The Influenza A PB1-F2 and N40 Start Codons Are Contained within an RNA Pseudoknot.

Influenza A is a negative-sense RNA virus with an eight-segment genome. Some segments encode more than one polypeptide product, but how the virus accesses alternate internal open reading frames (ORFs) is not completely understood. In segment 2, ribosomal scanning produces two internal ORFs, PB1-F2 and N40. Here, chemical mapping reveals a Mg(2+)-dependent pseudoknot structure that includes the PB1-F2 and N40 start codons. The results suggest that interactions of the ribosome with the pseudoknot may affect the level of translation for PB1-F2 and N40.