Das 3′-Ende der tRNA und seine Rolle bei der Proteinbiosynthese

C-terminal modifications of a protein by UAG-encoded incorporation of puromycin during in vitro protein synthesis in the absence of release factor 1

Deactivation of release factor 1 by polyclonal antibodies in an in vitro translation system, which was used to express the esterase gene, led to the reversible elimination of naturally occurring termination. This technique allowed the antibiotic puromycin to be used as an acceptor substrate for the peptidyl residue in the peptidyl-transferase reaction. This resulted in more than 80 % yield of protein with C-terminally incorporated puromycin. pCpPuromycin that was either conjugated with the Cy3 fluorophor or biotin by N4 alkylation of cytosine, also acted as an acceptor substrate for the peptidyl-transferase reaction and was incorporated into the protein C terminus. The resulting conjugates possessed Cy3-specific fluorescence and affinity to streptavidin-coated surfaces, respectively. This left the enzymatic activity of the reporter protein unaffected. It was also shown that extension of puromycin on its 5'-hydroxyl end by up to ten deoxyoligonucleotides also allowed conjugation with the C terminus of in vitro translated protein when RF1-dependent termination was suppressed. However, the conjugation yield decreased upon addition of more than six nucleotides.

Models of the elongation cycle: an evaluation

The central process for the transfer of the genetic information from the nucleic acid world into the structure of proteins is the ribosomal elongation cycle, where the sequence of codons is translated into the sequence of amino acids. The nascent polypeptide chain is elongated by one amino acid during the reactions of one cycle. Essentially, three models for the elongation cycle have been proposed. The allosteric three-site model and the hybrid-site model describe different aspects of tRNA binding and do not necessarily contradict each other. However, the alpha-epsilon model is not compatible with both models. The three models are evaluated in the light of recent results on the tRNA localization within the ribosome: the tRNAs of the elongating ribosome could be localized by two different techniques, viz. an advanced method of small-angle neutron scattering and cryo-electron microscopy. The best fit with the biochemical and structural data is obtained with the alpha-epsilon model.

Conserved nucleotides of 23 S rRNA located at the ribosomal peptidyltransferase center

Two nucleotides of the 23 S rRNA gene were mutated; the nucleotides correspond to the first two positions of the universally conserved sequence PsiGG2582 at the peptidyltransferase ring of 23 S rRNA. The ribosomes containing the altered 23 S rRNA were analyzed. Previously, it was shown that ribosomal assembly was indistinguishable from that in wild-type cells, that the flow of the corresponding 50 S subunit into the polysome fraction was not restricted, but that the ribosomes were strongly impaired in poly(Phe) synthesis (C. M. T. Spahn, J. Remme, M. A. Schäfer, and K. H. Nierhaus (1996) J. Biol. Chem. 271, 32849-32856). Here we apply assay systems exclusively testing the puromycin reaction of ribosomes carrying plasmid-born rRNA, a dipeptide assay using the minimal P site donor pA(fMet) and a translocation system not depending on the puromycin reaction. The mutations in helix 90 exclusively abolish or severely impair the ribosome capability to catalyze AcPhe-puromycin formation. A possible explanation of these observations is that G2581 and Psi2580 (and possibly also G2582) are part of the binding site of C75 of peptidyl-tRNA in the P site. The results suggest that in this case, however, such an interaction would disobey canonical base pairing.

Mutational analysis of two highly conserved UGG sequences of 23 S rRNA from Escherichia coli

The 23 S-type rRNA contains two phylogenetically conserved UGG sequences, which have the potential to bind the universal CCA-3'-ends of tRNAs at the ribosomal peptidyltransferase center by base pairing. The first two positions, UG, of these sequences at the helix-loop 80 (U2249G2250) and helix-loop 90 (Psi2580G2581) and some related nucleotides were tested by site-directed mutagenesis for their involvement in ribosomal function, i.e. peptidyltransferase. The plasmid-derived mutated 23 S rRNA comprised about 50% of the total 23 S rRNA. None of the single mutations caused an assembly defect, and all 50 S subunits carrying an altered 23 S rRNA could freely exchange with the pools of 70S ribosomes and polysomes. The mutations at the helix-loop 80 region hardly affected bacterial growth. However, mutations at the helix 90 caused severe growth effects and severely impaired the in vitro protein synthesis, showing that this 23 S rRNA region is of high importance for ribosomal function.

Aminoacylation of tRNAs as critical step of protein biosynthesis

Isoleucyl-tRNA synthetases isolated from commercial baker's yeast and E coli were investigated for their sequences of substrate additions and product releases. The results show that aminoacylation of tRNA is catalyzed by these enzymes in different pathways, eg isoleucyl-tRNA synthetase from yeast can act with four different catalytic cycles. Amino acid specificities are gained by a four-step recognition process consisting of two initial binding and two proofreading steps. Isoleucyl-tRNA synthetase from yeast rejects noncognate amino acids with discrimination factors of D = 300-38000, isoleucyl-tRNA synthetase from E coli with factors of D = 600-68000. Differences in Gibbs free energies of binding between cognate and noncognate amino acids are related to different hydrophobic interaction energies and assumed conformational changes of the enzyme. A simple hypothetical model of the isoleucine binding site is postulated. Comparison of gene sequences of isoleucyl-tRNA synthetase from yeast and E coli exhibits only 27% homology. Both genes show the 'HIGH'- and 'KMSKS'-regions assigned to binding of ATP and tRNA. Deletion of 250 carboxyterminal amino acids from the yeast enzyme results in a fragment which is still active in the pyrophosphate exchange reaction but does not catalyze the aminoacylation reaction. The enzyme is unable to catalyze the latter reaction if more than 10 carboxyterminal residues are deleted.