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 formyltransferase (MTF) is important for the initiation of protein synthesis in eubacteria such as Escherichia coli. In addition to the determinants for formylation present in the initiator tRNA, the nature of the amino acid attached to the tRNA is also important for formylation. We showed previously that a mutant tRNA aminoacylated with lysine was an extremely poor substrate for formylation. As a consequence, it was essentially inactive in initiation of protein synthesis in E. coli. In contrast, the same tRNA, when aminoacylated with methionine, was a good substrate for formylation and was, consequently, quite active in initiation. Here, we report on the isolation of suppressor mutations in MTF which compensate for the formylation defect of the mutant tRNA aminoacylated with lysine. The suppressor mutant has glycine 178 changed to glutamic acid. Mutants with glycine 178 of MTF changed to aspartic acid, lysine, and leucine were generated and were found to be progressively weaker suppressors. Studies on allele specificity of suppression using different mutant tRNAs as substrates suggest that the Gly178 to Glu mutation 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.
Li
Y
Y
Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
Ramesh
V
V
Mangroo
D
D
Taneja
C
C
RajBhandary
U L
UL
D000215:Acylation; D000483:Alleles; D001483:Base Sequence; D015500:Chloramphenicol O-Acetyltransferase / Q000235:genetics; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics*; D016153:Genes, Suppressor; D019877:Hydroxymethyl and Formyl Transferases / Q000235:genetics*; D008239:Lysine / Q000378:metabolism*; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D009690:Nucleic Acid Conformation; D012358:RNA, Transfer, Met / Q000378:metabolism*
D012358:RNA, Transfer, Met; D019877:Hydroxymethyl and Formyl Transferases; C021561:methionyl-tRNA formyltransferase; D015500:Chloramphenicol O-Acetyltransferase; D008239:Lysine