The dynamics of alterations in biological properties of A/PR/8/34 virus strain was studied in the process of prolonged adaptation to the growth in chick embryos (CE) at a lowered (25 degrees C) cultivation temperature. The variant selected after 59 passages and subsequent cloning at the above temperature retained the high reproductive capacity in CE at optimal temperature (34 degrees C) - a characteristic of the original strain. Unlike to the latter, it showed a distinctly reduced ability to reproduce at 40 degrees C and a lower level of pathogenicity for white mice and CE. Analysis of genes of the cloned cold-adapted A/PR/8/34 strain detected 5 ts mutations in genes 1, 3, 5, 6 and 7 coding for P3, P2, NP, NA and M proteins, respectively.
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