Identification of a novel translation factor necessary for the incorporation of selenocysteine into protein

Abstract

During the biosynthesis of selenoproteins in both prokaryotes and eukaryotes, selenocysteine is cotranslationally incorporated into the nascent polypeptide chain through a process directed by a UGA codon that normally functions as a stop codon. Recently, four genes have been identified whose products are required for selenocysteine incorporation in Escherichia coli. One of these genes, selC, codes for a novel transfer RNA species (tRNAUCA) that accepts serine and cotranslationally inserts selenocysteine by recognizing the specific UGA codon. The serine residue attached to this tRNA is converted to selenocysteine in a reaction dependent on functional selA and selD gene products. By contrast, the selB gene product (SELB) is not required until after selenocysteyl-tRNA biosynthesis. Here we present evidence indicating that SELB is a novel translation factor. The deduced amino-acid sequence of SELB exhibits extensive homology with the sequences of the translation initiation factor-2 (IF-2) and elongation factor Tu (EF-Tu). Furthermore, purified SELB protein binds guanine nucleotides in a 1:1 molar ratio and specifically complexes selenocysteyl-tRNAUCA, but does not interact with seryl-tRNAUCA. Thus, SELB could be an amino acid-specific elongation factor, replacing EF-Tu in a special translational step.

Diagnostic tool for the identification of MLL rearrangements including unknown partner genes

“…The second identified gene, SELB, encodes an elongation factor necessary for the incorporation of selenocysteine at certain UGA stop codons (31). However, the fusion between MLL intron 9 and SELB intron 1 produce per se disrupted ORFs and, thus, incompatible exon-exon fusions for both the MLL⅐SELB and the SELB⅐MLL fusion genes.…”

Identification of a protein component of a mammalian tRNASec complex implicated in the decoding of UGA as selenocysteine

“…As selenocysteine-containing proteins are found in all three kingdoms, to what extent are the mechanistic features inherent in one line of decent conserved in another? Current evidence supports a model in which the recognition of UGA as a selenocysteine codon, rather than a stop codon, is dependent upon the presence of mRNA secondary structures termed selenocysteine insertion sequence (SECIS) elements (Heider et al+, 1992; for review see Stadtman, 1996;Low & Berry, 1996;Atkins et al+, 1999)+ SECIS elements are RNA stemloop structures found immediately downstream of the selenocysteine-specific UGA codon in bacterial mRNAs, whereas, in mRNAs of eukaryotes and archaebacteria, SECIS elements are located in the 39 untranslated region, frequently at a considerable distance from the selenocysteine codon+ An outline of the biochemical pathway for selenocysteine insertion in bacteria has been elucidated by genetic and biochemical studies in which the products of the selA, sel B, sel C, and sel D genes are shown to be required for the insertion of selenocysteine at the di-rection of SECIS elements (Leinfelder et al+, 1988a(Leinfelder et al+, , 1988b(Leinfelder et al+, , 1989Forchhammer et al+, 1989Forchhammer et al+, , 1990)+ A key feature of this pathway involves the function of a specialized selenocysteine tRNA (tRNA Sec ; product of the sel C gene), which is initially charged with serine and subsequently converted to selenocysteyl-tRNA Sec (Baron & Böck, 1995)+ In correspondence to these factors, only tRNA Sec and the selenophosphate synthetase gene (homolog of sel D) have been identified in eukaryotes (Lee et al+, 1989;Low et al+, 1995)+ An important unanswered question is how, for eukaryotes, do the SECIS elements in the 39 untranslated region signal, at a distance, the cotranslational incorporation of selenocysteine at UGA codons? In particular, an understanding of the molecular link between tRNA Sec and SECIS RNA elements has remained elusive, in part, because the eukaryotic homolog of SELB has not yet been identified+ In a model proposed by Ringquist et al+ (1994) it is held that SELB delivers bacterial selenocysteyl-tRNA Sec to a ribosome-SECIS RNA complex+ This model implicitly accounts for the recognition of a selenocysteine codon by coupling the required SECIS RNA recognition event to the delivery of the charged tRNA+ In eukaryotes, the known elongation factor, EF-1a, fails to deliver selenocysteyltRNA Sec to the ribosome, and a partially characterized factor is implicated for this role (Jung et al+, 1994;Yamada, 1995)+ Recent progress has been made in the identification and cloning of a mammalian protein, dbpB, which recognizes SECIS RNA elements (Shen et al+, 1998)+ Nonetheless, a variety of proteins that have been detected in association with mammalian SECIS RNA elements, or by autoimmune antibodies that recognize tRNA Secprotein complexes, have yet to be fully characterized …”

Discrimination against misacylated tRNA by chloroplast elongation factor Tu

“…It is bound by E. coli EF-Tu only with dissociation constants comparable to the initiator tRNAfMe' [36], probably due to the unusual structure of tRNASe" which differs from the consensus tRNA structure by an unusually long acceptor stem [37]. Only after conversion to selenocysteinyl-tRNASec does binding occur by a specialized translation factor, the seZB gene product [38], which is thus able to distinguish between two different amino acid side chains bound to the same tRNA. Thus, as for chloroplast EF-Tu, a mechanism has evolved allowing the biochemical conversion of a tRNA-bound amino acid and still ensuring the fidelity required in protein biosynthesis.…”