RIBONUCLEOTIDE REDUCTASE IN THE ARCHAEON PYROCOCCUS-FURIOSUS - A CRITICAL ENZYME IN THE EVOLUTION OF DNA GENOMES

Ribonucleotide reductase (RNR), the enzyme responsible for deoxyribonu cleotide synthesis, has been isolated from Pyrococcus furiosus, a deep ly branching hyperthermophilic, strictly anaerobic archaeon. Its gene has been cloned, sequenced, and shown to harbor two insertions encodin g inteins, The purified enzyme absolutely requires adenosylcobalamin f or activity, a trait that defines it as a member of class II (adenosyl cobalamin-dependent) prokaryotic RNRs. On the other hand, the archaeal RNR has significant amino acid sequence homology with class I (aerobi c non-heme iron-dependent) and class III (anaerobic iron-sulfur depend ent) RNRs present in eukaryotes and bacteria, respectively. It is prop osed that this enzyme may be the closest possible relative of the orig inal RNR, which allowed the key ''RNA world'' to ''DNA world'' transit ion, and that the different classes of present-day RNRs are the produc ts of divergent evolution.