• bacteriophage;
  • cystovirus;
  • RNA-directed RNA polymerase;
  • de novo initiation;
  • transcription;
  • replication


We have determined the structure of P2, the self-priming RdRp from cystovirus ϕ12 in two crystal forms (A, B) at resolutions of 1.7 Å and 2.1 Å. Form A contains Mg2+ bound at a site that deviates from the canonical noncatalytic position seen in form B. These structures provide insight into the temperature sensitivity of a ts-mutant. However, the tunnel through which template ssRNA accesses the active site is partially occluded by a flexible loop; this feature, along with suboptimal positioning of other structural elements that prevent the formation of a stable initiation complex, indicate an inactive conformation in crystallo. Proteins 2013; 81:1479–1484. © 2013 Wiley Periodicals, Inc.