A Genetic Analysis ofVarious Functions oftheTyrRProtein ofEscherichia coli

TheTyrRprotein isinvolved inbothrepression andactivation ofthegenes oftheTyrRregulon. Correction ofanerror inapreviously published sequence hasrevealed aCro-like helix-turn-helix DNA-binding domain near thecarboxyl terminus. Site-directed mutagenesis inthis region hasgenerated anumberofmutants that canno longer repress oractivate. Deletions ofaminoacid residues 5to42produced aprotein that could repress butnot activate. Thecentral domain ofTyrRcontains anATP-binding site andishomologous withtheNtrCfamily of activator proteins. A mutation tosite A oftheATP-binding site andother mutations inthisregion affect tyrosine-mediated repression butdonotprevent activation orphenylalanine-mediated repression ofaroG. ThetyrRgeneofEscherichia coli encodes aprotein which regulates theexpression ofanumberofgenes involved inthe biosynthesis andtransport ofthearomatic aminoacids (13). Thesegenes, whicharedistributed ineight transcription units, constitute theTyrRregulon (41). Whenitbindsto DNA,theTyrRprotein recognizes sequences related tothe palindrome TGTAAAN6TTTACA.Suchsequences arereferred toasTyrRboxes. Invitro studies suggest thatthe actual binding site covers 22bp.TheTyrRboxbears some resemblance tothegeneral class ofoperators TGTN6-1OACA described byGicquel-Sanzey andCossart (24), withthe notable exception that TGTandACA intheTyrRboxare separated by12andnot10bp.TheTyrRprotein most frequently acts asarepressor byusing tyrosine asacofactor. Thus,thetranscription units aroL-aroM, aroF-tyrA, tyrP, tyrB, andaroPareallrepressed bytheTyrRprotein inthe presence oftyrosine (41). A commonfeature foreachof these transcription units istwoadjacent TyrRboxeswhich, except inaroP, overlap theRNA polymerase-binding site. In