Regional localization of the selenocysteine tRNA gene (TRSP) on human chromosome 19

Selenoproteins: molecular pathways and physiological roles

Selenium is an essential micronutrient with important functions in human health and relevance to several pathophysiological conditions. The biological effects of selenium are largely mediated by selenium-containing proteins (selenoproteins) that are present in all three domains of life. Although selenoproteins represent diverse molecular pathways and biological functions, all these proteins contain at least one selenocysteine (Sec), a selenium-containing amino acid, and most serve oxidoreductase functions. Sec is cotranslationally inserted into nascent polypeptide chains in response to the UGA codon, whose normal function is to terminate translation. To decode UGA as Sec, organisms evolved the Sec insertion machinery that allows incorporation of this amino acid at specific UGA codons in a process requiring a cis-acting Sec insertion sequence (SECIS) element. Although the basic mechanisms of Sec synthesis and insertion into proteins in both prokaryotes and eukaryotes have been studied in great detail, the identity and functions of many selenoproteins remain largely unknown. In the last decade, there has been significant progress in characterizing selenoproteins and selenoproteomes and understanding their physiological functions. We discuss current knowledge about how these unique proteins perform their functions at the molecular level and highlight new insights into the roles that selenoproteins play in human health.

Early embryonic lethality caused by targeted disruption of the mouse selenocysteine tRNA gene (Trsp)

Selenoprotein biosynthesis is mediated by tRNASec, which inserts selenocysteine at UGA codons in a complex, context-specific manner. This opal suppressor serves in the conversion of serine to selenocysteine as well. The mouse tRNASec gene (Trsp) maps to a proximal segment of chromosome 7. We constructed mice carrying a targeted deletion of the Trsp gene. The heterozygous mutants were viable, fertile, and appeared normal. Although the level of tRNASec was reduced to about 50%-80% of the wild type in most organs, one of the selenoproteins, glutathione peroxidase, remained unaffected in the levels of its mRNA, protein, and enzyme activity, indicating that the haploid amount of tRNASec is not limiting in its biosynthesis. In contrast, the homozygous mutants died shortly after implantation, and the embryos were resorbed before 6.5 days post coitum. When the preimplantation embryos were placed in culture, however, the trophoectoderm cells showed outgrowths and the inner cell mass cells of the homozygous embryos were able to proliferate. These results indicate that Trsp expression is essential for early development of the embryo, and its lack causes peri-implantation lethality. However, the lethality does not appear to be due to a cell-autonomous function of tRNASec.

Cloning, structural analysis and mapping of the mouse selenocysteine tRNA([Ser]Sec) gene (Trsp)

The tRNA([Ser]Sec) molecule mediates the synthesis of selenoproteins by incorporating selenocysteine into specific UGA codons upon translation of mRNAs that encode selenocysteine-containing proteins. The mouse gene encoding tRNA([Ser]Sec) (Trsp) was isolated from a genomic library and sequenced. The mouse sequence is colinear with its tRNA product, and contains a C to T transition relative to the homologous genes in other vertebrates except rat. Transcriptional control motifs found 5' to the tRNA coding region included a TATA element, a PSE element and an SPH motif which is associated with an octamer motif. A Northern hybridization analysis showed highest expression in the testis, followed by thymus, spleen, kidney, ovary, brain, liver, heart and skeletal muscle. Surprisingly, the expression level was lowest in embryonic stem cells. These results suggest a tissue-specific transcriptional control. Using restriction fragment length variants (RFLVs) in interspecific backcross mice between Mus musculus (C3H strain) and Mus spretus, the Trsp gene was mapped to the proximal region of mouse Chr 7, cosegregating with octamer-binding transcription factor-2 (Otf2).

A human DNA segment encompassing leucine and methionine tRNA pseudogenes localized on chromosome 6

A human genomic clone, designated LHtlm8, that strongly hybridized to a mammalian leucine tRNA(IAG) probe, was found to encompass a pair of tRNA pseudogenes that are transcribed in a homologous cell extract. A leucine tRNA(AAG) pseudogene (TRLP1) is 2.1-kb upstream and of opposite polarity to a methionine elongator tRNA(CAU) pseudogene (TRMEP1). TRLP1 has three nucleotide variations (97% identity) from its cognate leucine tRNA(IAG), while TRMEP1 has a 78% identity with its cognate tRNA. Similar to a number of other eukaryotic tRNA pseudogenes, presumptive precursor tRNA transcripts are generated from the two pseudogenes in vitro, but possibly due to their aberrant and unstable secondary and tertiary structures, no detectable mature tRNA products are observed. The two tRNA pseudogenes are encompassed within a 9.6-kb EcoRI fragment that has been assigned to the chromosomal locus, 6pter-q13, by Southern blot hybridization of human-rodent somatic cell hybrid DNAs with probes derived from the cloned tRNA pseudogenes and flanking sequences. A 4.4-kb EcoRI fragment also harbored in clone LHtlm8 was mapped to human chromosome 11, suggesting that the two EcoRI fragments were inadvertantly ligated together during construction of the genomic library.