Recognition of UGA as a selenocysteine codon in type I deiodinase requires sequences in the 3' untranslated region.

Selenocysteine is incorporated cotranslationally at UGA codons, normally read as stop codons, in several bacterial proteins and in the mammalian proteins glutathione peroxidase (GPX), selenoprotein P and Type I iodothyronine 5' deiodinase (5'DI). Previous analyses in bacteria have suggested that a stem-loop structure involving the UGA codon and adjacent sequences is necessary and sufficient for selenocysteine incorporation into formate dehydrogenase and glycine reductase. We used the recently cloned 5'DI to investigate selenoprotein synthesis in eukaryotes. We show that successful incorporation of selenocysteine into this enzyme requires a specific 3' untranslated (3'ut) segment of about 200 nucleotides, which is found in both rat and human 5'DI messenger RNAs. These sequences are not required for expression of a cysteine-mutant deiodinase. Although there is little primary sequence similarity between the 3'ut regions of these mRNAs and those encoding GPX, the 3'ut sequences of rat GPX can substitute for the 5'DI sequences in directing selenocysteine insertion. Computer analyses predict similar stem-loop structures in the 3'ut regions of the 5'DI and GPX mRNAs. Limited mutations in these structures reduce or eliminate their capacity to permit 5'DI translation. These results identify a 'selenocysteine-insertion sequence' motif in the 3'ut region of these mRNAs that is essential for successful translation of 5'DI, presumably GPX, and possibly other eukaryotic selenocysteine-containing proteins.     

Bile acids induce energy expenditure by promoting intracellular thyroid hormone activation

While bile acids (BAs) have long been known to be essential in dietary lipid absorption and cholesterol catabolism, in recent years an important role for BAs as signalling molecules has emerged. BAs activate mitogen-activated protein kinase pathways, are ligands for the G-protein-coupled receptor (GPCR) TGR5 and activate nuclear hormone receptors such as farnesoid X receptor alpha (FXR-alpha; NR1H4). FXR-alpha regulates the enterohepatic recycling and biosynthesis of BAs by controlling the expression of genes such as the short heterodimer partner (SHP; NR0B2) that inhibits the activity of other nuclear receptors. The FXR-alpha-mediated SHP induction also underlies the downregulation of the hepatic fatty acid and triglyceride biosynthesis and very-low-density lipoprotein production mediated by sterol-regulatory-element-binding protein 1c. This indicates that BAs might be able to function beyond the control of BA homeostasis as general metabolic integrators. Here we show that the administration of BAs to mice increases energy expenditure in brown adipose tissue, preventing obesity and resistance to insulin. This novel metabolic effect of BAs is critically dependent on induction of the cyclic-AMP-dependent thyroid hormone activating enzyme type 2 iodothyronine deiodinase (D2) because it is lost in D2-/- mice. Treatment of brown adipocytes and human skeletal myocytes with BA increases D2 activity and oxygen consumption. These effects are independent of FXR-alpha, and instead are mediated by increased cAMP production that stems from the binding of BAs with the G-protein-coupled receptor TGR5. In both rodents and humans, the most thermogenically important tissues are specifically targeted by this mechanism because they coexpress D2 and TGR5. The BA-TGR5-cAMP-D2 signalling pathway is therefore a crucial mechanism for fine-tuning energy homeostasis that can be targeted to improve metabolic control.     

Low pressure storage of seeds

Summary Storage of celery, cabbage and onion seed for 14 weeks under conditions of reduced atmospheric pressure decreased the deleterious effects of high temperature and high relative humidity upon the germination of these short-lived seeds.Acknowledgments. The authors gratefully acknowledge the financial support of the National Research Council of Canada.