ABSTRACT
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.
Subject(s)
Harringtonines/pharmacology , Peptide Chain Elongation, Translational/drug effects , Animals , Cell-Free System , Codon/physiology , Escherichia coli/metabolism , Homoharringtonine , Humans , In Vitro Techniques , Kinetics , Liver/metabolism , Models, Chemical , Placenta/metabolism , RNA, Transfer, Phe/metabolism , Rats , Ribosomes/metabolismABSTRACT
The order of relative affinity of three different functional form of tRNA (aminoacyl-tRNA, peptidyl-tRNA and deacylated tRNAOH) was established for three sites of the ribosome. The affinity increases for A-site in consecutive order: tRNAOH less than AcPhe-tRNA less than aa-tRNA; for P-site with messenger RNA: AcPhe-tRNA less than aa-tRNA less than tRNAOH; without messenger RNA: aa-tRNA less than AcPhe-tRNA less than tRNAOH; for E-site: (AcPhe-tRNA, aa-tRNA) much less than much less than tRNAOH. The dependence of association constants versus magnesium concentration for all forms of tRNA conforms the equation: delta lg Ka/delta lg[Mg2+] = n. Number "n" varies in the range 1 to 8 depending on the site of adsorption the form of tRNA and the presence of mRNA. Such magnesium dependence of affinity of tRNAs shows that the electrostatic interactions play an important role in the recognition of functional forms of tRNA by ribosomal sites.
Subject(s)
RNA, Transfer/metabolism , Ribosomes/metabolism , Binding Sites , Escherichia coli/metabolism , RNA, Transfer, Amino Acyl/metabolism , Templates, Genetic , ThermodynamicsABSTRACT
A spectral division method of two conformational states of spin-labeled macromolecules is presented. The method is suitable in conditions of highly anisotropic motion of spin label and is based on titration of experimental spectra of spin-labeled macromolecule by theoretical ones. Theoretical spectra simulation uses the Freed theory and spin-Hamiltonian parameters, derived from independent experiments. Nomogrammes and formula for calculation of order parameter Sz and correlation time tau c in temperature-viscosity experiment are available. The method was applied to spectral division of two conformational states of spin-labeled tRNAPhe from E. coli and spectral parameters Sz and tau c were obtained for both states. ESR spectra of these conformational states at t degree = 20 degrees differ strongly from one another by order parameter Sz. The first conformer, that is characterised by a greater order parameter has no globular conformational transition (in terms of changes of the hydrodynamic macromolecule radius) between 2 degrees and 20 degrees, but local conformational changes take place in this temperature region.
Subject(s)
Escherichia coli/genetics , RNA, Transfer, Amino Acyl/genetics , Electron Spin Resonance Spectroscopy , Kinetics , Mathematics , Nucleic Acid Conformation , Spin LabelsABSTRACT
Purified Phe-tRNAPhe revealed higher affinity to the donor (D) site of vacant 70S . poly(U) complex than to the acceptor (A) site, independent on Mg2+ concentration. As a result, in excess of ribosomes Phe-tRNAPhe binds exclusively to the D site. This was proved using the tests in the presence of tetracycline and puromycin. Preferential binding of Phe-tRNAPhe to the D site was used to measure equilibrium association constants of this interaction at different temperatures and Mg2+ concentrations. A large value of reaction enthalpy (ca. -26 Kcal/mole) was found.
