Sequence Analysis of the NS Gene of an NS Mutant of Influenza B Virus.

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By repeated back-crosses of influenza virus A/Aichi/2/68 with wild type B/Yamagata/1/73, we isolated a B type virus mutant, clone 201, bearing mutations in the NS gene ( RNA segment 8 ). Clone 201 grew efficiently both in MDCK cells and in fertile hens' eggs, and induced a severe cytopathic effect in MDCK cells early after infection, which was ascribed to the function of the mutated NS gene of clone 201 by reassortment experiment.Sequencing analysis of the NS gene of clone 201 revealed that there is a deletion of 13 bases from position 374 - 386 of the plus sense RNA, and, due to the frame shift downstream, the translation of <NS_1> stops at position 425, giving rise to an <NS_1> polypeptide consisting of 127 amino acids, more than a half shorter than the <NS_1> of wild type B/Yamagata ( 281 amino acids ). In addition to the deletion, 10 point mutations were noticed, including 4 silent mutations.The sequencing data are consisting with the findings that RNA segment 8 of 201 migrates slightly faster than the wild type counterpart in the polyacrylamide gel, and that 201 lacks the protein band for the <NS_1> of B/Yamagata ( Molecular Weight 39,800 ) and a heavy band is present instead at the position of <NS_2> of B/Yamagata ( Molecular Weight 19,000 ).Whether severe cytolysis induced by clone 201 is due to the large carboxy terminal deletion of <NS_1> or to the point mutation(s) is yet to be determined.