GDP polyribonucleotidyltransferase domain of vesicular stomatitis virus polymerase regulates leader-promoter escape and polyadenylation-coupled termination during stop-start transcription

 The unconventional mRNA capping enzyme, the GDP polyribonucleotidyltransferase (PRNTase) domain within the vesicular stomatitis virus (VSV) L protein, possesses a crucial "priming-capping loop." This loop governs both the initiation of leader RNA synthesis and the capping of monocistronic mRNAs during VSV's unique stop-start transcription cycle. The authors’ investigation focused on specific basic amino acid residues within a helix structure directly linked to this loop. Through identifying single-point mutations, they unveiled previously undocumented defective traits in various stages of stop-start transcription. Mutations at residue R1183 (R1183A and R1183K) notably decreased leader RNA synthesis by impeding early elongation, but not de novo initiation or productive elongation. Conversely, mutations at residue R1178 (R1178A and R1178K) reduced the efficiency of mRNA synthesis termination at gene junctions, leading to increased abnormal polycistronic mRNA levels. Remarkably, both residues R1183 and R1178 were found to be nonessential for cap-forming activities. The lethal impact of the R1183K mutation on VSV contrasted with the attenuation caused by the R1178K mutation, which generated polycistronic mRNAs during infection. These findings underscore the multifaceted role of the PRNTase domain, extending beyond pre-mRNA capping, in ensuring precise stop-start transcription in VSV. 

(LA)