Old viruses have new tricks.

Only a few years ago, biologists stumbled upon the fact that cells use RNA snippets called microRNAs as tools to control the activity of genes. Now it appears that some viruses also carry codes for microRNAs that can control the genes of invaded cells.

Once inside a host cell, a virus tricks the cell into producing these microRNAs, which then shut down genes that protect against infection by that virus.

"This [discovery] is fabulous, because it opens up a whole new avenue for making antiviral drugs," says research team member Mark Prichard of the University of Alabama at Birmingham. Drugs that block the viral microRNAs could reactivate the cells' own defenses against the virus, Prichard says. "There are a lot of viruses that this strategy might work for," he says.

The researchers scanned the genome of cytomegalovirus and found a microRNA that targets a gene called major histocompatibility complex class I-related chain B (MICB). The protein produced by this gene enables immune system cells called natural killers to fight viral infections.

Prichard and his colleagues then exposed lab-grown human cells to the virus and to a viral mutant that lacked the microRNA code. Only the original virus caused a reduction in MICB activity, the researchers report in the July 20 Science.