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With the emergence of new viruses, such as the SARS virus and the avian influenza virus, the importance of investigations on the genetic basis of viral infections becomes clear. Sendai virus causes a localized respiratory tract infection in rodents, while a mutant, F1-R, causes a systemic infection. It has been suggested that two determinants are responsible for the systemic infection caused by F1-R [Okada et al (1998) Arch Virol 143:2343-2352]. The primary determinant of the pantropism is the enhanced proteolytic cleavability of the fusion (F) protein of F1-R, which allows the virus to undergo multiple rounds of replication in many different organs, whereas wild-type virus can only undergo multiple rounds of replication in the lungs. The enhanced cleavability of F1-R F was previously attributed to an amino acid change at F115 that is adjacent to the cleavage site at amino acid 116. Secondly, wild-type virus buds only from the apical domain of bronchial epithelium, releasing virus into the lumen of the respiratory tract, whereas F1-R buds from both apical and basolateral domains. Thus, virus is released into the basement membrane where it can easily gain access to the bloodstream for dissemination. The microtubule disruption is attributed to two amino acid differences in M protein. To confirm that the F and M gene mutations described above are solely responsible for the phenotypic differences seen in wild-type versus F1-R infections, reverse genetics was used to construct recombinant Sendai viruses with various combinations of the mutations found in the M and F genes of F1-R. Plaque assays were performed with or without trypsin addition. A recombinant virus containing all F1-R M and F mutations formed plaques in LLC-MK2 cells and underwent multiple cycles of replication without trypsin addition. To clarify which mutation(s) are necessary for plaque formation, plaque assays were done using other recombinant viruses. A virus with only the F115 change, which was previously thought to be the only change important for plaque formation of F 1-R F, did not confer upon the virus the ability to form plaques without the addition of trypsin. Another virus with the F115 and both M changes gave the same result. Therefore, more than one mutation in the F gene contributes to the ability of F1-R to form plaques without trypsin addition.

Mutations in Sendai virus variant F1-R that correlate with plaque formation in the absence of trypsin