N-formylmethionine fMet

The first phase of translation, initiation, involves several steps. First, two proteins, initiation factors IF-1 and IF-3, bind to the 30 S subunit (1). Another factor, IF-2, binds as a complex with GTP (2). This allows the subunit to associate with the mRNA and makes it possible for a special tRNA to bind to the start codon (3). In prokaryotes, this starter tRNA carries the substituted amino acid N-formylmethionine (fMet). In eukaryotes, it carries an unsubstituted methionine. Finally, the 50 S subunit binds to the above complex (4). During steps 3 and 4, the initiation factors are released again, and the GTP bound to IF-2 is hydrolyzed to GDP and Pj. 

A special initiator tRNA, tRNAme i (I stands for initiator) is used for beginning protein synthesis. In bacteria, this initiator tRNA carries the modified amino acid N-formylmethionine (fmet). The formylation reaction transfers the formyl group from formyl-tetrahydrofolate to... 

N-formylmethionine (fMet) Transformylase does not formylate methionyl tRNA because tRNA and tRNA are structurally different. A second feature is its ability to initiate polypeptide synthesis. 

Unambiguous. Each codon encodes only one amino acid. An exception is in prokaryotes. The translation start signal is N-formylmethionine (fMet). This enters the ribosome at the P-site (Chapter 70) where it is subject to wobble on the first position of the codon (Chapter 68). Thus, although AUG is the major start codon, a significant number of genes start with GUG, UUG or CUG). In both pro- and eukaryotes, AUG codes for all methionine molecules within the subsequent reading frames. 

Start codon (AUG) codes for Methionine N-formylmethionine (fMet)... 

The first amino acid in the synthesis of all bacterial polypeptides is N-formylmethionine (fMet), i.e. a modified methionine residue with a formyl group attached to its amino group. Two types of methionyl-tRNA exist one binds formylmethionine and the other, normal methionine. Both have the same anticodon and recognize the AUG codon characteristic of methionine. However, only formylmethionyl-tRNA binds at the initiating AUG codon while normal methionyl-tRNA is bound at all internal AUG codons. 

It has long been known that peptides of bacterial origin, such as N-formylat-ed oligopeptides, are potent activators of neutrophils. Bacterial protein biosynthesis is initiated by the codon AUG, which codes for polypeptide chains at the NH2 terminus to start with N-formylmethionine. However, very few mature bacterial proteins actually have this amino acid at the NH2 terminus because Af-formylmethionine is cleaved off by proteolytic processing. Sometimes just this amino acid is cleaved, but often several adjacent residues are also removed with it. These observations formed the basis for the chemical synthesis of a variety of N-formylated oligopeptides and an assessment of their ability to activate neutrophils in vitro. The most potent of these formylated peptides is TV-formylmethionyl-leucyl-phenylalanine (fMet-Leu-Phe). 

The methionine residue found at the amino-terminal end of E. coli proteins is usually modified. In fact, protein synthesis in bacteria starts with H-formylmethionine (fMet). A special tRNA brings formylmethionine to the ribosome to initiate protein synthesis. This initiator tRNA (abbreviated as tRNAf) differs from the one that inserts methionine in internal positions (abbreviated as tRNA ). The subscript "f indicates that methionine attached to the initiator tRNA can be formylated, whereas it cannot be formyl-ated when attached to tRNA. In approximately one-half of E. coli proteins, N-formylmethionine is removed when the nascent chain is 10 amino acids long. 

Our own involvement in this area of research came about in an indirect way. A major interest in the laboratory had been the role of vitamin Bja in methyl group transfer from N -methyl-H4-folate to homocysteine to yield methionine, a problem that was an extension of studies on the isolation of the vitamin coenzyme while in Horace A. Barker s laboratory in 1958. When the exciting report by Clark and Marcker appeared showing that W-formylmethionine-tRNA (fMet-rRNA) was the initiator or protein synthesis, it was a natural extension of our work to examine the formation of fMet-tRNA, since this reaction also involved both a one-carbon transfer from a reduced folate derivative and the amino acid methionine. Herbert Dickerman, a postdoctoral fellow in the laboratory at that time, was able to purify the transformylase enzyme from E. coU extracts that catalyzed eqn. (1) ... 

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