A SINGLE AMINO-ACID SUBSTITUTION IN THE LARGE SUBUNIT OF HERPES-SIMPLEX VIRUS TYPE-1 RIBONUCLEOTIDE REDUCTASE WHICH PREVENTS SUBUNIT ASSOCIATION

The herpes simplex virus type 1 temperature-sensitive (ts) mutant ts1207 does not induce detectable levels of ribonucleotide reductase activity at the non-permissive temperature (NPT, 39.5°C). The ts lesion prevents the association of the enzyme's large (RR1) and small (RR2) subunits to give an active holoenzyme and maps within the gene specifying RR1. Here, it is shown that the ts mutant phenotype is due to the substitution of an asparagine for the wild-type (wt) serine at RR1 position 961, which is located within a region highly conserved between herpesviral and cellular RR1 subunit polypeptides. This ts1207 asparagine is predicted to alter a wt α-helix to a β-strand. We have used synthetic oligopeptides, corresponding to the wt amino acid sequence of the mutation site, and antisera raised against them to determine whether this region is involved in subunit association. Neither the oligopeptides nor the antisera inhibit the enzyme activity, or the reconstituted activity formed by mixing intact RR2 and RR1 subunits present in partially purified extracts of cells infected at the NPT with ts1207 or ts1222 (an HSV-1 mutant with a lesion in the RR2 subunit), respectively. We infer from these results that the site of the mutation is unlikely to be positioned at the surface of RR1 and hence is probably not directly involved in subunit association. We suggest that the mutation site identifies an important RR1 region whose alteration in ts1207 changes the structure of a contact region(s) positioned at the RR1/RR2 interface.