ROLE OF LYSINE-195 IN THE KMSKS SEQUENCE OF ESCHERICHIA-COLI TRYPTOPHANYL-TRANSFER-RNA SYNTHETASE

Lysine 195 in the K-195 MSKS sequence of E. coli tryptophanyl-tRNA syn thetase (TrpRS) was replaced with alanine. The resulting K195A mutant TrpRS had essentially unchanged K-m values for ATP and Trp, but a 1500 -fold decreased k(cat) in a pyrophosphate-ATP exchange reaction. This large decrease in k(cat) reduces the rate of aminoacyladenylate format ion (step 1) to a rate comparable to the rate of aminoacylation of tRN A(Trp) (step 2) by the K195A mutant enzyme. Both the TIGN and KMSKS se quences are important for step 1 of class I aminoacyl-tRNA synthetase reactions.