Herpes simplex virus ribonucleotide reductase: expression in Escherichia coli and purification to homogeneity of a tyrosyl free radical-containing, enzymatically active form of the 38-kilodalton subunit

Abstract

Infection of mammalian cells with herpes simplex virus (HSV) induces a virus-encoded ribonucleotide reductase which is different from the cellular enzyme. This essential viral enzyme consists of two nonidentical subunits of 140 and 38 kilodaltons (kDa) which have not previously been purified to homogeneity. The small subunit of ribonucleotide reductases from other species contains a tyrosyl free radical essential for activity. We have cloned the gene for the small subunit of HSV-1 ribonucleotide reductase into a tac expression plasmid vector. After transfection of Escherichia coli, expression of the 38-kDa protein was detected in immunoblots with a specific monoclonal antibody. About 30 micrograms of protein was produced per liter of bacterial culture. The 38-kDa protein was purified to homogeneity in an almost quantitative yield by immunoaffinity chromatography. It contained a tyrosyl free radical which gave a specific electron paramagnetic resonance spectrum identical to that we have observed in HSV-infected mammalian cells and clearly different from that produced by the E. coli and mammalian ribonucleotide reductases. The recombinant 38-kDa subunit had full activity when assayed in the presence of HSV-infected cell extracts deficient in the native 38-kDa subunit.