The template specificity of bacteriophage Phi6 RNA polymerase.

Bacteriophage Φ6 contains three double-stranded RNA (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. Segment L is transcribed poorly because its transcript starts with GU instead of GG found on segments S and M. Transcription in vivo is modified by the binding of host protein YajQ to the outside the core particle so that segment L is transcribed well. This mechanism is the determinant of the temporal control of gene expression in Φ6. Mutants of Φ6 have been isolated that are independent of YajQ for transcription of segment L. The mutations are found in the gene of the viral polymerase or the major capsid protein or both. These mutants are capable of transcribing segment L with the GU start or GA or GC. The same is found to be true when YajQ is added to wild-type particles. Minus-strand synthesis has restrictions that are different from that of plus-strand synthesis, and YajQ or mutations to independence do not modify minus-strand synthesis behavior. Purified polymerase P2 is able to transcribe dsRNA, but transcription behavior of segment L by both wild-type and mutant polymerases is different from that seen in capsid structures. Adding YajQ to purified polymerase does not change its transcription specificity.