The role of host protein YajQ in the temporal control of transcription in bacteriophage Phi6.

Bacteriophage Phi6 contains three dsRNA genomic segments L, M, and S. The RNA is located inside a core particle composed of multiple copies of a major structural protein, an RNA-dependent RNA polymerase, a hexameric NTPase, and an auxiliary protein. The virion RNA polymerase in the core particle transcribes segments M and S in vitro. Yet early in infection, the transcription of L is highly active. Late in infection, transcription of L is low, and that of M and S is high. A host protein encoded by yajQ is responsible for the activation of L transcription. Knockout mutants of yajQ do not support the replication of Phi6, although they do support the replication of distantly related members of the Cystoviridae. Phi6 can mutate to independence of YajQ. This requires two mutations in the gene for the RNA-dependent RNA polymerase. YajQ acts indirectly on the polymerase by binding to P1, the major structural protein of the core. Previous studies have shown that the activity of the polymerase in the core is controlled by the conformation of the core particle structure.