Turnover of transfer RNA species during development of the posterior silkgland of Bombyx mori L

The role of cis-zeatin-type cytokinins in plant growth regulation and mediating responses to environmental interactions

Cytokinins (CKs) are well-established as important phytohormonal regulators of plant growth and development. An increasing number of studies have also revealed the function of these hormones in plant responses to biotic and abiotic stresses. While the function of certain CK classes, including trans-zeatin and isopentenyladenine-type CKs, have been studied in detail, the role of cis-zeatin-type CKs (cZs) in plant development and in mediating environmental interactions is less well defined. Here we provide a comprehensive summary of the current knowledge about abundance, metabolism and activities of cZs in plants. We outline the history of their analysis and the metabolic routes comprising cZ biosynthesis and degradation. Further we provide an overview of changes in the pools of cZs during plant development and environmental interactions. We summarize studies that investigate the role of cZs in regulating plant development and defence responses to pathogen and herbivore attack and highlight their potential role as 'novel' stress-response markers. Since the functional roles of cZs remain largely based on correlative data and genetic manipulations of their biosynthesis, inactivation and degradation are few, we suggest experimental approaches using transgenic plants altered in cZ levels to further uncover their roles in plant growth and environmental interactions and their potential for crop improvement.

Specific codon usage pattern and its implications on the secondary structure of silk fibroin mRNA

We have identified two distinctive regions of the repetitive unit nucleotide sequence of fibroin mRNA of Bombyx mori. The codon usage for the major amino acids, glycine, alanine and serine is distinctly different in these two regions, indicating that it is determined by the fibroin mRNA or gene structure but not by the tRNA population. Comparative computer analyses of nucleotide substitutions in the unit sequence suggest that selection has operated on the codon usage to optimize the secondary structure characteristic of the fibroin mRNA.

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.

Differential synthesis rates of tRNA species in the silk gland of Bombyx mori are required to promote tRNA adaptation to silk messages

In studying the mechanism of tRNA adaptation to silk fibroin and sericin mRNA translation, we investigated the accumulation rates of tRNA species rapidly labeled in vivo in the posterior and middle silk glands of the silkworm Bombyx mori during the last larval instar. We found that the rates of synthesis of total and individual tRNA species labeled with [3H]uridine during the growth and the secretion phases and separated by electrophoresis on polyacrylamide gels, are specific for each tRNA species and each tissue. We noted continuous variations of the tRNA levels designed to reflect changes in the population of mRNAs. The tRNA adaptation to silk mRNAs is completed roughly at the end of the growth phase and at the beginning of the secretion phase (day four of the Vth instar). Its stabilization during the secretion phase occurs by a reversal of the relative rates of predominant tRNA species, mainly tRNAGly. The sum total of the synthetic rates for each tRNA species corroborates their known adaptive distribution during the secretion phase of silk proteins. In addition, when coupled to the similarity of their turnover rates, observed previously, the data support a transcriptional control of tRNA genes.

Transfer RNA in reticulocyte maturation

The disappearance of tRNA during the maturation of rabbit reticulocytes under the stress of phenylhydrazine-induced hemolysis was studied. The tRNA content of reticulocytes and of erythrocytes derived from them was compared. The results show that tRNA persists longer after reticulocyte maturation than ribosomes and than the ability to incorporate amino acids into protein. Considerable uniformity of tRNA degradation was noted with about 15% of the tRNA for most amino acids remaining after reticulocyte maturation. The half-life of tRNA in the maturing cells is estimated to be 50--60 h. There is little tRNA lacking the 3'-terminal pCpCpA moiety in cells derived from reticulocytes.

Effect of starvation on tRNA synthesis, amino acid pool, tRNA charging levels and aminoacyl-tRNA synthetase activities in the posterior silk gland of Bombyx mori L

Changes in the translational machinery components of the Bombyx mori posterior silk gland were analysed during starvation and refeeding and compared to the regularly fed larvae. During starvation, tRNA and ribosomal RNA synthesis are stopped. The amounts of different RNA classes and of the different tRNA species slow down at the same rate. Thus various tRNA show similar half-lifes and the preexisting tRNA adaptation to fibroin mRNA translation persists during starvation. Similarly, the tRNA/rRNA ratio is constant during starvation and refeeding (12 tRNA molecules for one ribosome) as in silk glands of control animals. Aminoacyl-tRNA synthetases and tRNA charging levels are decreased during starvation. The maximal tRNA charging level obtained during maximal protein synthesis in control animals is regained after 24 h refeeding of starved larvae. Changes observed in the free amino acid pool are not similar from one amino acid to another and levels reached after starvation do not differ strongly from the controls. Our results suggest that the production of translation apparatus components is coordinated and adjusted to the protein synthesis activity. Whether this coordination occurs in the silk gland is discussed on the basis of the "metabolic regulation", primarily described in prokaryotes and Yeast. Transfer RNA charging levels seem to play a key role in the process of regulation and could be implicated in the mechanism of tRNA adaptation if this phenomenon results as expected from a transcriptional control.

Studies on tRNA adaptation, tRNA turnover, precursor tRNA and tRNA gene distribution in Bombyx mori by using two-dimensional polyacrylamide gel electrophoresis

Eighteen out of twenty amino acids have been used for identifying tRNAs from the silkworm Bombyx mori L. fractionated on two-dimensional polyacrylamide gel electrophoresis. 43 spots out of 53 have been identified. This mapping confirms previous results and brings new answers to some questions on the regulation of tRNA biosynthesis. 1. In addition to quantitative adaptation of tRNAs to the composition of silk proteins (fibroin from the posterior silk gland, sericin from the middle part) and of iso-tRNAs from posterior silk gland to the major codons of fibroin mRNA, we also observe adaptation of tRNA from various tissues to the average amino acid content of proteins from fat body, gut, gonads and carcass of the silkworm. 2. In the silk gland, turnover rates of several tRNA species are similar. The selective accumulation of tRNAs needed for decoding fibroin and sericin mRNAs which takes place during the Vth larval instar, cannot be explained by the occurrence of a preferential degradation of some tRNA species. 3. Under given conditions for incubating silk glands, it is possible to obtain an accumulation of precursor tRNA species, which are enriched in pre-tRNAAla and pre-tRNAGly in the posterior silk gland and pre-tRNASer in the middle part. 4. The distribution of tRNA genes is not random. tRNA genes for glycine, alanine and serine are prominent. Selective transcription of batteries of iso-tRNA genes could explain our data.

Quantitative data on the Bombyx mori L. silkworm: a review

This paper summarizes a variety of quantitative data on the silkworm Bombyx mori, collected in the literature, to help building models on silk gland differentiation. The properites of the silk gland and their changes especially during the last larval instar have been reviewed (size, DNA, RNA amino acids, enzymes). The components of the silk (fibroin and sericin) are also studied (molecular weight, composition). Thus translation and transcription rates have been estimated. The relevant data on the fat body and the haemolymph are also given, as well as some characteristics of the oocyte/egg system.

The adaptation of the silkgland cell to the production of fibroin in Bombyx mori L

At the end of the larval life, the posterior silk gland of Bombyx mori is highly specialized in the biosynthesis of a specific protein : silk fibroin. The successive steps of fibroin production : amino supply, synthesis and secretion are described. Their analysis shows that fibroin synthesis is important enough to orient the overall cellular activities. Thus, the terminal differentiation of the posterior silk gland cell corresponds to the cells adaptation to the production of fibroin. Cytological and biochemical studies of the silk gland development show that specialization occurs discontinuously; the fourth molt, when fibroin is no more produced, is a phase of regression of the cellular adaptation whereas cell differentiation proceeds during the growth phase of the following fifth intermolt. After the spinning of the cocoon, the cells are lysed and disappear entirely at the nymphal stage. Biometrical analysis of silk production of different Bombyx strains in relation with the development of the proteosynthesis apparatus leads to the conclusion that the specific messenger RNA content determines the amount of synthetized fibroin. At maximum secretion, the mRNAF recruits almost all the cell ribosomes. The variations of the size of the proteosynthesis machinery are sufficient to explain the differences of productivity of the various silkworm strains. Different experimental factors affect silk production. Topical applications of juvenile hormone induce an increase of the RNA content and a consequent rise of the amount of secreted protein. In contrast, starvation reduces the silk production by acting at both transcriptonal and translational levels. Current researches on this system are devoted to the study of the differential gene expression, with particular interest to the regulation of the transcription of the specific fibroin messenger RNA.

Histidine transfer RNA levels in Friend leukemia cells: stimulation by histidine deprivation

Friend leukemia cells incubated with sublethal concentrations of histidinol for 5 to 6 days show up to twofold increases in their relative concentrations of histidine transfer RNA and no change in the relative concentrations of leucine transfer RNA. A similar effect is seen when cells are grown to stationary phase in the presence of 0.2 times the amount of histidine in Eagle's minimum essential medium. These observations support the theory that the concentrations of specific transfer RNA's are regulated by a mechanism that is sensitive to the extent of their aminoacylation.