ANALYSIS OF AN ESCHERICHIA-COLI MUTANT TYRR PROTEIN WITH IMPAIRED CAPACITY FOR TYROSINE-MEDIATED REPRESSION, BUT STILL ABLE TO ACTIVATE AT SIGMA(70) PROMOTERS

In Escherichia coil, TyrR represses and activates transcription of ope rons required for tyrosine, phenylalanine and tryptophan biosynthesis and uptake. The TyrR central domain is homologous with NtrC and some o ther bacterial regulatory proteins, although TyrR regulates sigma(70), not sigma(54), promoters. We isolated a central domain TyrR mutant (T yrR E274Q) by substitution of a normally conserved amino acid. The mut ant was unable to bring about tyrosine-mediated repression of aroF, ar oL, tyrB, and tyrP and had diminished capability for tyrosine- and phe nylalanine-mediated repression of aroP. In contrast, it was able to ef fect wild-type levels of phenylalanine-mediated repression of aroG, tr yptophan-mediated repression of aroP and transcriptional activation of mtr and tyrP, The binding of purified TyrR E274Q to ATP (a requiremen t for tyrosine binding) and to the strong TyrR box of tyrP operator DN A were normal, but tyrosine binding and tyrosine-dependent hexamerizat ion were significantly impaired, These properties are consistent with the proposal that self association is essential for tyrosine-mediated repression by TyrR but not for tyrosine- or phenylalanine-mediated act ivation. E(274) of TyrR must participate in either the binding of tyro sine, or the coupling of ATP binding with a conformational change that alters the affinity of the ATP-dependent aromatic amino acid-binding site.