A collection of temperature-sensitive mutants of influenza virus was examined in an attempt to define their defective functions when grown at the restrictive temperature and to detect alterations in the physical properties of virus grown at the permissive temperature. Three mutants were found to be defective in the synthesis of haemagglutinin. The haemagglutinin of the fourth mutant was shown to be functionally and antigenically normal but it could not be detected at the cell surface. A similar absence of function at the cell surface was demonstrated for the neuraminidase of two other mutants. Two genetically identical haemagglutinin-defective mutants were also partially defective in the incorporation of uridine into virus RNA but this pleiotropy did not extend to the neuraminidase. Although immunofluorescence showed that all mutants were able to synthesize ribonucleoprotein, the ribonucleoprotein of one mutant was unable to migrate from the nucleus. The heat-sensitivity of one mutant could not be correlated with defective function.

The physical properties of mutant virus particles did not differ from the parent strain except that the infectivity of one mutant was more labile and another mutant had an altered affinity for erythrocytes of different species. These defects did not correlate with known defects in multiplication.


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