Journal of Virology, October 1998, p. 8021-8031, Vol. 72, No. 10
0022-538X/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Department of Virology I,
Received 16 March 1998/Accepted 30 June 1998
Nucleotide sequences of all eight RNA segments of 10 human H3N2
influenza viruses isolated during a 5-year period from 1993 to 1997 were determined and analyzed phylogenetically in order to define the
evolutionary pathways of all genes in a parallel fashion. It was
evident that the hemagglutinin and neuraminidase genes of these viruses
evolved essentially in a single lineage and that amino acid changes
accumulated sequentially with respect to time. In contrast, amino acid
differences in the internal proteins were erratic and did not
accumulate over time. Parallel analysis of the phylogenetic patterns of
all genes revealed that the evolutionary pathways of the six internal
genes were not linked to the surface glycoproteins. Genes coding for
the basic polymerase-1, nucleoprotein, and matrix proteins of 1997 isolates were closest phylogenetically to those of earlier isolates of
1993 and 1994. Furthermore, all six internal genes of four viruses
isolated in the 1995 epidemic season consistently divided into two
distinct branch clusters, and two 1995 isolates contained PB2 genes
apparently originating from those of viruses before 1993. It was
apparent that the lack of correlation between the topologies of the
phylogenetic trees of the genes coding for the surface glycoproteins
and internal proteins was a reflection of genetic reassortment among
human H3N2 viruses. This is the first evidence demonstrating the
occurrence of genetic reassortment involving the internal genes of
human H3N2 viruses. Furthermore, internal protein variability coincided with marked increases in the activity of H3N2 viruses in 1995 and 1997.
*
Corresponding author. Mailing address: Department of
Virology I, National Institute of Infectious Diseases, Toyama 1-23-1, Shinjuku-ku, Tokyo 162, Japan. Phone: (3) 5285-1111. Fax: (3) 5285-1155. E-mail: knerome{at}nih.go.jp.
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