[Translation termination factor of eRFI of the ciliate Blepharisma japonicum recognizes all three stop codons].

We have determined the type of stop codon specificity of Blepharisma japonicum translation termination factor eRF1 in an in vitro reconstituted eukaryotic translation system and in in vivo assay (the dual reporter system). We have shown that B. japonicum eRF1 retained specificity towards all three stop codons although efficiency of peptydyl-tRNA hydrolysis in the presence of UGA is reduced in an in vitro assay. We suggest that since the heterotrich B. japonicum represents the earliest diverged lineage on phylogenetic tree of ciliates, B. japonicum has the universal genetic code as ancestor group for all ciliates.

[Protein S3 fragments neighboring mRNA during elongation and translation termination on the human ribosome].

Protein S3 fragments were determined that crosslink to modified mRNA analogues in positions +5 to +12 relative to the first nucleotide in the P-site binding codon in model complexes mimicking states of ribosomes at the elongation and translation termination steps. The mRNA analogues contained a Phe codon UUU/UUC at the 5'-termini that could predetermine the position of the tRNA(Phe) on the ribosome by the location of P-site binding and perfluorophenylazidobenzoyl group at a nucleotide in various positions 3' of the UUU/UUC codon. The crosslinked S3 protein was isolated from 80S ribosomal complexes irradiated with mild UV light and subjected to cyanogen bromide-induced cleavage at methionine residues with subsequent identification of the crosslinked oligopeptides. An analysis of the positions of modified oligopeptides resulting from the cleavage showed that, in dependence on the positions of modified nucleotides in the mRNA analogue, the crosslinking sites were found in the N-terminal half of the protein (fragment 2-127) and/or in the C-terminal fragment 190-236; the latter reflects a new peculiarity in the structure of the mRNA binding center in the ribosome, unknown to date. The results of crosslinking did not depend on the type of A-site codon or on the presence of translation termination factor eRF1.

[How translation termination factor eRF1 Euplotes does not recognise UGA stop codon].

In universal-code eukaryotes, a single class-1 translation termination factor eRF1 decodes all three stop codons, UAA, UAG, and UGA. In some ciliates with variant genetic codes one or two stop codons are used to encode amino acid(s) and are not recognized by eRF1. In Stylonychia, UAG and UAA codons are reassigned as glutamine codons, and in Euplotes, UGA is reassigned as cysteine codon. In omnipotent eRF1s, stop codon recognition is associated with the N-terminal domain of eRF1. Because variant-code ciliates most likely evolved from universal code ancestor(s), structural features should exist in ciliate eRF1s that restrict their stop codon recognition. To find out amino acid residues which confer UAR-only specificity to Euplotes aediculatus eRF1, eRFI chimeras were constructed by swapping eRF1 E. aediculatus N-terminal domain sequences with the matching ones from the human protein. In these chimeras the MC-domain was from human eRF1. Functional analysis of these chimeric eRFI highlighted the crucial role of the two regions (positions 38-50 and 123-145) in the N-terminal domain of E. aediculatus eRF1 that restrict E. aediculatus eRF1 specificity toward UAR codons. Possibly, restriction of eRF1 specificity to UAR codons might have been an early event occurring in independent instances in ciliate evolutionary history, possibly facilitating the reassignment of UGA to sense codons.

[C-domain of translation termination factor eRF1 neighbors stop codon at the 80S ribosomal A site].

Positioning of stop codon and the adjacent triplet downstream of it with respect to the components of human 80S termination complex was studied with the use of mRNA analogues that bore stop signal UPuPuPu (Pu is A or G) and photoactivatable perfluoroaryl azide group. This group was attached to one of nucleotides of the stop signal or 3' of it (in positions +4 to +9 with respect to the first nucleotide of the P site codon). It was shown that upon mild UV irradiation the mRNA analogues crosslinked to components of model complexes imitating state of 80S ribosome in the course of translation termination. It was found that termination factors eRF1 and eRF3 do not affect mutual arrangement of stop signal and the 18S rRNA. Factor eRF1 was shown to cross-link to modified nucleotides in positions +5 to +9 (ability of eRF1 to cross-link to stop codon nucleotide in position +4 was shown earlier). Fragments of eRF1 containing cross-linking sites of the mRNA analogues were determined. In fragment 52-195 (containing the N-domain and a part of the M-domain) we have found cross-linking sites of the analogues that bore modifying groups on A or G in positions +5 to +9 or at the terminal phosphate of nucleotide in position +7. For mRNA analogues bearing modifying groups on G site of cross-linking from positions +5 to +7 was found in the eRF1 fragment

[Influence of individual domains of the translation termination factor eRF1 on induction of the GTPase activity of the translation termination factor eRF3].

Translation termination in eukaryotes is governed by two proteins, belonging to the class-1 (eRF1) and class-2 (eRF3) polypeptide release factors. eRF3 catalyzes hydrolysis of GTP to GDP and inorganic phosphate in the ribosome in the absence of mRNA, tRNA, aminoacyl-tRNA and peptidyl-tRNA but needs the presence of eRF1. It's known that eRF1 and eRF3 interact with each other in vitro and in vivo via their C-terminal regions. eRF1 consists of three domains - N, M, and C. In this study we examined the influence of individual domains of the human eRF1 on induction of the human eRF3 GTPase activity in the ribosome in vitro. It was shown that none of the N-, M-, C- and NM-domains induces eRF3 GTPase activity in presence of the ribosomes. MC-domain does induce GTPase activity of eRF3 but four times less efficient than full-length eRF1, therefore, MC-domain (and very likely M-domain) binds to the ribosome in the presence of eRF3. Based on these data and taking into account the data available in literature, a conclusion was drawn that the N domain of eRF1 is not essential for eRF1-dependent induction of the eRF3 GTPase activity. A working hypothesis is formulated, postulating that GTPase activity eRF3 during the translation termination is associated with the intermolecular interactions of GTP/GDP, GTPase center of the large ribosomal subunit (60S), MC-domain of eRF1, C-terminal region and GTP-binding domains of eRF3, but without participation of the N-terminal region of eRF3.

[The tRNA anticodon is recognized by aminoacyl-tRNA-synthetase]. / Antikodon tRNK uznaetsia aminoatsil-tRNK-sintetazoi.

Data concerning the regions of tRNA molecules recognized by cognate aminoacyl-tRNA synthetases and obtained recently by the new methods (the synthesis of mutant tRNA genes and their transcription in vitro, chimeric suppressor tRNA, RNA engineering) are briefly discussed. The results of several laboratories are in full agreement with the hypothesis proposed earlier on the role of a tRNA anticodon as a specific region by which the enzyme identifies various tRNAs. More than half of tRNAs belong to the group recognized by the aminoacyl-tRNA synthetases by this mechanism. The behavior of natural and artificially constructed tRNAs which contain modified anticodons, but conserve amino acid specificity in the reaction of aminoacylation, indicates that in some tRNAs the role of the anticodon in the tRNA identity is minor if any. This group of tRNA is recognized by the aminoacyl-tRNA synthetases predominantly via the double helical region of the molecule. The authors discuss the significance of the absence of topographic conservatism in the mechanism by which tRNAs are recognized by the aminoacyl-tRNA synthetases.

[Reverse transcription of influenza virion RNA without exogenous primer]. / Obratnaia transkriptsiia virionnoi RNK virusa grippa bez vneshnei zatravki.

Reverse transcriptase has been shown to transcribe virion DNA of influenza A virus without an exogeneous primer. At least six virion RNA segments are transcribed with the formation of complementary 4S DNA product. A possible primer function of hairpin structures at the 3'-end of virion RNA segments is discussed.

[Enzymatic synthesis and characterization of DNA complementary to ceruloplasmin mRNA from rat liver]. / Fermentativnyi sintez i kharakteristika DNK, komplementarnoi tseruloplazminovoi mRNK iz pecheni krysy.

Poly(A) containing rat liver 21S RNA homogeneous in polyacrylamide gel electrophoresis under denaturing conditions and stimulating the synthesis of ceruloplasmin in a cell-free proteinsynthesizing system, was used as a template for reverse transcription in the presence of T10 primer and highly purified reverse transcriptase from avian myeloblastosis virus. The cDNA made this way was characterized by means of hybridization kinetics with mRNA, by melting of the hybrids formed and by chain length measurements. To increase the degree of representativity, the ceruloplasmin mRNA was fragmented by mild alkaline treatment, enzymatically polyadenylated and transcribed. The cDNA made was fully characterized and the kinetic complexity measured by hybridization with the mRNA was found to be equal to 2300 nucleotides as compared with the value of 3000 nucleotides is expected from gel electrophoresis data. The observed difference may indicate the presence of repeated sequences in the given mRNA. The sufficient representativitness of the synthesized cDNA and its specificity with respect to ceruloplasmin mRNA allows to use it as a molecular probe to study the ceruloplasmin gene structure.

[Nuclear precursors for ceruloplasmin-coding messenger RNA from rat liver]. / Iadernye predshestvenniki informatsionnoi RNK, kodiruiushchei tseruloplazmin, iz pecheni krysy.

The distribution of the sequences coding for ceruloplasmin (CP) in rat liver heterogeneous nuclear RNA (hnRNA) was studied using highly specific CP cDNA as a hybridization probe. The content of CP-coding sequences in poly(A)-containing and poly(A)-free subfractions of hnRNA was shown to be respectively 1 and 27 equivalents of CP mRNA molecule per one hepatocyte. The gel electrophoresis of hnRNA under strongly denaturing conditions with the subsequent transfer of RNA to diazobenzyloxymethyl paper and hybridization with [32P]-cDNA probe showed that CP mRNA sequences were of multiple molecular weight distribution. In particular, 9.0, 6.6, 2.4 and 1.6 megadalton fractions of non-polyadenylate hnRNA carried CP-coding sequences while the only hand that hybridized to CP cDNA was detected in polyadenylated hnRNA. This band was of a molecular weight 1.1-1.2 megadaltons corresponding to that of cytoplasmic CP mRNA. The hybridization of high molecular weight hnRNA with full-length CP cDNA followed by the determination of the size of cDNA fragments protected against SI nuclease demonstrated that coding sequences of CP pre-mRNA are interrupted by intervening sequences.

[Highly conserved region 1816-1831 of the 18S ribosomal RNA is close to the first nucleotide of the A-site codon in elongation and termination of translation]. / Vysokokonservativnyi uchastok 1816-1831 18S ribosomnoi RNK sblizhen s pervym nukleotidom kodona v A-uchastke ribosomy pri élongatsii i terminatsii transliatsii.

Two mRNA analogs, pUUCUAAA (with stop codon UAA) and pUUCUCAA (with Ser codon UCA) containing a perfluoroarylazido group at U4, were used to study the position relative to the 18S rRNA for the first nucleotide of the codon located in the A site of the human 80S ribosome. To place UAA or UCA in the A site, UCC-recognizing tRNAPhe was bound in the P site. With each analog, crosslinking was detected for highly conserved fragment 1816-1831, which contains invariant dinucleotide A1823/A1824 and is in helix 44 at the 3' end of the 18S rRNA. Since 18S rRNA modification did not depend on whether the U4 photoreactive group was in the sense or stop codon, it was assumed that polypeptide chain release factor 1 directly recognizes the trinucleotide of a stop codon located in the A site.