The influenza polymerase consists of three proteins dubbed PB1, PB2 and PA, that work with viral RNA and nucleoprotein to transcribe and replicate the influenza genome in a host cell. Earlier work by Doudna and Mehle with avian influenza had shown that a mutation in the viral protein PB2 - whereby glutamic acid is replaced at a certain position on the amino acid chain with lysine - enables the virus to jump from birds to humans. When glutamic acid is retained in PB2, its presence suppresses the polymerase from performing in human cells.

"That's why we were surprised when we looked at the gene sequences for the current H1N1 polymerase," Mehle says. "The viruses were replicating in people yet they retained the inhibitory glutamic acid in PB2."

In their investigation, Mehle and Doudna found that the 2009 H1N1 virus has acquired the SR polymorphism in its PB2 protein that enhances polymerase activity in human cells. To confirm that the SR polymorphism was a new pathway for the virus to infect humans, they introduced the mutation into the PB2 protein of the avian influenza. As with swine influenza, the polymerase activity and viral replication of the avian virus became enhanced in humans.

"The SR polymorphism mutation in PB2 accomplishes the same goal as the change from glutamic acid to lysine," Mehle explains. "The fact that all of the 2009 H1N1 isolates contain this second mutation supports the notion that it is important for transmission into humans, although we don't yet know the relative importance of the changes in the polymerase versus mutations elsewhere in the virus."

Mehle and Doudna are now conducting a series of biochemical and structural studies to get a comprehensive understanding of this polymerase mutation and why it evolved. Such studies are necessary before effective new antivirals can be developed.

"We need to identify what is unique about human cells that requires mutations in the influenza polymerase, possibly providing new avenues to exploit in developing therapeutic strategies," Mehle says.

Source: DOE/Lawrence Berkeley National Laboratory