Science 12 March 2026
13 March 2026
A team of researchers has finally cracked a decades-old mystery in virology: how low-pathogenic avian influenza viruses transform into highly pathogenic strains. The study, published in Science, identifies a mechanical "trap" within the virus's own replication machinery as the culprit.
For years, we know that the hallmark of highly pathogenic avian influenza (HPAI) is a specific change in the hemagglutinin (HA) gene. This change involves the insertion of a multibasic cleavage site (MBCS), which allows the virus to spread systemically throughout a bird's body rather than remaining confined to the respiratory or gastrointestinal tracts. But how the virus physically gains these extra genetic "links"?
The "Polymerase Trapping" Mechanism
The researchers discovered that the transition is driven by a combination of specific RNA sequences and physical structures. The process can be simplified into three key steps:
1. The virus typically starts with a "monobasic" site rich in purines (adenine and guanine). These sequences are naturally "slippery" for the virus’s replication enzyme, the polymerase.
2. As the polymerase moves along the RNA template to create a copy, the "entering" and "exiting" strands of the template can occasionally base-pair with each other, forming a temporary bridge or "stem".
3. This bridge physically traps the polymerase, preventing it from moving forward. In its attempt to break free, the polymerase "stutters" or backtracks, replicating the same small stretch of genetic code multiple times.
This "stuttering" results in the insertion of extra nucleotides, effectively building the lethal multibasic cleavage site. The study explains why these deadly mutations are almost exclusively seen in H5 and H7 subtypes. Other subtypes (like H6) often lack the specific "slippery" genetic sequences or the exact RNA structures needed to trigger the trap. The researchers suggest that similar "polymerase-trapping" structures may influence the evolution of other dangerous RNA viruses, such as Ebola or Lassa fever.
