Recognition of bacterial initiator tRNA by an initiation factor

Effect of the amino acid attached to Escherichia coli initiator tRNA on its affinity for the initiation factor IF2 and on the IF2 dependence of its binding to the ribosome

We show that the nature of the amino acid in the formylaminoacyl-tRNA influences initiation factor (IF) 2 dependence of its ribosome binding and that this IF2 dependence reflects the relative affinity of the formylaminoacyl-tRNA for the initiation factor IF2. We compared the template-dependent ribosome binding activities, in the presence of initiation factors, of wild type and anticodon sequence mutants of Escherichia coli initiator tRNAs that carry formylmethionine (fMet), formylglutamine (fGln), or formylvaline (fVal). The fGln-tRNA bound less well than fMet-tRNA whereas the fVal-tRNA bound as well as fMet-tRNA. The rate and extent of binding of fGln-tRNA to the ribosome was significantly increased by further addition of purified initiation factor IF2. In contrast, the binding of fVal-tRNA or fMet-tRNA was not affected much by the addition of IF2. Using gel mobility shift assay, we have measured the apparent Kd values of the IF2.formylaminoacyl-tRNA binary complexes. These are 1.8, 3.5, and 10.5 microM for fMet-tRNA, fVal-tRNA, and fGln-tRNA, respectively.

Mechanism of translational initiation in prokaryotes. Evidence for a direct effect of IF2 on the activity of the 30 S ribosomal subunit

Initiation factor IF2 from either Escherichia coli or Bacillus stearothermophilus was found to possess the previously undetected property of stimulating the template-dependent ribosomal binding of aminoacyl-tRNAs with free alpha-NH2 groups. IF1, which had no detectable activity alone, was found to stimulate the activity of E. coli IF2 and, to a lesser extent, that of B. stearothermophilus IF2. Since in the absence of ribosomes not even a weak interaction between the two IF2 molecules and the aminoacyl-tRNAs was detected, the present findings indicate that IF2 can act at the ribosomal level stimulating aminoacyl-tRNA binding without prior formation of a binary complex with the aminoacyl-tRNA. IF2 does not appear to open or strengthen a weak A-site binding, but rather to enhance aminoacyl-tRNA binding to a 30 S site equivalent to the P-site by slowing down the rate of aminoacyl-tRNA dissociation from ribosomes.

Translation initiation factor 2 alters transcriptional selectivity of Escherichia coli ribonucleic acid polymerase

Preparations of translation initiation factor IF-2 strongly stimulate the production of rRNA by Escherichia coli RNA polymerase but have little effect on the synthesis of other RNA species. The factor alters the sedimentation characteristics of RNA polymerase holoenzyme. We suggest a mechanism by which IF-2 alters the pattern of transcriptional selectivity of RNA polymerase.

Recognition of tRNA Trp by initiation factors from Escherichia coli

Binding of acetyl or formyltryptophanyl-tRNA Trp from Escherichia coli or beef liver to E. coli ribosomes is strongly stimulated by E. coli initiation factors and requires GTP. The N-acylated tryptophan is puromycin reactive. Polypeptide chain initiation with acetyltryptophan dependent on poly(U,G) has been demonstrated and is highly dependent on added initiation factors. tRNA Trp appears, therefore, to share some structural features with tRNAfMet of significance to the process of polypeptide chain initiation.

The involvement of a complex between formylmethionyl-tRNA and initiation factor IF-2 in prokaryotic initiation

A complex between initiation factor IF-2 and fMet-tRNA can be formed under ionic conditions, which are optimal for initiation complex formation. The complex can be retained on cellulose nitrate filters after fixing with glutaraldehyde. The IF-2 - FMet-tRNA complex formation is not influenced by GTP and GDP. Other nucleoside di of triphosphates also have no effect. Evidence is presented that this complex acts as an intermediate in polypeptide chain initiation. The IF-2 - fMet-tRNA complex formation is not influenced by initiation factors IF-1 and IF-3. The binary complex can be bound to the 30-S subunit in the absence of GTP, which indicates that there is no concomittant binding of the IF-2 - fMet-tRNA complex and the nucleotide moiety to the 30-S subunit. The binding of the binary complex is stimulated by GTP. The influence of some inhibitors of initiation on the IF-2 - fMet-tRNA complex formation has been tested. Aurin tricarboxylic acid appeared to be a strong inhibitor, whereas the sulfhydryl reagents N-ethylmaleimide and p-chloromercuribenzoate had no effect.

Derivatives of guanosine triphosphate with ribose 2'-hydroxyl substituents. Interactions with the protein synthetic enzymes of Escherichia coli

This report describes the preparation of four methylated and phosphorylated derivatives of GTP, 2'-O-methylguanosine 5'-triphosphate (PPP-Me2' Guo), and guanosine 2'-monophosphate 5'-triphosphate (PPP-Guo-2'P), 3'-O-methylguanosine 5'-triphosphate (PPP-Me3'Guo), and guanosine 3'-monophosphate 5'-triphosphate (PPP-Guo-3'P). These compounds were compared to GTP in their ability to support reactions catalyzed by Escherichia coli initiation factor 2(IF-2), elongation factor Tu (EF-Tu), and elongation factor G )EF-G). As with previously studied GTP analogues, the nucleotide specificities of IF-2-dependent N-formylmethionylpuromycin formation and EF-Tu-dependent Ac-Phe2-tRNA formation were similar. There was little difference between the reactions supported by GTP, PPP-Me2' Guo, PPP-Me3' Guo, and PPP-Guo-3'P, but PPP-Guo-2'P was a poor substrate with both enzymes. A spectrum of activity was observed in EF-G-dependent formation of N-acetylphenylalanylphenylalanylpuromycin. While PPP-Me2' Guo was almost as effective as GTP in supporting translocation, PPP-Guo-2'P was a very poor substrate, having even less activity than guanosine 3'-diphosphate 5'-triphosphate. Intermediate activities were observed with PPP-Me3' Guo and PPP-Guo-3'P, the former nucleotide being more active than the latter.

Cooperative effects of initiation factors and fMet-tRNA in the formation of the 40-S initiation complex

In this paper the mode of action of IF-1 in 40-S initiation complex formation was studied with MS2 RNA as messenger. Using initiation factors IF-2 and IF-3 labeled in vitro it appeared that IF-1 did not influence the binding of these factors in the absence of fMet-tRNA. However, in the presence of fMet-tRNA it was found that the enhancement of the fMet-tRNA binding by IF-1 was accompanied with an equimolar increase in binding of IF-2. Moreover, it appeared that also in absence of IF-1, fMet-tRNA binding is coupled with an equimolar enhancement of the IF-2 binding, which suggests the existence of a preribosomal complex between IF-2 and fMet-tRNA. The apparent Km values for both the binding of fMet-tRNA and IF-2 to 30-S subunits were determined and appeared to be equal, which makes a functioning of such a preribosomal complex in protein initiation very likely. The participation of GTP in this complex will be discussed. Functions of IF-1 in dissociation and recycling of IF-2, described by others, and the stimulation on the 30-S subunit level might well be explained as pleiotropic effects of one basic action of IF-1, i.e. a conformational change of 30-S subunits.

Interaction of poly(A) and mRNA with eukaryotic initiator met-tRNA-f binding factor: identification of this activity on reticulocyte ribonucleic acid protein particles

Messenger ribonucleoprotein particles (mRNPs) have been isolated from rabbit reticulocyte polysomes. One of the proteins of the mRNP complex has many properties of a specific eukaryotic initiation factor, the soluble Met-tRNA-f binding protein. A purified preparation of this factor, in addition to binding Met-tRNA-f, binds poly(A) and globin mRNA. The binding of these substrates is greater than that obtained with other natural or artificial polyribonucleotides. The mRNP fraction also binds poly(A) and Met-tRNA-f. Since the factor which binds initiator tRNA also binds poly(A) and mRNA, and this activity can be found on mRNPs, there may be a relationship between initiator tRNA binding and messenger binding in early events of eukaryotic initiation.

Lack of inhibition by L-1-chloro-4-phenyl-3-toluene-p-sulphonamidobutan-2-one (L-1-tosylamido-2-phenylethyl chloromethyl ketone) of the formation of bacterial polypeptide chain initiation complex

The chymotrypsin inhibitor l-1-chloro-4-phenyl-3-toluene-p-sulphonamidobutan-2-one does not inhibit the function of the initiation factor during the formation of the polypeptide chain initiation complex in vitro. Since the inhibitor has been shown previously to inhibit polypeptide chain elongation by reacting with elongation factor EF-Tu, the inhibitor can be used to investigate the initiation and elongation steps of protein biosynthesis separately.

Factor-promoted dissociation of free ribosomes in a rabbit reticulocyte lysate system: inhibition and requirement for an energy source

A factor that promoted the dissociation of ribosomes into ribosomal subunits in a complete globin-synthesizing system was isolated from rabbit reticulocyte ribosomes. The factor stimulated both globin synthesis and the association of the small ribosomal subunit with polyribosomes. The dissociation of free ribosomes by the factor could be followed as an independent reaction in the presence of a high concentration of cycloheximide, which inhibited translation and the resulting accumulation of runoff subunits. This reaction required an energy source and was inhibited by P-(5'-guanylyl)-methylenebisphosphonate. These data suggest that energy generated by hydrolysis of GTP may be a requirement for the dissociation reaction. Aurintricarboxylic acid inhibited the factor-mediated dissociation of free ribosomes but not the factor-mediated preservation of subunits formed on peptide-chain termination.

Protein initiation in eukaryotes: formation and function of a ternary complex composed of a partially purified ribosomal factor, methionyl transfer RNA, and guanosine triphosphate

A protein factor contained in a 1 M KCl extract of L-cell ribosomes and partially purified by chromatography on DEAE-cellulose forms a specific ternary complex with rat-liver Met-tRNA(f) and GTP. The complex is measured by its quantitative retention on nitrocellulose membranes. Complex assembly is optimal at 100 mM KCl and 0.2 mM MgCl(2), and is independent of mRNA and of ribosomes. The GTP requirement can be replaced over 65% by its methylene analogue GDPCH(2)P, indicating that GTP hydrolysis is not involved. Complex formation is inhibited by 10 muM aurintricarboxylic acid, but is unaffected by 100 muM pactamycin, 100 muM fusidic acid, or by excess uncharged methionine tRNA(f). The ternary complex is relatively stable and appears at the void volume during filtration on Sephadex G-100. At 1-3 mM MgCl(2) and in the presence of other factors, the ternary complex is implicated in protein initiation by (i) its capacity to bind to the 40S ribosomal subunit to form a 48S complex; and (ii) the subsequent association of the 48S complex with a 60S subunit to form a functional "80S complex."

Initiation factor 2-dependent ribosomal binding of N-formylmethionyl-transfer RNA without added guanosine triphosphate

Evidence is presented that suggests that GTP (or 5'-guanylylmethylene diphosphonate) is not essential for either formation of an AUG-directed, initiation factor IF-2-dependent initiation complex on the 30S subunit or for positioning of fMet-tRNA on the peptidyl site. However, recycling of limiting amounts of IF-2 requires both GTP and the 50S subunit. The formation of the complex IF-2-30S as an intermediate in polypeptidechain initiation is suggested.

A complex between initiation factor IF2, guanosine triphosphate, and fMet-tRNA: an intermediate in initiation complex formation

Evidence is presented that suggests the formation of a complex between polypeptide chain-initiation factor IF 2, GTP, and fMet-tRNA(f). This complex transfers both fMet-tRNA(f) and GTP to 30S ribosomal subunits in the presence of ApUpG and initiation factor IF 1. The resultant 30S initiation complex reacts with 50S subunits to form a 70S initiation complex. fMet-tRNA(f) in this 70S complex reacts with puromycin to form fMet-puromycin. These results suggest that [IF 2, GTP, fMet-tRNA(f)] is an intermediate in the initiation of protein synthesis in Escherichia coli.