Influenza virus inhibits cleavage of the HSP70 pre-mRNAs at the polyadenylation site.

Influenza virus infection is known to shut off the expression of host genes. To study the mechanism, we examined the effects of influenza A/Udorn/72 virus infection on the heat induction of a major heat shock protein, HSP70, in Madin-Darby canine kidney cells. The induction of HSP70 protein synthesis was progressively suppressed with postinfection time when heat shock was applied. Northern hybridization analysis revealed the appearance of longer, heterogeneous HSP70 transcripts in the range of 2.7 to 30 kb with a concomitant decrease in the amount of the mature 2.7-kb mRNAs in the nucleus of the infected cells. Such longer beta-actin transcripts were also observed but with much less intensity. The longer HSP70 transcripts contained the downstream sequence of the polyadenylation site, as demonstrated by RNase protection with an antisense RNA probe containing the sequence through the polyadenylation sites. This clearly proved that influenza virus infection inhibits the polyadenylation-site cleavage of the pre-mRNAs by the host cleavage and polyadenylation machinery. One temperature-sensitive mutant virus carrying a temperature-sensitive mutation on the NS1 gene failed to inhibit the cleavage at the nonpermissive temperature, indicating that the NS1 protein is involved in the inhibition of the pre-mRNA cleavage. This is the first report of the down-regulation of cellular mRNA maturation at the point of polyadenylation-site cleavage by virus infection and identifies a new mechanism by which the influenza virus shuts off host gene expression. Copyright 1999 Academic Press.