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Comparative Studies of Wild-type and Cold-mutant (Temperature-sensitive) Influenza Viruses: Independent Segregation of Temperature-sensitivity of Virus Replication from Temperature-sensitivity of Virion Transcriptase Activity during Recombination of Mutant A/Ann Arbor/6/60 with Wild-type H3N2 Strains

Hunnein F. Maassab

^* World Health Organization Influenza Center, Bureau of Laboratories, Center for Disease Control, Public Health Service, U.S. Department of Health, Education, and Welfare, Atlanta, Georgia 30333 U.S.A.
Department of Epidemiology, School of Public Health, University of Michigan, Ann Arbor, Michigan 48104 U.S.A.

RNA 1* (see end of Summary) of a cold-adapted and temperature-sensitive (ts) influenza virus mutant A/Ann Arbor/6/60 has a different mobility from RNA 1 of wild-type (wt) A/Ann Arbor/6/60 when subjected to electrophoresis through acrylamide/agarose gels in the absence of denaturing agents. Detection of this lesion in RNA 1 of the mutant virus was dependent on the temperature of the gel during electrophoresis. Because RNA 1 is believed to code for a protein involved in virus-specific RNA synthesis we compared phenotypes of virion transcriptases in the wt and mutant viruses. The enzyme of the mutant virus was found to be about 40% less active at 40 °C than the enzyme of the wt virus when related to their activities at 31 °C. Two cold-adapted ts recombinants which derive their RNA 1 from the mutant A/Ann Arbor/6/60 have virion transcriptases with a phenotype similar to that of their mutant parent. Three different cold-adapted ts recombinants, however, which also derive their RNA 1 from the mutant A/Ann Arbor/6/60, have virion transcriptases with a phenotype similar to that of wt virus. We conclude, therefore, that the conditional-lethal ts property of A/Ann Arbor/6/60 mutant and its recombinants is independent of the phenotypic marker observed for the A/Ann Arbor/6/60 mutant virion transcriptase, and that the lesion in RNA 1 of the mutant may also be unrelated to the observed difference between virion transcriptases of the mutant and wt A/Ann Arbor/6/60 viruses. The phenotypes of the virion transcriptases in recombinants did, however, correlate with the derivation of their RNA 2. This suggests that the increased temperature-sensitivity of virion transcriptase of the A/Ann Arbor/6/60 mutant is caused by either (1) a lesion (not necessarily conditionally lethal) that occurred in its RNA 2 during the course of cold-adaptation, or (2) a lesion in another gene whose product is a component of the virion transcriptase complex, but which lesion is only expressed phenotypically when there is a synergistic interaction in the transcriptase complex with the product of A/Ann Arbor/6/60 RNA 2.

* Our nomenclature for RNAs 1, 2 and 3 relates to results of electrophoresis under reference conditions where urea is omitted from gels and water at a temperature of 38 to 39 °C is circulated through the heat exchanger of the electrophoresis apparatus. Application of this nomenclature to describe in the text results obtained with other conditions of electrophoresis has only been done where control experiments have been performed which verify the validity of such an extrapolation.

Received 30 September 1978; accepted 25 January 1979.

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