The packaging signal of influenza viral RNA molecules.

The packaging signal present in influenza viral RNA molecules is shown not to constitute a separate structural element, but to reside within the 5'-bulged promoter structure, as caused by the central unpaired residue A10 in its 5' branch. Upon insertion of two uridine residues in the 3' branch opposite A10, the minus-strand viral RNA (vRNA) promoter is converted into a 3'-bulged structure, whereas the plus-strand cRNA promoter instead adopts the 5'-bulged conformation. In this promoter variant it is exclusively the cRNA that is found packaged in the progeny virions. Upon insertion of only a single uridine nucleotide opposite 5'A10, the two debulged structures of the vRNA and cRNA promoters are rendered identical, and both vRNA and cRNA molecules are packaged indiscriminately, in a 1:1 ratio, but at lower rates. We propose that the binding interactions of viral polymerase with either of the two differently bulged vRNA and cRNA promoter structures result in two different conformations of the enzyme protein. Only the 5' bulged RNA-associated polymerase conformation appears to be recognized for nuclear export, which depends on nuclear matrix protein M1 and nonstructural protein NS2. And the respective wild-type vRNP- or insertion mutant cRNP complex is observed to enter the cytoplasm and hence is included in the viral encapsidation process, which takes place at the plasma membrane.