Physical mapping and DNA sequence analysis of the rifampicin resistance locus in vaccinia virus

Rifampicin has been shown to inhibit the maturation of poxviruses at a discrete step in envelope formation (Moss et al., 1969; Pennington et al., 1970; Nagayama et al., 1970; Grimley et al., 1970). A rifampicin-resistant vaccinia virus mutant (RifR) was selected for its ability to grow in the presence of 100 micrograms/ml of rifampicin. Utilizing intact DNA or endonuclease restricted cloned DNA subfragments derived from the RifR mutant virus, the locus specifying rifampicin resistance was physically mapped by marker rescue analysis leftward of the unique XhoI site within the HindIII D fragment. DNA sequencing of a 445 bp fragment encompassing this region revealed an AT to GC transition when compared with the equivalent wild-type DNA fragment. Analysis of the six potential open reading frames within the 445-bp fragment indicated only one available open reading frame. On this basis, the rifampicin-resistant vaccinia virus mutant was shown to have a codon transition from asparagine to aspartic acid.