Properties of RNA polymerase activity associated with infectious bursal disease virus and characterization of its reaction products.

An RNA-dependent RNA polymerase activity of infectious bursal disease virus (IBDV) could be demonstrated without any special treatment of the virus particles. Ca2+ ions had to be removed from the reaction mixture. Mg2+ (4 mM) was essential for the polymerase activity which was optimal at pH 8.5 and 40 degrees C. The RNA products synthesized in vitro were 24S single-stranded (ss) RNA and 14S double-stranded (ds) RNA which remained closely associated with IBDV particles and which could only be released by proteolytic degradation of the virus. The positions of the two bands in polyacrylamide gels and hybridization with virion RNA identified the 14S RNA as the two genomic dsRNA segment. The 24S ssRNA also formed two bands, did not self-anneal, hybridized with virion RNA, and induced in vitro translation of virus-specific polypeptides. Therefore, this product was considered to be newly transcribed mRNA.