The length of the aminoacyl-acceptor stem of the selenocysteine-specific tRNA(Sec) of Escherichia coli is the determinant for binding to elongation factors SELB or Tu.
Mutations in selC, which reduce the 8-base pair aminoacyl-acceptor helix to the canonical 7-base pair length (tRNA(Sec)(delAc] or which replace the extra arm of tRNA(Sec) by that of a serine acceptor tRNA species (tRNA(Sec)(ExS), block the function in selenoprotein synthesis in vivo (Baron, C., Heider, J., and Böck, A. (1990) Nucleic Acids Res. 18, 6761-6766). tRNA(Sec), tRNA(Sec)(delAc), and tRNA(Sec)(ExS) were purified and analyzed for their interaction with purified seryl-tRNA synthetase, selenocysteine synthase and translation factors SELB and EF-Tu. It was found that seryl-tRNA synthetase displays 10-fold impaired Km and Kcat values for tRNA(Sec) in comparison to tRNA(Ser), decreasing the overall charging efficiency (Kcat/Km) of tRNA(Sec) to 1% of that characteristic for tRNA(Ser). tRNA(Sec)(ExS) was a less efficient substrate for the enzyme (Kcat/Km 0.2% of the tRNA(Ser) value) whereas the tRNA(Ser)(delAc) variant was charged with an approximately 2-3-fold improved rate compared to wild-type tRNA(Sec). Both mutant tRNA variants, when charged with L-serine, were able to interact with selenocysteine synthase to give rise to selenocysteyl-tRNA with tRNA(Sec)(ExS) being as efficient as wild-type tRNA(Sec). Seryl-tRNA(Sec)(delAc), on the other hand, was selenylated very slowly. Reduction of the length of the aminoacyl-acceptor stem to 7 base pairs prevented the interaction with translation factor SELB but allowed binding to EF-Tu, irrespective of whether tRNA(Sec)(delAc) was charged with serine or selenocysteine. The aminoacyl-acceptor helix of tRNA(Sec), therefore, is a major determinant directing binding to SELB and precluding interaction with EF-Tu.
Baron
C
C
Lehrstuhl für Mikrobiologie, Universität Munich, Federal Republic of Germany.
Böck
A
A
D001426:Bacterial Proteins / Q000378:metabolism*; D001483:Base Sequence; D002855:Chromatography, Thin Layer; D004926:Escherichia coli / Q000378:metabolism*; D005796:Genes; D006868:Hydrolysis; D008969:Molecular Sequence Data; D009154:Mutation; D009690:Nucleic Acid Conformation; D010444:Peptide Elongation Factor Tu / Q000378:metabolism*; D010445:Peptide Elongation Factors / Q000378:metabolism*; D010957:Plasmids; D012333:RNA, Messenger / Q000235:genetics; D012345:RNA, Transfer, Amino Acid-Specific / Q000235:genetics / Q000378:metabolism*; D012260:Ribonucleases; D014166:Transferases / Q000378:metabolism
D001426:Bacterial Proteins; D010445:Peptide Elongation Factors; D012333:RNA, Messenger; D012345:RNA, Transfer, Amino Acid-Specific; C061878:SelB protein, Bacteria; C036928:tRNA, selenocysteine-; D014166:Transferases; C060600:selenium transferase; D012260:Ribonucleases; D010444:Peptide Elongation Factor Tu