary for the activation of gene expression by the T yrR protein. Amino acid substitutions have been made at positions 2, 4 , 5, 6, 7, 8, 9, 10, and 16. TyrR mutants with amino acid substitution s V-5-->P (VP5), VF5, CS7, CR7, DR9, RI10, RS10, and ER16 show no or v ery little activation of expression of either mtr or tyrP. In each cas e, however, the ability to repress aroF is unaltered. Amino acid subst itutions at positions 4, 6, and 8 have no effect on activation. Small internal deletions of residues 10 to 19, 20 to 29, or 30 to 39 also de stroy phenylalanine- or tyrosine-mediated activation of mtr and tyrP. In. these mutants repression of aroF is also unaltered. In activation- defective tyrR mutants, expression of mtr is repressed in the presence of tyrosine. This tyrosine-mediated repression is trpR dependent and implies an interaction between TrpR and TyrR proteins in the presence of tyrosine.