PARAMYXOVIRUS RNA EDITING AND THE REQUIREMENT FOR HEXAMER GENOME LENGTH

Paramyxoviruses cotranscriptionally edit their P gene mRNA by the prog rammed insertion of G residues into a short G run contained within a l arger purine run, via pseudo-templated transcription. The templates fo r paramyxovirus transcription are genome nucleocapsids in which each n ucleoprotein subunit is associated with 6 nt, and only genomes whose l engths are multiples of 6 are found naturally or are replicated effici ently in transfected cell systems. We have examined the effect of vary ing total genome length on the frequency and number of insertions into the mRNA editing site in a transfected cell system, using constructs that generate mini-genome analogues. We found that, as long as the pur ine run sequence and the region immediately upstream were unaltered, e diting occurred during mRNA synthesis independent of the precise lengt h of the mini-genome, However, when mini-genome constructs whose lengt hs were not multiples of 6 were used, insertions (or deletions) occurr ed during antigenome synthesis within the purine run, which strikingly restored the hexamer length. Genome length correction due to changes in the antigenome purine run length occurred only when the mini-genome was not a multiple of 6, and these changes were only poorly affected by mutations in the mRNA editing site and the region immediately upstr eam, Our results suggest that the mRNA editing site is a natural hotsp ot for viral polymerase slippage during genome replication, and that t his site serves the dual and complementary function of maintaining hex amer genome length. The unusual requirement of paramyxoviruses for gen omes of precise hexamer length may have evolved to maintain genome sta bility against insertions in the mRNA editing site during replication.