Studies on the mode of action of hygromycin B, an inhibitor of translocation in eukaryotes

Hygromycin B is an unusual aminoglycoside antibiotic active against both prokaryotic and eukaryotic cells. Hygromycin B at 0.38 mM concentration completely halts yeast cell growth in rich media, presumably by preventing protein synthesis by cytoplasmic ribosomes. Polypeptide synthesis in cell-free extracts from rabbit reticulocytes, wheat germ and yeast is strongly blocked by low concentrations of hygromycin B. The antibiotic inhibits peptide chain elongation by yeast polysomes by preventing elongation factor EF-2-dependent translocation, although it does not affect either the formation of the EF-2-GTP-ribosome complex or the EF-2- and ribosome-dependent GTP hydrolysis which takes place uncoupled from translocation. The inhibition of translocation by hygromycin B might result from the stabilization of peptidyl-tRNA bound to the ribosomal acceptor site, since the stability of [3H]Phe-tRNA-EF-1-poly(U)-ribosome and [3H]Phe-tRNA-poly(U)-ribosome complexes is increased in the presence of hygromycin B. The inhibition of polyphenylalanine synthesis by reticulocyte ribosomes and enzymic translocation of peptidyl-tRNA by yeast polysomes can be reversed by increasing concentrations of EF-2 suggesting a relationship between the binding sites of EF-2 and hygromycin B on the ribosome. Neither non-enzymic translocation, that takes place in the presence of high potassium concentrations, nor the peptide bondforming step are affected by hygromycin B.

Genetics and biochemistry of cryptopleurine resistance in the yeast Saccharomyces cerevisiae

Protein synthesis by ribosomes from several cryptopleurine-resistant yeast mutants is also resistant to emetine and tubulosine. These mutants can be classified into two different types: Class I mutants which display high levels of resistance to emetine and tubulosine and Class II mutants that are only weakly resistant to tubulosine and are slightly more sensitive to emetine than those of Class I. Apparently all mutants have similar levels of resistance to cryptopleurine. The distinct phenotypes of Class I and Class II strains are expressed through their 40S ribosomal subunit. Genetic analysis has shown that the mutations to cryptopleuring resistance are allelic and that in a particular case (strain CRY6) the pleiotropic phenotype is a result of the expression of the cry1 locus. It is suggested that Class I and Class II mutants arise from two independent mutational events within The cry1 allel. In heterozygous (+/cry1) diploids both the sensitive and the resistant genes are expressed as shown by studies of the action of cryptopleurine on polyphenylalanine-synthesizing systems derived from each parental sensitive and resistant haploid strain and heterozygous diploid strains. The apparent dominance of sensitivity over resistance which may be observed in vivo in heterozygous (+/cry1) diploids has been explained in terms of the mode of action of the inhibitors.

Inhibition of translation in eukaryotic systems by harringtonine

The Cephalotaxus alkaloids harringtonine, homoharringtonine and isoharringtonine inhibit protein synthesis in eukaryotic cells. The alkaloids do not inhibit, in model systems, any of the steps of the initiation process but block poly(U)-directed polyphenylalanine synthesis as well as peptide bond formation in the fragment reaction assay, the sparsomycin-induced binding of (C)U-A-C-C-A-[3H]Leu-Ac, and the enzymic and the non-enzymic binding of Phe-tRNA to ribosomes. These results suggest that the Cephalotaxus alkaloids inhibit the elongation phase of translation by preventing substrate binding to the acceptor site on the 60-S ribosome subunit and therefore block aminoacyl-tRNA binding and peptide bond formation. However, the Cephalotaxus alkaloids do not inhibit polypeptide synthesis and peptidyl[3H]puromycin formation in polysomes. Furthermore, these alkaloids strongly inhibit [14C]trichlodermin binding to free ribosomes but hardly affect the interaction of the antibiotic with yeast polysomot interact with polysomes and therefore only inhibit cycles of elongation. This explains the polysome run off that has been observed by some workers in the presence of harringtonine.

[A case of hepatocarcinoma (author's transl)]

A case of hepatocarcinoma is presented in a five year old male treated with right hemihepatectomy and chemotherapy has been followed during nine months. The rarity of this histologic variety in childhood is emphasized, and so are the clinical, biological and pathological features.

Cryptopleurine resistance: genetic locus for a 40S ribosomal component in Saccharomyces cerevisiae

Mutants resistant to the phenanthrene alkaloids tylophorine, tylocrebrine, and cryptopleurine have been isolated from the yeast Saccharomyces cerevisiae. A single recessive nuclear gene was responsible for resistance. This resistance locus, cry, was closely linked to the mating locus (2.2 centimorgans). The resistance was ribosomal and was due to an altered 40S ribosomal subunit. The mode of action of these drugs has been examined, and they appear to inhibit the translocation phase of protein synthesis.