Abstract
Aminoacyl tRNA synthetases (aaRS) are crucial enzymes that join amino acids to their cognate tRNAs, thereby implementing the genetic code. These enzymes fall into two unrelated structural classes whose evolution has not been explained. The leading hypothesis, proposed by Rodin and Ohno, is that the two classes originated as a pair of sense-antisense genes encoded on opposite strands of a single DNA molecule. This unusual idea obtained its main support from reports of a "Rosetta stone": a locus where genes for heat shock protein 70 (HSP70) and an Nicotinamide adenine dinulecotide-specific glutamate dehydrogenase (NAD-GDH), which are structurally homologous to the two classes of aaRS, overlap extensively on complementary DNA strands. This remarkable locus was first characterized in the oomycete Achlya klebsiana and has since been reported in many other species. Here we present evidence that the open reading frames on the antisense strand of HSP70 genes are spurious, and we identify a more probable candidate for the gene encoding the oomycete NAD-GDH enzyme. These results cast extensive doubt on the Rosetta Stone argument.
Highlights
Aminoacyl tRNA synthetases are the enzymes that join amino acids to their cognate tRNAs prior to translation, thereby implementing the genetic code
Aphanomyces euteiches expressed sequence tags (ESTs; sequenced by Gaulin et al 2008) homologous to the A. klebsiana heat shock protein 70 (HSP70)/antisense-open reading frames (ORFs) (AS-ORF) genomic locus were identified by BlastN searches at National Centre for Biotechnology Information (NCBI), using as a query the sequence of the whole A. klebsiana genomic locus (6,575 bp made by merging GenBank accession numbers U02504 and U02505; LeJohn, Cameron, Yang, and Rennie 1994)
The experimental evidence that the A. klebsiana AS-ORF codes for Nicotinamide adenine dinulecotide-specific glutamate dehydrogenase (NAD-GDH) hinges on the specificity of the polyclonal antibody used in these studies
Summary
Aminoacyl tRNA synthetases (aaRS) are the enzymes that join amino acids to their cognate tRNAs prior to translation, thereby implementing the genetic code. Their work was based on an earlier report that heat shock protein 70 (HSP70) and an Nicotinamide adenine dinulecotide-specific glutamate dehydrogenase (NAD-GDH) are encoded as a sense–antisense pair by a single DNA sequence in A. klebsiana (LeJohn, Cameron, Yang, and Rennie 1994). Because canonical dehydrogenases are structurally similar to Class I aaRS, and HSP70 has structural homology to Class II aaRS, Carter and Duax (2002) concluded that the A. klebsiana gene proved that such structurally divergent proteins could be encoded on opposite strands of the same DNA molecule. We present evidence that the antisense ORF is spurious, even though it is present in many species, and we identify a different oomycete gene as a more probable candidate for the locus encoding the NAD-GDH enzyme
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