A vaccinia virus isatin-beta-thiosemicarbazone resistance mutation maps in the viral gene encoding the 132-kDa subunit of RNA polymerase.

The mutation in a vaccinia virus mutant resistant to inhibition by isatin-beta-thiosemicarbazone was mapped by marker rescue. DNA from the resistant mutant was cloned into cosmid and plasmid vectors and tested for its ability to convert wild-type vaccinia virus to IBT resistant virus in a helper-mediated marker rescue protocol. Resistance was mapped in this way to a 0.9-kb DNA fragment derived from the HindIII A fragment of vaccinia genome. Southern blot hybridization using this DNA as a probe demonstrated that the 0.9-kb fragment is contained within the DNA sequence encoding the second largest subunit of vaccinia RNA polymerase, rpo132. Thus, mutation of rpo132 can cause resistance to IBT in vaccinia virus.