Use of a bacterial expression vector to identify the gene encoding a major core protein of vaccinia virus

The DNA sequence of a vaccinia virus late gene contains an open reading frame that corresponds to the 28,000-dalton (28K) polypeptide made by in vitro translation of hybrid-selected mRNA. To further characterize the protein product of this late gene, we cloned a segment of DNA containing part of the open reading frame into a bacterial expression vector. The fusion protein produced from this vector, containing 151 amino acids of the predicted vaccinia virus protein, was used to immunize rabbits. The resulting antiserum specifically bound to a major 25K structural protein that is localized in the core of vaccinia virions, as well as to a 28K protein found in infected cells. Pulse-chase experiments indicated that the 25K core protein is originally made as a 28K precursor.