An international group of virologists led by Mathilde Richard of Erasmus Medical Centre has uncovered the molecular mechanism by which low-pathogenic avian influenza viruses become highly pathogenic (Science, 12 March). It had been known for over forty years that H5 and H7 influenza viruses can make the jump from wild birds to poultry through the insertion of certain cleavage sites into the haemagglutinin gene, but how such an insertion arises had remained unclear. The Rotterdam virologists and their colleagues now show that transient RNA structures are involved. Such an RNA structure causes viral polymerases to stall at specific sequences, after which the insertion required to become highly pathogenic can take place. The virologists suggest that a similar mechanism may also play a role in other influenza viruses or RNA viruses.
