Biological functions of the NS1 protein of an influenza B virus mutant which has a long carboxyl terminal deletion.

To clarify the function of the NS gene of a highly cytolytic mutant of influenza virus B/Yamagata/1/73 which expresses an NS1 protein with a long carboxyl terminal deletion (clone 201), we prepared a single gene reassortant (201 L-77) and a control reassortant (YL-20) in which all the genes were of wild type influenza virus B/Lee/40 origin except NS gene which was derived from either clone 201 or wild type B/Yamagata. Comparative studies have revealed that 201 L-77 destructed infected cells more severely and much earlier after infection than did YL-20, although both produced comparable amount of infectious virus. The highly cytolytic reassortant 201 L-77 produced a small plaque, while the weakly cytolytic reassortant YL-20 produced a large plaque in MDCK cells. There was little difference between the two reassortants in the time course and the amount of synthesis of viral proteins within the infected cells. However, the mode of synthesis of viral RNA (vRNA) by 201 L-77 was greatly altered compared with YL-20.