Further analysis of Marek's disease virus horizontal transmission confirms that U_L44 (gC) and U_L13 protein kinase activity are essential, while U_S2 is nonessential

Marek's disease virus (MDV) causes a devastating disease in chickens characterized by the development of lymphoblastoid tumors in multiple organs and is transmitted from the skin of infected chickens. We have previously reported that the U_S2, U_L44 (glycoprotein C [gC]), and U _L13 genes are essential for horizontal transmission of MDV in gain-of-function studies using an a priori spread-deficient virus that was based on an infectious clone from the highly virulent RB-1B virus (pRB-1B). To precisely determine the importance of each individual gene in the process of chicken-to-chicken transmission, we used the transmission-restored clone that readily transmits horizontally and mutated each individual gene in loss-of-function experiments. Two independent U_S2-negative mutants transmitted horizontally, eliminating U_S2 as being essential for the process. In contrast, the absence of gC expression or mutating the invariant lysine essential for U_L13 kinase activity abolished horizontal spread of MDV between chickens.