NS GENE IS A POSSIBLE DETERMINANT OF APAT HOGENICITY OF COLD-ADAPTED DONOR OF ATTENUATION A LENINGRAD/134/47/57 (II2N2) AND ITS REASSORTANTS/

Phenotypical properties of single-gene reassortants of attenuated cold -adapted strain A/Leningrad/135/47/57 (H2N2) and strain A/PR8/34 virul ent for laboratory animals were studied. Only the group of reassortant s inheriting NS gene from cold-adapted virus was fully attenuated for various animal species, similarly as reassortants with 6/2 genomic for mula containing all the 6 internal protein genes from strain A/Leningr ad/ 134/47/57. Reassortant 25A-1 single-gene for NS was temperature-se nsitive (ts) on mammalian cells but formed plaques at 40 degrees C on chicken kidney cells. Reassortants with genomic formula 6/2 were tempe rature-sensitive in all types of cells used. Reassortant 25A-1 could s ynthesize normal amounts of polypeptides in MDCK cells at 39 degrees C , whereas protein synthesis of reassortants with 6/2 genomic formula w as noticeably reduced at this temperature. Hence, a similar level of a ttenuation of both reassortant groups appears to be due to various mol ecular mechanisms. Possible role of NS2 gene mutation in attenuation o f strain A/Leningrad/134/47/57 and its reassortants is discussed.