Interaction of Ac-Phe-tRNA with e. coli ribosomal subunits. 1. Sparsomycin-induced formation of a complex containing 50 S and 30 S subunits but not mRNA

Characterization of sparsomycin resistance in Streptomyces sparsogenes

The antitumor antibiotic sparsomycin, produced by Streptomyces sparsogenes, is a universal translation inhibitor that blocks the peptide bond formation in ribosomes from all species. Sparsomycin-resistant strains were selected by transforming the sensitive Streptomyces lividans with an S. sparsogenes library. Resistance was linked to the presence of a plasmid containing an S. sparsogenes 5.9-kbp DNA insert. A restriction analysis of the insert traced down the resistance to a 3.6-kbp DNA fragment, which was sequenced. The analysis of the fragment nucleotide sequence together with the previous restriction data associate the resistance to srd, an open reading frame of 1,800 nucleotides. Ribosomes from S. sparsogenes and the S. lividans-resistant strains are equally sensitive to the inhibitor and bind the drug with similar affinity. Moreover, the drug was not modified by the resistant strains. However, resistant cells accumulated less antibiotic than the sensitive ones. In addition, membrane fractions from the resistant strains showed a higher capacity for binding the drug. The results indicate that resistance in the producer strain is not connected to either ribosome modification or drug inactivation, but it might be related to an alteration in the sparsomycin permeability barrier.

Mechanism of action of sparsomycin in protein synthesis

Before CI isomerizes to C*I, we detect a competitive phase of inhibition (Ki = k5/k4 = 0.05 microM) which eventually, by increasing the concentration of I, becomes linear mixed noncompetitive and involves C*I in place of CI. The equilibration of C and I according to reaction 2 is much slower than the equilibration between C and S in reaction 1 (time-dependent inhibition). The inactivation plots obey reaction 2 and allow us to estimate k6 as equal to 2.2 min-1. The isomerized C*I, free of excess I, can be studied as a mixture with complex C. From the kinetics of the regeneration of C from C*I, in the presence of puromycin, we can estimate k7 to be between 0.22 min-1 and 0.06 min-1. Although the isomerized C*I survives after adsorption on cellulose nitrate filter disks, it does not survive after gel chromatography on a Sepharose CL-4B column but is converted quantitatively to complex C containing D of unchanged reactivity. This result does not support the proposed [Flynn, G. A., & Ash, R. J., (1990) Biochem. Biophys. Res. Commun. 166, 673-680] chemical reaction between D and I toward new products. The isomerized C*I can be obtained not only from the already-made complex C but also de novo from D, R, and M. In the latter case, the reactions which lead to C are represented by the following hypothetical scheme: D + R + M in equilibrium with DRM or C (binding reaction). When C*I is formed de novo, this reaction is coupled to reaction 2 and the ultimate product is a mixture of C and C*I.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of translation in bacterial and eukaryotic systems by the antibiotic anthelmycin (hikizimycin)

Anthelmycin inhibits protein synthesis on both pro- and eukaryotic ribosomes by preventing the peptide bond-forming reaction. The drug is structurally similar to certain other 4-aminohexosyl cytosine antibiotics including blasticidin S, gougerotin, amicetin and bamicetin although unlike these compounds anthelmycin lacks an aminoacyl moiety. It is proposed that anthelmycin inhibits the ribosomal peptidyl transferase centre by associating with a site that overlaps the (related) ribosomal receptor site(s) for the other four inhibitors.

Inhibitors of protein synthesis V. Irreversible interaction of antibiotics with an initiation complex

The initiation complex (t-complex) formed in a cell-free system (E. coli) from Ac-Phe-tRNA, poly(U) and washed ribosomes in the presence of initiation factors (ribosomal wash) and GTP, contains the Ac-Phe-tRNA bound quantitatively in a puromycin-reactive state. The t-complex is irreversibly inactivated by spiramycin with respect to its reactivity toward puromycin. The inactivated t-complex retains all of the Ac-Phe-tRNA bound, but it does not react with puromycin (2 x10-minus-3M) within 32 min at 25 degrees. In the case of another inhibitor protein synthesis, sparsomycin, the permanently "modified" t-complex not only retains all the bound Ac-Phe-tRNA but it can still react with puromycin. In the continuous presence of sparsomycin (1 x 10-minus-7M) the bound Ac-Phe-tRNA reacts quantitatively at a rate which is one-tenth the rate at which the t-complex reacts with puromycin, at low (6.25 x 10-minus-5M) or high (2 x 10-minus-3M) concentrations. These results are not in agreement with current views according to which aparsomycin binds to the ribosome reversibly at a single site with a KI in the range of 10-minus6-10-minus-7 M and according to which this stie is at the A'-site (puromycin site) of peptidyl transferase.

The involvement of sulphydryl groups in the peptidyl transferase centre of eukaryotic ribosomes

Treatment of mammalian ribosomes with N-ethylmaleimide enhances up to 100% the ribosome efficiency in the "fragment reaction assay" for peptide bond formation by increasing the affinity of the substrate C-A-C-C-A-Leu-Ac for the donor site. This stimulation in peptidyl transferase activity was not observed when yeast ribosomes were treated in a similar manner. Stimulation of the peptidyl transferase activity of mammalian ribosomes was also observed by treatment with either p-chloromercuribenzoic acid or 5,5'-dithiobis-(2-nitrobenzoic acid) or 5,5'-dithiobis-(2-nitropyridine) or the maleimide-derived antibiotic showdomycin. N-Ethylmaleimide treatment also enhances C-A-C-C-A-Leu binding to the acceptor site of the peptidyl transferase centre. However, neither binding of N-Ac-Phe-tRNA in the presence of ethanol, nor binding of Phe-tRNA to the ribosomes is stimulated by N-ethylmaleimide. The antibiotic tenuazionic acid (a selective inhibitor of peptide bond formation by mammalian ribosomes) appears to require for its inhibitory effect the ribosome sulphydryl residues, since its inhibitory action on the fragment reaction is greatly decreased in ribosomes treated with N-ethylmaleimide.