Analysis of an Escherichia coli mutant TyrR protein with impaired capacity for tyrosine‐mediated repression, but still able to activate atσ⁷⁰ promoters

In Escherichia coli, TyrR represses and activates transcription of operons required for tyrosine, phenylalanine and tryptophan biosynthesis and uptake. The TyrR central domain is homologous with NtrC and some other bacterial regulatory proteins, although TyrR regulates σ⁷⁰, not σ⁵⁴, promoters. We isolated a central domain TyrR mutant (TyrR E274Q) by substitution of a normally conserved amino acid. The mutant was unable to bring about tyrosine‐mediated repression of aroF, aroL, tyrB, and tyrP and had diminished capability for tyrosine‐ and phenylalanine‐mediated repression of aroP. In contrast, it was able to effect wild‐type levels of phenylalanine‐mediated repression of aroG, tryptophan‐mediated repression of aroP and transcriptional activation of mtr and tyrP. The binding of purified TyrR E274Q to ATP (a requirement for tyrosine binding) and to the strong TyrR box of tyrP operator DNA were normal, but tyrosine binding and tyrosine‐dependent hexamerization 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 activation. E²⁷⁴ of TyrR must participate in either the binding of tyrosine, or the coupling of ATP binding with a conformational change that alters the affinity of the ATP‐dependent aromatic amino acid‐binding site.