The phosphoprotein (P) of vesicular stomatitis virus (VSV) is an essential subunit of the viral RNAdependent RNA polymerase (RdRp) complex. It is phosphorylated at two different domains. Using defective interfering (DI) RNA or minigenomic RNA templates, we previously demonstrated that phosphorylation within the amino-terminal domain I is essential for transcription, whereas phosphorylation within the carboxyterminal domain II is necessary for replication. For the present study, we examined the role of the phosphorylation of residues in these domains in the life cycle of VSV. Various mutant P coding sequences were inserted into a full-length cDNA clone of VSV, and the virus recovery, kinetics of growth, and mRNA and protein synthesis were examined. We observed that virus recovery was completely abolished when all three phosphate acceptor sites in domain I or both sites in domain II were replaced with alanine. Single or double mutations in domain I (with the exception of P60/64) or single mutations in domain II had no adverse effect on virus recovery. VSVP227, carrying alanine at position 227, showed reduced kinetics of virus growth but increased kinetics of viral mRNA synthesis in infected cells. More interestingly, this particular virus exhibited a significantly reduced cytopathic effects and apoptosis in infected cells, implying that P may be involved in these processes. Furthermore, we found that DI RNAs of different sizes were generated by high-multiplicity passaging of various mutant VSVs, indicating that the viral RdRp may play a significant role in the process of DI particle generation. Taken together, our results suggest that the phosphorylation of residues in domains I and II of VSV P is indispensable for virus growth.Vesicular stomatitis virus (VSV) is the prototypic rhabdovirus and has a negative-stranded RNA genome of 11,161 nucleotides (50). Within the virus and in infected cells, the genomic RNA is tightly encapsidated by the nucleocapsid (N) protein, forming the viral nucleocapsid, which serves as the template for transcription and replication by the associated viral RNA-dependent RNA polymerase (RdRp) (50). The RdRp is a complex that contains the virally encoded large (L) protein and the phosphoprotein (P protein). While the L protein contains the catalytic center for the polymerization of nucleotides, the P protein serves as an essential subunit of the RdRp. The P protein is multifunctional: in addition to playing a major role in polymerase functions, it binds to the L protein and stabilizes it from proteolytic degradation (5, 12), it complexes with the newly synthesized N protein for the efficient encapsidation of nascent RNA (9, 39, 48), and it interacts with terminal sequences of the viral genome for viral RNA synthesis (27, 29).Through mutational and biochemical studies, three functionally homologous domains have been identified in the P proteins (Fig. 1A) of both the Indiana (P I ) and New Jersey (P NJ ) serotypes of VSV (20,47). The amino-terminal domain I (residues 1 to approximately 150) ...