Seryl-tRNA synthetase fromEscherichia col:implication of itsN-terminal domain inaminoacylation activity and specificity

Escherichia coliseryl-tRNA synthetase (SerRS) a dimeric class 11aminoacyl-tRNA synthetase withtwo structural domains charges specifically thefiveisoacceptor tRNAser aswell asthetRNAs(seIC product) ofE.coli. TheN-terminal domainisa60Alongarmlike coiled coil structure built of2longantiparallel a-h helices, whereastheC-terminal domainisa c-,B structure. A deletion oftheN-terminal armofthe enzymedoesnotaffect theaminoacidactivation step ofthereaction, butreduces dramatically aminoacylation activity. TheKcatlKm valueforthemutant enzymeisreduced bymorethan4orders ofmagnitude, withanearly 30foldincreased Kmvalue fortRNAser. Anonlyslightly truncated mutant form(16aminoacids ofthetipofthearmreplaced byaglycine) hasan intermediate aminoacylation activity. Bothmutant synthetases havelost their specificity fortRNA"rand charge alsonon-cognate type1tRNA(s). Ourresults support thehypothesis that class 11synthetases have evolved fromanancestral catalytic coreenzymeby addingnon-catalytic N-terminal orC-terminal tRNA binding (specificity) domains which actasdeterminants forcognateandanti-determinants