Characterization of Marek's disease virus insertion and deletion mutants that lack US1 (ICP22 homolog), US10, and/or US2 and neighboring short-component open reading frames.

We report the characterization of Marek's disease virus (MDV) strains having mutations in various genes that map to the unique short (US) region of the viral genome. A deletion mutant (GA delta 4.8lac) lacks 4.8 kbp of US region DNA, the deleted segment having been replaced by the lacZ gene of Escherichia coli. This deletion results in the loss of the MDV-encoded US1, US10, and US2 homologs of herpes simplex virus type 1, as well as three putative MDV-specific genes, Sorf1, Sorf2, and Sorf3. Two mutants containing lacZ insertions in the US1 and US10 genes have been constructed, and we have previously reported a US2lac insertion mutant (J. L. Cantello, A. S. Anderson, A. Francesconi, and R. W. Morgan, J. Virol. 65:1584-1588, 1991). The isolation of these mutants indicates that the relevant genes are not required for growth of MDV in chicken embryo fibroblasts. The mutants had early growth kinetics indistinguishable from those of their parent viruses; however, 5 to 7 days after being plated, the US1 insertion mutant (US1lac) and the GA delta 4.8lac deletion mutant showed a 5- to 10-fold decrease in virus growth. This decrease in virus accumulation correlated with a 30 to 50% decrease in plaquing efficiency when these viruses were plated onto established versus fresh chicken embryo fibroblast monolayers compared with a 10 to 15% decrease seen for the parent viruses and for the US10lac or US2lac insertion mutants. Finally, GA delta 4.8lac could be reisolated from chickens, indicating that the deleted genes are not required for the infection of chickens following intra-abdominal inoculation of an attenuated serotype 1 MDV.