Antiviral activity of distamycin A against vaccinia virus is the result of inhibition of postreplicative mRNA synthesis.

Distamycin A has been described as an inhibitor of the cellular pathogenesis of vaccinia virus in culture. Distamycin is an antibiotic that specifically targets the minor groove of DNA. We show here that distamycin is a potent inhibitor of vaccinia virus replication. Pulse-labeling experiments showed that most major late proteins failed to accumulate in the presence of the antibiotic. We characterized the effect of distamycin on vaccinia virus nucleic acid biosynthesis with the goal of determining the inhibitor's target. Early gene transcription was unaffected. DNA synthesis proceeded at normal rates, but DNA accumulated in large masses in the cytoplasm with no evidence of virion assembly. Transcription from the intermediate class promoter for the I1L gene was partially reduced by distamycin; however, transcription from the intermediate promoters for the three late transcription factor genes was severely inhibited. The accumulation of the late transcripts for the viral F17R and A10L genes also was severely impaired and was shown to be a direct inhibition of late promoter activity. These results indicate that inhibition of postreplicative intermediate and late transcription is the basis for inhibition of vaccinia virus by distamycin and indicate that DNA minor-groove ligands hold promise for effective anti-poxvirus drugs.