Kirromycin, an inhibitor of protein biosynthesis that acts on elongation factor Tu

Kirromycin, a new inhibitor of protein synthesis, is shown to interfere with the peptide transfer reaction by acting on elongation factor Tu (EF-Tu). All the reactions associated with this elongation factor are affected. Formation of the EF-Tu.GTP complex is strongly stimulated. Peptide bond formation is prevented only when Phe-tRNA(Phe) is bound enzymatically to ribosomes, presumably because GTP hydrolysis associated with enzymatic binding of Phe-tRNA(Phe) is not followed by release of EF-Tu.GDP from the ribosome. This antibiotic also enables EF-Tu to catalyze the binding of Phe-tRNA(Phe) to the poly(U).ribosome complex even in the absence of GTP. EF-Tu activity in the GTPase reaction is dramatically affected by kirromycin: GTP hydrolysis, which normally requires ribosomes and aminoacyl-tRNA, takes place with the elongation factor alone. This GTPase shows the same K(m) for GTP as the one dependent on Phe-tRNA(Phe) and ribosomes in the absence of the antibiotic. Ribosomes and Phe-tRNA(Phe), but not tRNA(Phe) or Ac-Phe-tRNA(Phe), stimulate the kirromycin-induced EF-Tu GTPase. These results indicate that the catalytic center of EF-Tu GTPase that is dependent upon aminoacyl-tRNA and ribosomes is primarily located on the elongation factor. In conclusion, kirromycin can substitute for GTP, aminoacyl-tRNA, or ribosomes in various reactions involving EF-Tu, apparently by affecting the allosteric controls between the sites on the EF-Tu molecule interacting with these components.

Protein synthesis in Bacillus subtilis: differential effect of potassium ions on in vitro peptide chain initiation and elongation

The preparation and fractionation of a highly active and stable in vitro protein-synthesizing system from Bacillus subtilis is described. Potassium satisfied the requirement for a monovalent ion when the initiation factor-dependent binding of formyl-methionyl-transfer ribonucleic acid and synthesis of formyl-methionyl-puromycin were assayed, whereas it inhibited the reactions for polyphenylalanine synthesis. On the other hand, the ammonium ion satisfied the requirement for all assayed reactions. The in vitro experimental evidence suggested that potassium is an inhibitor of one or a few specific reactions involved in peptide chain elongation in B. subtilis.

Selective inhibition of initiating ribosomes by spectinomycin

Spectinomycin at low concentrations inhibits initiating ribosomes but not ribosomes already engaged in chain elongation. The initiating ribosomes are blocked in some step after formation of the initiation complex, probably the first translocation. Cells inhibited by spectinomycin accumulate polysomes, and these have proved to be unstable polyinitiation complexes: they can be pulse-labeled with methionine but not with valine, and they disappear after addition of rifampicin. Hence, the blocked ribosomes evidently are released and then reinitiate. In heterozygotes this cyclic reinitiation by the sensitive ribosomes, each blocking an initiation site for an average of 10-15 min, can explain (just as with streptomycin) the dominance of sensitivity over resistance.

Activity of colicin E3-treated ribosomes in initiation and in chain elongation

Colicin E3 (called E3 here) is shown to act on polysomal ribosomes as well as on free ribosomes. This treatment severely inhibits subsequent chain elongation, both on preformed polysomes completing their nascent chains and on ribosomes translating poly(U). Elevated Mg(++) concentration relieves this inhibition. However, elevated Mg(++) concentration does not relieve the inhibition of ribosomes initiating on natural messenger; hence damage by E3 appears to have an additional, specific effect on some stage in initiation. The effect of damage by E3 resembles that of streptomycin in several respects. This similarity suggests that the two agents may act on closely related sites on the ribosome.

Mechanism of action of the mycotoxin trichodermin, a 12,13-epoxytrichothecene

Trichodermin is a member of a group of closely related compounds-the 12,13-epoxytrichothecenes-that form a medically and economically important class of mycotoxins produced by fungi that spoil fruit and grain. Our studies show that trichodermin is a very potent inhibitor of protein synthesis in mammalian cells. Since ribosomes remain in polyribosomes in inhibited cells, trichodermin inhibits the elongation and/or termination processes of protein synthesis. In vitro, trichodermin is a potent inhibitor of the peptidyl transferase activity required for elongation and/or termination. An in vitro comparison of the effects of three peptidyl transferase inhibitors on elongation and termination indicates that anisomycin acts primarily on elongation while trichodermin and sparsomycin act primarily on termination. A new in vivo test to distinguish elongation inhibitors from termination inhibitors confirms that trichodermin inhibits primarily the termination process. Thus trichodermin inhibits protein synthesis by blocking the activity of peptidyl transferase required for termination. These studies suggest that the toxicosis caused by one of the 12,13-epoxytrichothecenes is due to its action as a protein synthesis inhibitor involving the peptidyl transferase activity of the eukaryotic ribosomes.

Influence of cholesteryl 14-methylhexadecanoate on some ribosomal functions required for peptide elongation

1. Polyribosomes and ribosomal subunits from rat liver were adsorbed on a cellulosic ion-exchange adsorbent, freeze-dried and extracted with organic solvents. The activity of extracted particles in peptide elongation was tested in the presence of purified peptideelongation factors. 2. Chloroform-methanol mixture (2:1, v/v) extracted 1.87+/-0.15 pmol of cholesteryl 14-methylhexadecanoate/pmol of the smaller ribosomal subunit and 0.92+/-0.11 pmol/pmol of the larger subunit. 3. In the presence of transferase I, extracted polyribosomes and 40S subunits bound more phenylalanyl-tRNA than did control non-extracted particles. The same binding as in control mixtures was obtained with extracted particles supplemented with cholesteryl 14-methylhexadecanoate in quantities corresponding to those extracted. 4. The polymerization of phenylalanine was greatly decreased with extracted polyribosomes and subunits and addition of the cholesteryl ester could not fully restore the original activity. 5. Extraction significantly decreased the activity of the P site of peptidyl transferase and normal activity was recovered after the addition of the ester. The A site of peptidyl transferase in extracted polyribosomes showed an increased activity when compared with non-extracted polyribosomes. 6. Cholesteryl 14-methylhexadecanoate apparently affects the function of the ribosomal A site and peptidyl transferase site and probably also that of the guanosine triphosphatase site and P site. The presence of different amounts of the ester in polyribosomes may be one of the mechanisms modulating peptide elongation at the ribosomal level.

Mechanism of inhibition of eukaryotic protein synthesis by trichothecene fungal toxins

The 12,13-epoxytrichothecenes, a group of sesquiterpenoid fungal antibiotics, inhibit protein synthesis in eukaryotic cells but do not share a common mode of action. Trichodermin stabilizes polyribosomes, prevents their disaggregation by puromycin, and also prevents the release of nascent peptides from ribosomes by puromycin. Nivalenol, T-2 toxin, and verrucarin A cause rapid and almost quantitative breakdown of polyribosomes in H-HeLa cells, a process which is inhibited by anisomycin, cycloheximide, or trichodermin. Similar effects of trichodermin, nivalenol, and verrucarin A are also observed in yeast spheroplasts. We conclude that nivalenol, T-2 toxin, and verrucarin A are potent and highly selective inhibitors of polypeptide chain initiation in eukaryotes, whereas trichodermin inhibits chain elongation and (or) termination. We have compared the structural formulae of various trichothecenes and suggest that the presence of substituents on carbon-15 of the common trichothecene ring may be important in determining the precise modes of action of this group of compounds.

Influence of polyamine limitation on the chain growth rates of beta-galactosidase and of its messenger ribonucleic acid

The rates of elongation of beta-galactosidase and its messenger ribonucleic acid (RNA) were estimated in a polyamine-deficient mutant of Escherichia coli through an analysis of the kinetics of enzyme induction. The chain growth of beta-galactosidase was calculated from the time required after the appearance of an amino terminal fragment of 60 amino acids (auto-alpha) until completed enzyme began to accumulate. The elongation rate of beta-galactosidase messenger RNA was estimated from the time after induction at which streptolydigen-resistant, enzyme-forming capacity first appeared. Upon polyamine starvation, the rate of polypeptide elongation slowed from 17 to 10 amino acids per s and the messenger RNA elongation rate decreased from 47 to 30 nucleotides per s. These reductions in polymerization rates were proportional to the decrease in cellular growth rate produced by polyamine starvation. It was concluded that, although it is quite unlikely that polyamine levels are involved in regulation of cell growth, they may be acting as cofactors in the synthesis of RNA or protein, or both.

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.

Adenine reversal of ethionine-induced inhibition of protein synthesis: a comparison with computer-generated data

In vivo labeling kinetics of (a) nascent peptides on liver polysomes, and (b) proteins of the 100 000 × g supernatant have been determined for ethionine-intoxicated female rats given adenine, to reverse the effect of ethionine. When compared to computer-generated data the results indicate that the reversal effect of adenine is not simply through the increase in the previously reduced rate of initiation. In addition to increasing the rate of initiation, adenine, soon after its administration, reduces the rate of elongation. The rate of elongation subsequently returns to normal levels.

A specific inhibitor of polypeptide-chain initiation in Escherichia coli

An inhibitor of polypeptide-chain initiation was isolated from E. coli cells. This protein inhibits formation of the 30S or 70S initiation complex with either fMet-tRNA(f) as initiator and AUG, MS2 RNA, or late T4 RNA as messenger, or acPhe-tRNA as initiator and poly(U) as messenger. Chain elongation, e.g., poly(U) translation at high Mg(2+) concentration, is not inhibited. The inhibitor is rendered ineffective when active aminoacylation of tRNA is taking place, e.g., during natural mRNA translation. This inhibitor is distinct from the so-called interference (i) factors, which interfere exclusively with the action of initiation factor 3. Since the new inhibitor can apparently be turned on and off, it may have a regulatory function in translation.