Alien sensing by GC content

Abacterial protein silences foreign DNA by recognizing low GC content, as shown by William Navarre, Ferric Fang (University of Washington, Seattle, WA), and colleagues. The silencing might allow the cell to experiment evolutionarily with intruding DNA.

The silencing protein is a Salmonella histone-like protein called H-NS. The group was searching for direct targets of this known repressor when they noticed that the H-NS binding sites were GC poor (∼47%) compared with the rest of the chromosome (∼52%). A GC-poor foreign gene that the group recombined into Salmonella was also repressed by H-NS.

Most bacteriophage and other bacteria are lower in GC content than Salmonella and its relatives, so invading DNA is an obvious target for H-NS. “It's like a primitive immune system,” says Fang. “Reduce their expression, and the foreign...