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 Mg(2+) 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.
Keywords: RNA-directed RNA polymerase; bacteriophage; cystovirus; de novo initiation; replication; transcription.
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