Transcribing and replicating particles in a double-stranded RNA virus from Leishmania

During the replicative cycle of many double-stranded RNA viruses, transcription of particles with a double-stranded RNA genome alternates with replication of particles containing a single-stranded genome. In virions infecting some strains of Leishmania guyanensis the putative transcriptase and replicase activities of the RNA-dependent RNA polymerase were previously detected in vitro. Northern hybridization to RNA of known polarity demonstrates that the single-stranded RNA products are of positive polarity and, by definition, are the products of the viral transcriptase. Re-evaluation of previously published data in the light of these findings suggests that transcription in Leishmania viruses is conservative. Sedimentation in sucrose gradients revealed two types of viral particles; single-stranded RNA particles comprised a small fraction of the virus population and sedimented more slowly than the peak of double-stranded RNA particles. In agreement with the replicative model of other dsRNA viruses, these single-stranded particles co-purified with the viral replicase activity that resulted in double-stranded RNA synthesis. In virus-infected promastigote extracts replicase activity decreased with increasing parasite density in culture, suggesting a correlation between cell division and viral replication.