You may not see it, but an evolutionary battle is being waged all around us between bacteria and the viruses that target them. This drawing depicts an exciting new discovery on how these viruses – called bacteriophages – bypass the bacterial defenses.

At the heart of this is the CRISPR-Cas9 system, which has recently become famous for revolutionizing genetic engineering. Originally evolved as the bacterial immune system, this tool can bind to and chop up invading viral DNA using a protein called Cas9. In response, viruses have evolved anti-CRISPR proteins that protect them by blocking Cas9.

This paper reveals how one of these anti-CRISPR proteins, called AcrIIA6, can suppress the bacterial defense system. AcrIIA6, shown as handcuffs, captures and inhibits two Cas9 molecules (red), while a naturally resistant variant (green) is able to cleave its target DNA. These findings are an exciting glimpse into the molecular basis of these interactions, and will allow precise control of CRISPR-Cas9-based genome editing for many diverse applications.

Congratulations to the labs of Adeline Goulet and Yannick Doyon & collaborators!