Initiation of translation with native 40-S ribosomal subunits

Stimulation by aurintricarboxylic acid of tobacco mosaic virus-specific RNA synthesis and production of informosome-like infection-specific ribonucleoprotein

It was shown that aurintricarboxylic acid (ATA), a well-known inhibitor of protein synthesis, markedly stimulates the synthesis of tobacco mosaic virus (TMV)-specific RNA species of the intermediate (I) class (apparent molecular weights 1.1-1.3 x 10(6) and 0.6-0.8 x 10(6)). No stimulation by ATA of full-length genomic TMV RNA or the subgenomic TMV RNA coding for TMV coat protein was detected. Informosome-like infection-specific ribonucleoprotein (vRNP) particles different from mature TMV particles were found in the TMV-infected cells (Yu. L. Dorokhov, N. M. Alexandrova, N. A. Miroshnichenko, and J. G. Atabekov, 1983, Virology 127, 237-252). It is shown here that [3H]uridine incorporation into vRNP RNAs was markedly stimulated in the presence of ATA. vRNP can be released from the TMV-specific polyribosomes by EDTA treatment, which suggests that it is involved in the translation process. The results of the pulse-chase experiments (including those in which TMV RNA synthesis is blocked by 2-thiouracil) suggest that vRNP does not serve as a precursor for mature virion.

Selective inhibition of the polypeptide chain elongation in eukaryotic cells

The effect of Cephalotaxus alkaloids--homoharringtonine and cephalotaxine--on translation in a cell-free system from rabbit reticulocytes and on phenylalanine polymerisation by human ribosomes was studied. The effect of the alkaloids on the nonenzymatic and the eEF-1-dependent Phe-tRNA(Phe) binding to poly(U)-programmed 80S ribosomes, diphenylalanine synthesis accompanying nonenzymatic Phe-tRNA(Phe) binding and acetylphenylalanyl-puromycin formation was examined. Homoharringtonine was shown to inhibit the formation of diphenylalanine and acetylphenylalanyl-puromycin catalysed by human and rat liver ribosomes, but was inactive as an inhibitor on the E. coli elongation system. Neither nonenzymatic nor enzymatic Phe-tRNA(Phe) binding was noticeably affected by the alkaloid. It has been proposed that the site of homoharringtonine binding to 80S ribosomes should overlap or coincide with the acceptor site of the ribosomal peptidyl transferase centre. The association constant of homoharringtonine for 80S human ribosomes was estimated to be (2.57 +/- 0.33).10(7) M-1 in the presence of puromycin. Cephalotaxine did not exert a significant influence on the polypeptide chain elongation.

Increased ribosomal affinity for mRNA causes resistance to edeine in a mutant of Saccharomyces cerevisiae

The effect of edeine on the translation of mRNA or poly(U)-directed polyphenylalanine synthesis has been studied in an edeine-resistant mutant of Saccharomyces cerevisiae under three different experimental conditions: in the whole lysate system, in a micrococcal-nuclease-treated lysate, and in a high-salt-treated lysate. The results indicate that translation of messenger is more resistant to edeine in the whole lysate than in the depleted lysates; these observations suggest that resistance to edeine is associated with the presence of endogenous mRNA. It is shown that 40S mutant subunits have a higher affinity for polysomal RNA than 40S wild-type subunits. Since the mRNA binding is inhibited by 7-methylguanosine 5'-monophosphate, the interaction between polysomal RNA and 40S ribosomes is specific for mRNA. The data demonstrate that in each of the depleted lysates, with edeine initially present, the formation of the 80S initiation complex is inhibited. However, edeine inhibition of [3H]methionine binding to 80S ribosomes is overcome completely in the mutant extract by preincubation of this lysate with polysomal RNA. The results indicate that the mutant may carry a specific change in a messenger-binding factor or in a ribosomal protein thereby permitting an increased stability of the messenger-ribosome complex which consequently results in an increased resistance of the mutant lysate to edeine.

Mechanism of action of a 16-membered macrolide. Binding of rosaramicin to the Escherichia coli ribosome and its subunits

Rosaramicin is a new macrolide antibiotic with activity similar to that of erythromycin. In this paper, we describe the synthesis of [3H]dihydrorosaramicin and show by sucrose gradient centrifugation, gel exclusion chromatography and equilibrium dialysis that rosaramicin and its dihydroderivative bind specifically to the 70-S ribosome and 50-S ribosomal subunit of Escherichia coli. Binding to the 30-S subunit is not detectable. The parameters of the binding interaction were evaluated by equilibrium dialysis. The affinities of E. coli ribosomes for rosaramicin and dihydrorosaramicin are in good agreement with the minimal inhibitory concentration of these drugs for microorganisms.

Inhibitory effects of 'cap' analogues on globin mRNA and encephalomyocarditis RNA translation in a reticulocyte cell-free system

The cap analogues 7-methylguanosine 5'-phosphate [m7G(5')p], 7-methylguanosine 5'-triphosphate [m7G(5')ppp] and 2'-O-methylguanosine 5'-triphosphate [Gm(5')ppp] inhibit the translation of capped globin mRNA and encephalomyocarditis (EMC) RNA (a naturally uncapped mRNA) in a reticulocyte cell-free system. This inhibition occurs at the level of protein synthesis initiation and is of a competitive type since it can be overcome by increasing the mRNA concentration. However, the translation of globin mRNA is more sensitive to the inhibitory effects of the cap analogues m7G(5')p and m7G(5')ppp than translation of EMC RNA. The same spectra of specific inhibition is also observed with some other initiation inhibitors such as aurintricarboxylic acid, which inhibits mRNA binding, but not with pactamycin which does not affect mRNA interaction. A model is presented suggesting that this preferential inhibition by cap analogues could be explained mainly by the different affinities of globin mRNA and EMC RNA for the initiation complexes between 40-S subunits and Met-tRNAf. Moreover Gm(5')ppp cannot be considered simply as a cap analogue since it also affects some step prior to mRNA binding.