Deoxyribonucleotide Synthesis in Phycovirus-Infected Green Algae. A New Virus-Induced Ribonucleotide Reductase

Infection o f Chlorella-like green algae with freshwater phycoviruses is associated with a large and rapid de­ mand for D N A precursors which cannot be met by the algal deoxyribonucleotide-synthesizing enzymes. We have demonstrated in these cells an up to ten-fold in­ crease o f the key enzyme, ribonucleotide reductase, 1 —2 h post infection. The enzyme activity has been par­ tially enriched from cell extracts. In vitro, it differs from that o f uninfected algae in three characteristic parame­ ters, viz. eight-fold higher resistance to millimolar hy­ droxyurea concentrations, much higher optimum con­ centration o f an allosteric effector nucleotide, thymidine triphosphate, and an unusually low temperature opti­ mum at 20 °C. We conclude that the large D N A phyco­ viruses, like Herpes and pox viruses, code for their own specific ribonucleotide reductase.