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Interference between viruses occurs when infection by one virus results in the inhibition of replication of another virus. Adeno-associated virus (AAV2) is a human parvovirus with the unique characteristics of a dependence upon a helper virus for a productive infection and the ability to interfere with the replica-tion of the helper virus. Previously, we demonstrated that AAV2 Rep78 and Rep52 interact and inhibit cAMP-dependent protein kinase A (PKA) and its novel homolog PrKX. We hypothesized that modula-tion of PKA activity by AAV2 may be responsible for inhibition of helper virus replication. In this study we demonstrate that adenovirus replication is sensitive to PKA activity and that AAV2 Rep78/Rep52 proteins contain an inhibitory domain similar to that of the heat-stable PKA inhibitor. This domain, while not dir-ectly necessary for AAV2 replication and packaging, is necessary to preserve AAV2 replication ®tness during an Ad co-infection. Furthermore, a mutant AAV2 virus lacking this region fails to inhibit adeno-virus replication. Thus, inhibition of PKA activity by AAV2 constitutes a novel form of viral interference