ABSTRACT
The genomes of RSV (Fig. 1B) and the PIVs (PIV3 is shown in Fig. 1C) are approximately 15.5 kb in length and are organized into gene units that are separated by short intergenic sequences (IG, bold in Fig. 1C). Gene start (GS) sequences and gene end (GE) sequences direct initiation and termination of mRNA transcription for each gene unit. Transcription occurs in a sequential manner in which the viral polymerase scans the genome from the 30 end and transcribes each gene unit in turn. Polymerase drop-off between gene units leads to a transcriptional gradient in which gene units closer to the 30 end of the genome are transcribed more efficiently than downstream genes.
