Abstract
Respiratory syncytial virus (RSV) is a nonsegmented negative-strand RNA virus (NSV) and a leading cause of severe lower respiratory tract illness in infants and the elderly. Transcription of the ten RSV genes proceeds sequentially from the 3' promoter and requires conserved gene start (GS) and gene end (GE) signals. Previous studies using the prototypical GA1 genotype Long and A2 strains have indicated a gradient of gene transcription extending across the genome, with the highest level of mRNA coming from the most promoter-proximal gene, the first nonstructural (NS1) gene, and mRNA levels from subsequent genes dropping until reaching a minimum at the most promoter-distal gene, the polymerase (L) gene. However, recent reports show non-gradient levels of mRNA, with higher than expected levels from the attachment (G) gene. It is unknown to what extent different transcript stabilities might shape measured mRNA levels. It is also unclear whether patterns of RSV gene expression vary, or show strain- or genotype-dependence. To address this, mRNA abundances from five RSV genes were measured by quantitative real-time PCR (qPCR) in three cell lines and in cotton rats infected with RSV isolates belonging to four genotypes (GA1, ON, GB1, BA). Relative mRNA levels reached steady-state between four and 24 hours post-infection. Steady-state patterns were non-gradient and genotype-specific, where mRNA levels from the G gene exceeded those from the more promoter-proximal nucleocapsid (N) gene across isolates. Transcript stabilities could not account for the non-gradient patterns observed, indicating that relative mRNA levels more strongly reflect transcription than decay. Our results indicate that gene expression from a small but diverse set of RSV genotypes is non-gradient and genotype-dependent. We propose novel models of RSV transcription that can account for non-gradient transcription.
Highlights
Respiratory syncytial virus (RSV) can infect individuals repeatedly and is the most common pathogen associated with severe lower respiratory tract disease in children worldwide [1,2,3,4,5]
The polymerase must return from the matrix 2 (M2) gene end (GE) signal for full-length L mRNA to be made [28], suggesting that transcribing polymerases scan the RSV genome bidirectionally for a new gene start (GS) signal after terminating transcription
Oligonucleotide standards of known concentration were used to convert cycle threshold (CT) values measured by real-time PCR for mRNA targets (Fig 1A) to mRNA abundances
Summary
Respiratory syncytial virus (RSV) can infect individuals repeatedly and is the most common pathogen associated with severe lower respiratory tract disease in children worldwide [1,2,3,4,5]. Transcription in RSV and other NSV is sequential, with genes transcribed in their order of occurrence from the 3’ promoter of the genome [16,17,18,19,20,21,22]. Transcription is initiated at the GS signal which serves as a capping signal on the 5’ end of the nascent mRNA [21, 26, 27]. The GE signal of matrix 2 (M2) occurs downstream of the GS signal of the large polymerase (L) gene. The polymerase must return from the M2 GE signal for full-length L mRNA to be made [28], suggesting that transcribing polymerases scan the RSV genome bidirectionally for a new GS signal after terminating transcription. Scanning polymerase dynamics may be a universal feature of NSV transcription [21, 29,30,31]
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