The relative amount of an influenza A virus segment present in the viral particle is not affected by a reduction in replication of that segment

The principles of influenza A virus replication and packaging are not fully understood. In order to in-vestigate the signals required for these processes we have introduced mutations in the terminal non-coding region of an influenza A virus neuraminidase (NA) gene. Specifically, we have obtained two viruses, NA/X and NA/Y, which produce areduced amount of NA-specific genomic RNA in infected cells but not in the viral particle.These data indicate that (i) specific signals which affect the amount of RNA in the viral particle are distinct from those required for viral replication and (ii) the amount of packaged RNA is not strictly dependent on the amount of RNA produced uring replication. In addition, mutant NA/Y was shown to be effectively attenuated in mice. Thus, diminished replication of one viral segment might be a principle on which to base a live influenza virus vaccine. Influenza A viruses are single-stranded negative-sense RNA viruses that possess a segmented genome. Each segment has non-coding sequences at both ends of the gene (Lamb, 1989; Lamb & Horvath, 1991). These sequences contain the signals required for transcription, replication and packaging of the gene (Luytjes et al., 1989). Extensive mutational nalysis of these non-coding residues was carried out in vitro and in vivo. These data indicate that nucleotide positions 1-14 on the 3 ' end and 1-15 on the 5 ' end (nomenclature is always given as the negative sense) are important for effective transcription and replication (Parvin et al., 1989; Yamanaka et al.