REQUIREMENTS FOR THE SELF-DIRECTED REPLICATION OF FLOCK HOUSE VIRUS-RNA-1

The larger segment (RNA 1) of the bipartite, positive-sense RNA genome of the nodavirus flock house virus encodes the viral RNA-dependent RN A polymerase, Two nonstructural viral proteins are made during the sel f-directed replication of this RNA: protein A (110 kDa), the translati on product of RNA 1 itself, and protein B (11 kDa), the translation pr oduct of a subgenomic RNA (RNA 3) that is produced from RNA 1 during r eplication. To examine the roles of these proteins in RNA replication, specialized T7 transcription plasmids that contained wild-type or mut ant copies of flock house virus RNA 1 cDNA were constructed and used i n cells infected with the vaccinia virus-T7 RNA polymerase recombinant to make full-length transcripts that directed their own replication, Sequences in the primary transcripts that extended beyond the ends of the authentic RNA 1 sequence inhibited self directed RNA replication, but plasmids that were constructed to minimize these terminal extensio ns produced primary transcripts that replicated as abundantly as authe ntic RNA 1. Truncation or mutation of the open reading frame for prote in A eliminated self-directed replication, although the mutant RNA 1 r emained a competent template for replication by wild-type protein A su pplied in trans. These results showed that protein A was essential for RNA replication and that the process was not inseparably coupled to c omplete translation of the template. In contrast, protein B could be e liminated without inhibiting replication by mutations that disrupted t he second of the two overlapping open reading frames on RNA 3, Further more, a mutant of RNA 1 in which the first nucleotide of the RNA 3 reg ion was changed from G to U replicated at levels as high as those of t he wild type without making either RNA 3 or protein B. However, dimini shing replication levels were observed during subsequent replicative p assages of RNA from both the mutants that could not make protein B. Ro les for this protein that could account for the subtle phenotype of th ese mutants are discussed.