Evolutionary Lineages and Molecular Epidemiology of Influenza A, B, and C Viruses
Influenza viruses are classified into three types—A, B, and C—on the basis of their type-specific nucleoprotein and matrix protein antigens. The proteins of viruses belonging to different types do not readily cross-react in serologic tests. However, many structural and biochemical features are shared by type A, B, and C viruses. All influenza viruses possess a segmented genome consisting of single-stranded RNAs of negative polarity. Influenza A and B viruses have eight RNA segments. In the case of the A viruses, these RNAs code for at least ten proteins: the three polymerase proteins (PB2, PB1, and PA), the hemagglutinin (HA), the nucleoprotein (NP), the neuraminidase (NA), two matrix proteins (M1 and M2), and two nonstructural proteins (NS1 and NS2). Ten proteins have also been described for influenza B viruses; although an M2 protein does not appear to be expressed in B-virus-infected cells, an additional protein (NB) coded for by the neuraminidase gene is found. In contrast to influenza A and B viruses, influenza C viruses contain only seven RNA segments encoding eight known proteins, and the C viruses lack the gene coding for the neuraminidase.
KeywordsInfluenza Virus Mutation Rate Sialic Acid Polymerase Complex Polymerase Protein
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