The trace element selenium is found in proteins as selenocysteine (Sec), the 21st amino acid to participate in ribosome-mediated translation. The substrate for ribosomal protein synthesis is selenocysteinyl-tRNA Sec . Its biosynthesis from seryl-tRNA Sec has been established for bacteria, but the mechanism of conversion from Ser-tRNA Sec remained unresolved for archaea and eukarya. Here, we provide evidence for a different route present in these domains of life that requires the tRNA Sec -dependent conversion of O-phosphoserine (Sep) to Sec. In this two-step pathway, O-phosphoseryl-tRNA Sec kinase (PSTK) converts Ser-tRNA Sec to SeptRNA Sec . This misacylated tRNA is the obligatory precursor for a Sep-tRNA:Sec-tRNA synthase (SepSecS); this protein was previously annotated as SLA/LP. The human and archaeal SepSecS genes complement in vivo an Escherichia coli Sec synthase (SelA) deletion strain. Furthermore, purified recombinant SepSecS converts SeptRNA Sec into Sec-tRNA Sec in vitro in the presence of sodium selenite and purified recombinant E. coli selenophosphate synthetase (SelD). Phylogenetic arguments suggest that Sec decoding was present in the last universal common ancestor. SepSecS and PSTK coevolved with the archaeal and eukaryotic lineages, but the history of PSTK is marked by several horizontal gene transfer events, including transfer to non-Sec-decoding Cyanobacteria and fungi.aminoacyl-tRNA ͉ evolution ͉ formate dehydrogenase ͉ pyridoxal phosphate
The amide aminoacyl-tRNAs, Gln-tRNA Gln and Asn-tRNA Asn , are formed in many bacteria by a pretranslational tRNAdependent amidation of the mischarged tRNA species, Glu-tRNA Gln or Asp-tRNA Asn . This conversion is catalyzed by a heterotrimeric amidotransferase GatCAB in the presence of ATP and an amide donor (Gln or Asn). Helicobacter pylori has a single GatCAB enzyme required in vivo for both Gln-tRNA Gln and Asn-tRNA Asn synthesis. In vitro characterization reveals that the enzyme transamidates Asp-tRNA Asn and Glu-tRNA Gln with similar efficiency (k cat /K m of 1368.4 s ؊1 /mM and 3059.3 s ؊1 /mM respectively). The essential glutaminase activity of the enzyme is a property of the A-subunit, which displays the characteristic amidase signature sequence. Mutations of the GatA catalytic triad residues (Lys 52 , Ser 128 , Ser 152 ) abolished glutaminase activity and consequently the amidotransferase activity with glutamine as the amide donor. However, the latter activity was rescued when the mutant enzymes were presented with ammonium chloride. The presence of Asp-tRNA Asn and ATP enhances the glutaminase activity about 22-fold. H. pylori GatCAB uses the amide donor glutamine 129-fold more efficiently than asparagine, suggesting that GatCAB is a glutamine-dependent amidotransferase much like the unrelated asparagine synthetase B. Genomic analysis suggests that most bacteria synthesize asparagine in a glutamine-dependent manner, either by a tRNA-dependent or in a tRNA-independent route. However, all known bacteria that contain asparagine synthetase A form Asn-tRNA Asn by direct acylation catalyzed by asparaginyl-tRNA synthetase. Therefore, bacterial amide aminoacyl-tRNA formation is intimately tied to amide amino acid metabolism.
The potential of early interventions for dementia has increased interest in cognitive impairments less severe than dementia. However, predictors of the trajectory from intact cognition to dementia have not yet been clearly identified. The purpose of this study was to determine whether known risk factors for dementia increased the risk of mild cognitive impairments or progression from mild cognitive impairments to dementia. A polytomous logistic regression model was used, with parameters governing transitions within transient states (intact cognition, mild cognitive impairments, global impairment) estimated separately from parameters governing the transition from transient to absorbing state (dementia or death). Analyses were based on seven annual examinations (1991-2002) of 470 Nun Study participants aged > or = 75 years at baseline and living in the United States. Odds of developing dementia increased with age primarily for those with low educational levels. In these women, presence of an apolipoprotein E gene *E4 allele increased the odds more than fourfold by age 95 years. Age, education, and the apolipoprotein E gene were all significantly associated with mild cognitive impairments. Only age, however, was associated with progression to dementia. Thus, risk factors for dementia may operate primarily by predisposing individuals to develop mild cognitive impairments; subsequent progression to dementia then depends on only time and competing mortality.
Glutamyl-tRNA synthetases (GluRSs) occur in two types, the discriminating and the nondiscriminating enzymes. They differ in their choice of substrates and use either tRNA Glu or both tRNA Glu and tRNA Gln . Although most organisms encode only one GluRS, a number of bacteria encode two different GluRS proteins; yet, the tRNA specificity of these enzymes and the reason for such gene duplications are unknown. A database search revealed duplicated GluRS genes in >20 bacterial species, suggesting that this phenomenon is not unusual in the bacterial domain. To determine the tRNA preferences of GluRS, we chose the duplicated enzyme sets from Helicobacter pylori and Acidithiobacillus ferrooxidans. H. pylori contains one tRNA Glu and one tRNA Gln species, whereas A. ferrooxidans possesses two of each. We show that the duplicated GluRS proteins are enzyme pairs with complementary tRNA specificities. The H. pylori GluRS1 acylated only tRNA Glu , whereas GluRS2 was specific solely for tRNA Gln . The A. ferrooxidans GluRS2 preferentially charged tRNA UUG Gln . Conversely, A. ferrooxidans
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