Temperature-sensitive (ts) mutants of influenza virus strain A/Udorn/72 (H3N2 subtype) with lesions in RNA segment 8 exhibited intrasegmental complementation, and were divided in two complementation groups (H1 and H2) on MDCK cells. The nucleotide sequence of segment 8 was determined for three of these mutants. The H1 strains, ICR1629 and SPC45, have a single amino acid substitution in the coding region of the non-structural protein NS1, whereas the H2 strain, ICR516, has a substitution in the NS2-coding region. With both NS1 ts mutants, the synthesis of two late proteins, the matrix protein (M1) and haemagglutinin (HA), was greatly reduced and NS1 synthesis also decreased at 40 °C (non-permissive temperature) compared to that at 34 °C (permissive temperature). The synthesis of each virus-specific RNA was analysed using a quantitative hybridization method. However, at 40 °C, the levels of individual mRNAs including those for the late proteins, were almost the same as those at 34 °C, and attained the wild-type levels later in the infection (5 h post-infection) when the synthesis of the late proteins and the NS1 protein was severely reduced. The observations suggest that the NS1 protein, which is a nuclear protein, is involved in some post-transcriptional processes in the synthesis of the late proteins and the NS1 protein.


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