[Regulatory role of tRNA in synthesis of chicken egg proteins. I. Comparative study of transfer ribonucleic acids extracted from the liver and oviducts of the laying hen]

Polyacrylamide gel mapping of chicken tRNA: comparison of polysome-bound and whole-cell tRNA from normal and avian sarcoma virus-infected chicken embryo fibroblasts

Analysis of the tRNA population from chicken cells was performed by means of polyacrylamide gel mapping. About 60 species were detected; most of these were positively identified by their acceptor specificity. The comparison of polysome-bound and overall cellular tRNA gel patterns from normal and Rous sarcoma virus-infected chicken embryo fibroblasts led us to the following observations: some tRNA species were present in the same relative proportions in all the preparations, and within isoaccepting groups the same species was preponderant; however, although about 8% of whole-cell tRNA was recovered in polysomal preparations, amounts ranging from 3 to 30% were found for individual tRNA species. This points to the absence of a direct correlation between the amount of each mature tRNA species produced and the frequency with which it is used in this case of embryonic cells. No significant difference was observed between the whole-cell tRNA patterns from normal and infected cells. Thus, tRNA transcription appears unaltered when cells are transformed and virus producing. No change was observed in the extent of a post-transcriptional modification of tRNAPhe (the base Y). However, viral infection led to some changes in the relative proportions of individual species from polysomal preparations.

Effects of thyroidectomy on heart and liver rat tRNAs: study of chromatographic and electrophoretic behaviour

Modifications of tRNAs in various physiological or experimental conditions are well documented. We have compared isoacceptor tRNAs extracted from target organs (heart and liver) from thyroidectomized rats to those of control animals. Nine liver aminoacyl-tRNAs and eight heart aminoacyl-tRNAs from thyroidectomized and control rats were analysed by RPC-5 chromatography. Quantitative differences were demonstrated in the relative proportions of the various liver tRNA isoacceptors for glycine, lysine, methionine, phenylalanine and serine and of the heart isoacceptor tRNAs for glycine, lysine, methionine, phenylalanine and valine. A qualitative variation was noted only for tRNATyr from the heart and liver of thyroidectomized rats. Isoacceptor tRNAs were obtained at a high resolution using a two-dimensional polyacrylamide gel electrophoresis. Isoacceptor tRNAs corresponding to 14 amino acids for the liver and 12 amino acids for the heart were identified. Although for most of the tRNAs examined the number of isoacceptors remained unchanged, the number of spots corresponding to tRNAGlu and tRNAHis from the liver and tRNAAla from the heart was different after thyroidectomy. Furthermore the change in electrophoretic behaviour of tRNATyr from the liver of thyroidectomized rats suggests a structural modification of one of the isoacceptors in relation to the change in thyroid status. Thus, thyroid hormones appear to induce some modification of the isoacceptor tRNAs in their target organs.

Polyacrylamide gel mapping of chicken tRNA: comparison of polysome-bound and whole-cell tRNA from normal and avian sarcoma virus-infected chicken embryo fibroblasts

Analysis of the tRNA population from chicken cells was performed by means of polyacrylamide gel mapping. About 60 species were detected; most of these were positively identified by their acceptor specificity. The comparison of polysome-bound and overall cellular tRNA gel patterns from normal and Rous sarcoma virus-infected chicken embryo fibroblasts led us to the following observations: some tRNA species were present in the same relative proportions in all the preparations, and within isoaccepting groups the same species was preponderant; however, although about 8% of whole-cell tRNA was recovered in polysomal preparations, amounts ranging from 3 to 30% were found for individual tRNA species. This points to the absence of a direct correlation between the amount of each mature tRNA species produced and the frequency with which it is used in this case of embryonic cells. No significant difference was observed between the whole-cell tRNA patterns from normal and infected cells. Thus, tRNA transcription appears unaltered when cells are transformed and virus producing. No change was observed in the extent of a post-transcriptional modification of tRNAPhe (the base Y). However, viral infection led to some changes in the relative proportions of individual species from polysomal preparations.

Does quantitative tRNA adaptation to codon content in mRNA optimize the ribosomal translation efficiency? Proposal for a translation system model

Neither a dynamic nor an energetic approach of the translation process has taken into account that intracellular levels of iso-tRNA species are adapted or adjusted to the codon frequency of mRNA being decoded (Bombyx mori silk gland, rabbit reticulocyte). A critical study of available experimental data suggests that the average elongation rate of a protein is maximized in the presence of an adapted tRNA population, usually an homologous tRNA. In addition, the amount of synthesized protein parallels that of corresponding mRNA. Other evidences--including in vitro and in vivo elongation assays with fibroin mRNA--show that individual elongation rates are not uniform. Pauses occur at certain sites of the mRNA chain. The relative lifetime of these pauses depends on the tRNA pool used. Finally, it appears that translation accuracy also depends on the balanced tRNA population. We propose to explain these different effects by using a codon-anticodon recognition model, called "trial and error system" based on a stochastic processing of the ribosome. Accordingly, various acylated tRNA species which surround a ribosome randomly encounter the receptor A site. Every trapped tRNA species is tested for a proper pairing with the codon to be recognized at the level of a comparator or discriminator function. If the pairing is correct, transpeptidation becomes irreversible. If not, the aminoacyl-tRNA is rejected and another randomly trapped tRNA is processed in turn. Mathematical analysis of this model shows that the mean number of trials used for translating the whole sequence of a mRNA is minimized when the proportion of different iso-tRNA species is correlated with the square root of codon frequency. Quantitations of reticulocyte tRNA support such a parabolic relation. Our translation system model brings some light into the role of tRNA adaptation for optimizing translation efficiency, i.e. maximizing both speed and accuracy. Some consequences of the model are discussed.

Messenger RNA translation in the presence of homologous and heterologous tRNA

The effect of tRNA distribution in the cell on the rate of specific protein synthesis has been investigated. A tRNA-dependent cell-free protein-synthesizing system derived from Krebs-II ascites cells has been worked out. In this system it is possible to synthesize specific proteins (egg-white proteins or globin) by the translation of oviduct or reticulocyte messenger RNA in the presence of tRNA from homologous or heterologous tissues. The rate of translation of a give messenger RNA is optimal in the presence of tRNA from the homologous tissue. The lower level of protein synthesis obtained with an excess of heterologous tRNA can be overcome by adding the homologous tRNA. Nevertheless homologous isoaccepting species partly purified by a reversed-phase chromatography or a benzoylated DEAE-cellulose column cannot fully restore the optimal synthesis, eliminating the hypothesis according to which a particular tRNA species is involved in this phenomenon. The correct distribution of tRNA is necessary for optimal translation of a messenger RNA.