TRANSCRIPTION ELONGATION-FACTOR OF RESPIRATORY SYNCYTIAL VIRUS, A NONSEGMENTED NEGATIVE-STRAND RNA VIRUS

RNA synthesis by the paramyxovirus respiratory syncytial virus, a ubiq uitous human pathogen, was found to be more complex than previously ap preciated for the nonsegmented negative-strand RNA viruses. Intracellu lar RNA replication of a plasmid-encoded ''minigenome'' analog of vira l genomic RNA was directed by coexpression of the N, P, and L proteins . But, under these conditions, the greater part of mRNA synthesis term inated prematurely. This difference in processivity between the replic ase and the transcriptase was unanticipated because the two enzymes os tensively shared the same protein subunits and template. Coexpression of the M2 gene at a low level of input plasmid resulted in the efficie nt production of full-length mRNA and, in the case of a dicistronic mi nigenome, sequential transcription. At a higher Level, coexpression of the M2 gene inhibited transcription and RNA replication. The M2 mRNA contains two overlapping translational open reading frames (ORFs), whi ch were segregated for further analysis. Expression of the upstream OR F1, which encoded the previously described 22-kDa M2 protein, was asso ciated with transcription elongation, A model involving this protein i n the balance between transcription and replication is proposed. ORF2, which lacks an assigned protein, was associated with inhibition of RN A synthesis, We propose that this activity renders nucleocapsids synth etically quiescent prior to incorporation into virions.