Conformational changes involved in initiation of minus-strand synthesis of a virus-associated RNA.

Synthesis of wild-type levels of turnip crinkle virus (TCV)-associated satC complementary strands by purified, recombinant TCV RNA-dependent RNA polymerase (RdRp) in vitro was previously determined to require 3' end pairing to the large symmetrical internal loop of a phylogenetically conserved hairpin (H5) located upstream from the hairpin core promoter. However, wild-type satC transcripts, which fold into a single detectable conformation in vitro as determined by temperature-gradient gel electrophoresis, do not contain either the phylogenetically inferred H5 structure or the 3' end/H5 interaction. This implies that conformational changes are required to produce the phylogenetically inferred H5 structure for its pairing with the 3' end, which takes place subsequent to the initial conformation assumed by the RNA and prior to transcription initiation. The DR region, located 140 nucleotides upstream from the 3' end and previously determined to be important for transcription in vitro and replication in vivo, is proposed to have a role in the conformational switch, since stabilizing the phylogenetically inferred H5 structure decreases the negative effects of a DR mutation in vivo. In addition, high levels of aberrant transcription correlate with a specific conformational change in the Pr while maintaining the same conformation of the 3' terminus. These results suggest that a series of events that promote conformational changes is needed to expose the 3' terminus to the RdRp for accurate synthesis of wild-type levels of complementary strands in vitro.