Suppressor mutations in Escherichia coli methionyl-tRNA formyltransferase that compensate for the formylation defect of a mutant tRNA aminoacylated with lysine

The specific formylation of initiator methionyl-tRNA by methionyl-tRNA form yltransferase (MTF) is important for the initiation of protein synthesis in eubacteria such as Escherichia coli. In addition to the determinants for f ormylation present in the initiator tRNA, the nature of the amino acid atta ched to the tRNA is also important for formylation. We showed previously th at a mutant tRNA aminoacylated with lysine was an extremely poor substrate for formylation. As a consequence, it was essentially inactive in initiatio n of protein synthesis in E. coli. In contrast, the same tRNA, when aminoac ylated with methionine, was a good substrate for formylation and was, conse quently, quite active in initiation. Here, we report on the isolation of su ppressor mutations in MTF which compensate for the formylation defect of th e mutant tRNA aminoacylated with lysine. The suppressor mutant has glycine 178 changed to glutamic acid. Mutants with glycine 178 of MTF changed to as partic acid, lysine, and leucine were generated and were found to be progre ssively weaker suppressors. Studies on allele specificity of suppression us ing different mutant tRNAs as substrates suggest that the Gly178 to Glu mut ation compensates for the nature of the amino acid attached to the tRNA. We discuss these results in the framework of the crystal structure of the MTF .fMet-tRNA complex published recently.