PARAMYXOVIRUS MESSENGER-RNA EDITING LEADS TO G-DELETIONS AS WELL AS INSERTIONS

Paramyxoviruses are thought to edit their P gene mRNAs co-transcriptio nally, by, a mechanism in which the polymerase stutters and reads the same template base more than once. Sendai virus (SeV) and bovine parai nfluenza virus type 3 (bPIV3) are closely related viruses, but SeV edi ts its P gene mRNA with the insertion of a single G residue (at simila r to 50% frequency) within the sequence 5' A(6)G(3), whereas bPIV3 ins erts 1 to similar to 6 Gs at roughly equal frequency within the sequen ce 5' A(6)G(4). When SeV synthethic mini-genomes containing either SeV or bPIV3 P gene editing cassettes are expressed from cDNA in cells wh ich are also transfected with the SeV NP, P and L genes, the virus-spe cific editing patterns were reproduced. Since the bPIV3 editing patter n was reproduced in a system that is otherwise completely SeV, this su ggests that all the information for the virus-specific editing pattern s is due to the RNA sequence itself. Unexpectedly, the length of the t emplate C run was found to be critical, even though it varies from 3 t o 7 nucleotides in length in different viruses. Expanding this templat e C run first led to attenuation of the insertion phenotype, and then to deletions rather than insertions. A stuttering or slippage model to account for these events has been further refined to include a pressu re which displaces the nascent strand in a given direction once it has disengaged from the template, and the similarities of this model to t hose which account for readthrough of cellular RNA polymerase transcri ption blocks are discussed.