Complementation of a vaccinia virus host-range K1L gene deletion by the nonhomologous CP77 gene

We investigated the host-range restriction of a vaccinia virus (VV) K1L deletion mutant in rabbit kidney RK13 cells and the ability of the nonhomologous cowpox virus CP77 gene to overcome this block. Viral early mRNAs were made by K1L- VV but early protein synthesis was arrested consistent with a translational block. Replication of viral DNA did not occur and neither intermediate nor late mRNAs or proteins were detected. These results indicated that host-range restriction occurs earlier in RK13 cells than in Chinese hamster ovary cells (CHO) cells infected with CP77- VV, where the block occurs at translation of intermediate stage mRNA. We confirmed a report (Perkus et al., Virology 179, 276-286, 1990) that the CP77 gene, which allows VV replication in CHO cells, could replace the K1L gene for plaque formation in RK13 cells. However, the size of the plaques formed by K1L-CP77+ VV was smaller than those formed by K1L+CP77- VV. Single-step growth curves also showed that the CP77 gene could functionally replace the K1L gene, although formation of infectious virus was delayed and did not reach the same level as that of K1L+ VV. Most surprisingly, the dramatic shutoff of viral and host gene expression was similar in RK13 cells infected with K1L-CP77- VV and K1L-CP77+ VV and little difference was noted for the first 6 hr. Subsequently, in cells infected with the K1L-CP77+ VV, viral early protein synthesis was spontaneously resurrected and the replication cycle proceeded. Despite the absence of homology, K1L and CP77 gene products appear to be acting in a common virus/cell interaction pathway.