Biosynthesis of selenobases in transfer RNA by Escherichia coli

Biosynthesis of 5-methylaminomethyl-2-selenouridine, a naturally occurring nucleoside in Escherichia coli tRNA

A selenium-containing nucleoside, 5-methylaminomethyl-2-selenouridine (mnm5se2U), is present in lysine- and glutamate-isoaccepting tRNA species of Escherichia coli. The synthesis of mnm5se2U is optimum (4 mol/100 mol tRNA) when selenium is present at about 1 microM concentration and is neither decreased by a high (8 mM) level of sulfur in the medium nor increased by excessive (10 or 100 microM) levels of selenium. Lysine- and glutamate-isoaccepting tRNA species that contain 5-methylaminomethyl-2-thiouridine (mnm5s2U) coexist with the seleno-tRNAs in E. coli cells and a reciprocal relationship between the mnm5se2U- and the mnm5s2U-containing species is maintained under a variety of growth conditions. The complete 5-methylaminomethyl side chain is not a prerequisite for introduction of selenium at the 2-position. In E. coli mutants deficient in the ability to synthesize the 5-methylaminomethyl substituent, both the 2-thiouridine and the corresponding 2-selenouridine derivatives of intermediate forms are accumulated. Broken cell preparations of E. coli synthesize mnm5se2U in tRNAs by an ATP-dependent process that appears to involve the replacement of sulfur in mnm5s2U with selenium.

Selenium-containing tRNAs from Clostridium sticklandii. Cochromatography of seleno-tRNA I with L-VALYL-tRNA

It has been previously shown that Clostridium sticklandii specifically synthesized three readily separable 75Se-labeled tRNAs, designated seleno-tRNAs I, II and III, and the partially purified seleno-tRNA II cochromatographed with L-prolyl-tRNA on DEAE-Sephadex A-50 (Chen, C.S. and Stadtman, T.C. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 1403-1407). In the present study a highly purified 75Se-labeled tRNA I was obtained by chromatography on benzoylated DEAE-cellulose, DEAE-Sephadex A-50 and Sepharose 4B. The 75Se-labeled tRNA I cochromatographed with an L-valine-accepting species on DEAE-Sephadex A-50 and Sepharose 4B. Addition of a 285-fold molar excess of unlabeled L-valine to the L-valine acceptor activity assay mixture markedly decreased the amount of L-[14C]valine bound to seleno-tRNA I.

Selenium-containing tRNAs from Clostridium sticklandii: cochromatography of one species with L-prolyl-tRNA

75Se-Labeled tRNAs were synthesized by Clostridium sticklandii cultures supplemented with 1 microM sodium [75Se]selenite or [75Se]selenocysteine. This process is highly specific for selenium; it occurred in the presence of 1.2 mM sodium sulfide and was not decreased by the further addition of a 500-fold molar excess of cysteine. The 75Se in these tRNAs was located in the polynucleotide portion of the molecules and not in esterified (alkali-labile) selenocysteine. Inhibition of cell multiplication by antibiotics that block either protein synthesis or DNA-dependent RNA synthesis did not prevent this 75Se incorporation. Three [75Se]tRNAs were separated from C. sticklandii cells labeled in the presence of chloramphenicol and were partially purified by chromatography on benzoylated DEAE-cellulose and DEAE-Sephadex A-50 columns. These were designated seleno-tRNAs I, II, and III according to their elution sequence from benzoylated DEAE-cellulose. Cochromatography of purified seleno-tRNA II on DEAE-Sephadex A-50 with an L-proline-accepting species suggests that it is a selenium-containing L-prolyl-tRNA.

Selenium biochemistry

The toxicity of selenium to animals and plants has been known and extensively documented since the 1930's, but it is only during the past 15 years that selenium has also been shown to be an essential micronutrient for animals and bacteria. Very little is known about the specific role or roles of selenium and, to date, there are only three enzyme-catalyzed reactions that have been shown to require the participation of a selenium-containing protein. These are the reactions catalyzed by (i) formate dehydrogenase of bacteria, (ii) glycine reductase of clostridia, and (iii) glutathione peroxidase of erythrocytes. The common denominator of these selenium-dependent processes is that they are all oxidation-reduction reactions. A fourth selenoprotein has been isolated from skeletal muscle of sheep but its catalytic function has not been identified. The form in which selenium occurs in these selenoproteins is unknown. The selenoprotein of clostridial glycine reductase contains selenium in a covalently bound form. Studies in progress indicate that this may be an organoselenium compound not previously detected in nature. Identification of the chemical nature of selenium in proteins participating in electron transport processes should enable us to determine its specific role and to understand the basic defects in certain cardiac and skeletal muscle degenerative diseases which are selenium-deficiency syndromes. The greater availability and ease of isolation of the selenoprotein of the bacterial glycine reductase system makes this the biological material of choice for studies on the mechanism of action of selenium. An added attractive feature of this system is that it can conserve the energy made available by the reductive deamination of glycine in a biologically useful form by synthesizing ATP.