Sendai virus gene start signals are not equivalent in reinitiation capacity: moderation at the fusion protein gene

In paramyxovirus transcription, viral RNA polymerase synthesizes each monoc istronic mRNA by recognizing the gene start (S) and end (E) signals flankin g each gene. These signal sequences are well conserved in the virus family; nevertheless, they do exhibit some variations even within a virus species. In Sendai virus (SeV) Z strain, the E signals are identical for all six ge nes but there are four (N, P/M/HN, F, and L) different S signals with one o r two nucleotide variations. The significance of these variations for in vi tro and in vivo replication has been unknown. We addressed this issue by Se V reverse genetics. The luciferase gene was placed between the N and P gene so that recombinant SeVs expressed luciferase under the control of each of the four different S signals. The S signal for the F gene was found to dri ve a lower level of transcription than that of the other three, which exhib ited comparable reinitiation capacities. The polar attenuation of SeV trans cription thus appeared to be not linear but biphasic. Then, a mutant SeV wh ose F gene S signal was replaced with that used for the P, M, and HN genes was created, and its replication capability was examined. The mutant produc ed a larger amount of F protein and downstream gene-encoded proteins and re plicated faster than wild-type SeV in cultured cells and in embryonated egg s. Compared with the wild type, the mutant virus also replicated faster in mice and was more virulent, requiring a dose 20 times lower to kill 50% of mice. On the other hand, the unique F start sequence as well as the other s tart sequences are perfectly conserved in all SeV isolates sequenced to dat e, including highly virulent fresh isolates as well as egg-adapted strains, with a virulence several magnitudes lower than that of the fresh isolates. This moderation of transcription at the F gene may therefore be relevant t o viral fitness in nature.