The gene encoding the late nonstructural 36K protein of vaccinia virus is essential for virus reproduction.

Two genetic markers--the thymidine kinase gene of herpes simplex virus, and the beta-galactosidase gene of Escherichia coli--were incorporated into the 36K protein gene (IL1 gene according to the nomenclature of the Copenhagen strain of vaccinia virus; Goebel et al., 1990) from the HindIII-P DNA fragment of the LIVP strain (variant of Lister strain) of vaccinia virus (VV). After recombination of the obtained integration plasmid pVZ64-TK with the VV genome (tk-), it was found that the resultant TK+ viruses were unstable with respect to the Lac+ phenotype. On the basis of hybridization of DNA fragments of selected clones, a scheme for the formation of hybrid viruses is proposed, and an approach to a simple phenotypical discrimination between essential and non-essential genes for VV viability is described.