A NET -RESISTANT HERPES-SIMPLEX VIRUS MUTANT(1 FRAMESHIFT PERMITS SYNTHESIS OF THYMIDINE KINASE FROM A DRUG)

Clinical resistance to antiviral drugs requires that a virus evade dru g therapy yet retain pathogenicity. Thymidine kinase (TK)-negative mut ants of herpes simplex virus ace resistant to the drug, acyclovir, but are attenuated for pathogenicity in animal models. However, numerous cases of clinical resistance to acyclovir have been associated with vi ruses that were reported to express no TK activity. We studied an acyc lovir-resistant clinical mutant that contains a single-base insertion in its fk gene, predicting the synthesis of a truncated TK polypeptide with no TK activity. Nevertheless, the mutant retained some TK activi ty and the ability to reactivate from latent infections of mouse trige minal ganglia. The mutant expressed both the predicted truncated polyp eptide and a low level of a polypeptide that comigrated with full-leng th TK on polyacrylamide gels and reacted with anti-TK antiserum, provi ding evidence for a frameshifting mechanism. In vitro transcription an d translation of mutant fk genes, including constructs in which report er epitopes could be expressed only if frameshifting occurred, also ga ve rise to truncated and full-length polypeptides. Reverse transcripta se-polymerase chain reaction analysis coupled with open reading frame cloning failed to detect alterations in tk transcripts that could acco unt for the synthesis of full-length polypeptide. Thus, synthesis of f ull-length TK was due to an unusual net +1 frameshift during translati on, a phenomenon hitherto confined in eukaryotic cells to certain RNA viruses and retrotransposons. Utilization of cellular frameshifting me chanisms may permit an otherwise TK-negative virus to exhibit clinical acyclovir resistance.