Studies on the half life time of rat liver transfer RNA species

tRNATyr has an unusually short half-life in Trypanosoma brucei

Following transcription, tRNAs undergo a series of processing and modification events to become functional adaptors in protein synthesis. Eukaryotes have also evolved intracellular transport systems whereby nucleus-encoded tRNAs may travel out and into the nucleus. In trypanosomes, nearly all tRNAs are also imported from the cytoplasm into the mitochondrion, which lacks tRNA genes. Differential subcellular localization of the cytoplasmic splicing machinery and a nuclear enzyme responsible for queuosine modification at the anticodon "wobble" position appear to be important quality control mechanisms for tRNATyr, the only intron-containing tRNA in T. brucei Since tRNA-guanine transglycosylase (TGT), the enzyme responsible for Q formation, cannot act on an intron-containing tRNA, retrograde nuclear transport is an essential step in maturation. Unlike maturation/processing pathways, the general mechanisms of tRNA stabilization and degradation in T. brucei are poorly understood. Using a combination of cellular and molecular approaches, we show that tRNATyr has an unusually short half-life. tRNATyr, and in addition tRNAAsp, also show the presence of slow-migrating bands during electrophoresis; we term these conformers: alt-tRNATyr and alt-tRNAAsp, respectively. Although we do not know the chemical or structural nature of these conformers, alt-tRNATyr has a short half-life resembling that of tRNATyr; the same is not true for alt-tRNAAsp We also show that RRP44, which is usually an exosome subunit in other organisms, is involved in tRNA degradation of the only intron-containing tRNA in T. brucei and is partly responsible for its unusually short half-life.

An extensive allelic series of Drosophila kae1 mutants reveals diverse and tissue-specific requirements for t6A biogenesis

N(6)-threonylcarbamoyl-adenosine (t6A) is one of the few RNA modifications that is universally present in life. This modification occurs at high frequency at position 37 of most tRNAs that decode ANN codons, and stabilizes cognate anticodon-codon interactions. Nearly all genetic studies of the t6A pathway have focused on single-celled organisms. In this study, we report the isolation of an extensive allelic series in the Drosophila ortholog of the core t6A biosynthesis factor Kae1. kae1 hemizygous larvae exhibit decreases in t6A that correlate with allele strength; however, we still detect substantial t6A-modified tRNAs even during the extended larval phase of null alleles. Nevertheless, complementation of Drosophila Kae1 and other t6A factors in corresponding yeast null mutants demonstrates that these metazoan genes execute t6A synthesis. Turning to the biological consequences of t6A loss, we characterize prominent kae1 melanotic masses and show that they are associated with lymph gland overgrowth and ectopic generation of lamellocytes. On the other hand, kae1 mutants exhibit other phenotypes that reflect insufficient tissue growth. Interestingly, whole-tissue and clonal analyses show that strongly mitotic tissues such as imaginal discs are exquisitely sensitive to loss of kae1, whereas nonproliferating tissues are less affected. Indeed, despite overt requirements of t6A for growth of many tissues, certain strong kae1 alleles achieve and sustain enlarged body size during their extended larval phase. Our studies highlight tissue-specific requirements of the t6A pathway in a metazoan context and provide insights into the diverse biological roles of this fundamental RNA modification during animal development and disease.

Posttranscriptional suppression of proto-oncogene c-fms expression by vigilin in breast cancer

cis-acting elements found in 3'-untranslated regions (UTRs) are regulatory signals determining mRNA stability and translational efficiency. By binding a novel non-AU-rich 69-nucleotide (nt) c-fms 3' UTR sequence, we previously identified HuR as a promoter of c-fms proto-oncogene mRNA. We now identify the 69-nt c-fms mRNA 3' UTR sequence as a cellular vigilin target through which vigilin inhibits the expression of c-fms mRNA and protein. Altering association of either vigilin or HuR with c-fms mRNA in vivo reciprocally affected mRNA association with the other protein. Mechanistic studies show that vigilin decreased c-fms mRNA stability. Furthermore, vigilin inhibited c-fms translation. Vigilin suppresses while HuR encourages cellular motility and invasion of breast cancer cells. In summary, we identified a competition for binding the 69-nt sequence, through which vigilin and HuR exert opposing effects on c-fms expression, suggesting a role for vigilin in suppression of breast cancer progression.

Estrogen induction of sexual behavior in female rats and synthesis of polyadenylated messenger RNA in the ventromedial nucleus of the hypothalamus

To test the hypothesis that estrogen stimulates sexual behavior by inducing transcription of polyadenylated messenger RNA, we studied the effects of cordycepin, an adenosine analog that disrupts polyadenylation, on the lordotic responses of ovariectomized female rats made sexually receptive with systemic injections of estradiol benzoate (EB) and progesterone (P). Cordycepin inhibited lordosis when infused into the ventromedial nucleus of the hypothalamus within an hour before the females received EB; its effectiveness varied linearly with dose. It did not significantly alter sexual behavior when infused into the medial preoptic area. A dose of cordycepin that decreased lordosis when infused 1 h before injection of 0.5 microgram EB did not affect the behavior when infused 1 h before injection of 500 micrograms P. Cordycepin does not suppress behavior by blocking estrogen uptake since it did not alter estrogen accumulation by hypothalamic cell nuclei. Cordycepin inhibits ribosomal RNA (rRNA) synthesis as well as polyadenylation. While this probably contributes to cordycepin's inhibitory effects on lordosis, it cannot fully account for them since a cytidine analog that inhibits rRNA synthesis without inhibiting polyadenylation did not mimic cordycepin's behavioral effects. Cordycepin may suppress synthesis of P receptors; however, this could not fully account for its behavioral effects since cordycepin also inhibited lordosis when the P receptor was bypassed by substituting methysergide for P. As assessed by protein synthesis autoradiography, cordycepin's effects are highly localized. The data support the notion that estrogen facilitates female sexual behavior by altering gene expression in the brain.

Synthesis of transfer RNA in rat liver after acute and chronic aflatoxin B1 administration

Synthesis of rat liver tRNA following acute and chronic administration of aflatoxin B1 was compared to protein synthesis. Twenty hours after acute administration of aflatoxin B1 total RNA synthesis was inhibited by 91% and tRNA synthesis by 77% while protein synthesis as assayed in vitro was inhibited by 75%. In the chronic (long-term) experiment at 7, 18 and 30 weeks after administration of aflatoxin B1 no inhibition of total RNA and tRNA synthesis was observed but protein synthesis in the cell-free system was inhibited by 16%. Thus, the inhibitory effects on protein synthesis observed in the long-term experiments are not correlated with the inhibition of tRNA synthesis.

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.

Mouse histidyl-tRNAs during pregnancy. Differentiation of activity profiles within and between organs

Optimal conditions for in vitro formation of His-tRNA were established. Transfer RNA of maternal mouse organs and total embryo at 13 and 17 days of pregnancy was acylated in vitro with [3H] or [14C] histidine and examined by reversed-phase plaskon chromatography. Most tissues show different radioactive profiles reflecting a varying activity of six to eight isoaccepting His-tRNA species. Quantitative differences in profile were observed for liver tRNA during pregnancy. Profiles of embryo and uterus, kidney, heart and muscle changes less, and that of brain did not change during pregnancy. The significance of these observations with respect to molecular differentiation of His-tRNAs during pregnancy is discussed.

Macromolecules and behavior II. Training induced alteration in leucine transfer RNA of goldfish brain

Goldfish were trained to swim upright after a polystyrene float had been attached ventrally. The effects of training on the amino acid acceptor activity and methylated albumin-kieselguhr column chromatography profile of several brain isoaccepting tRNA species were studied. Comparison of the kinetics of aminoacylation and chromatography profile of tRNAs from trained and control fish showed no differences for Arginyl-, glutamyl-, histidyl-, lysyl-, phenylalanyl-, seryl, threonyl- or valyl-tRNA. In contrast, training induced alterations in tRNA-Leu activity and activity profile. Training had no effect on brain amino acid pools or liver tRNA-Leu activity. Together, behavioral control experiments and plasma cortisol titers indicated that the training-induced increase in tRNA-Leu activity was not caused by stress, exertion, random disoriented swimming or the minor surgical procedures required. Rather, this activity change seems to correlate with some, as yet unidentified, aspect of behavioral training. The implications of these findings with respect to learning are discussed.