Genes are regulated in numerous ways; one is the addition of methyl groups to histone proteins, which bind and package up DNA. A study of a viral enzyme that methylates histones suggests that it functions by 'walking' from one methylation site to the next.
The enzyme, called vSET, is found in Paramecium bursaria chlorella virus 1 and suppresses gene expression in the host to aid infection. Hua Wei and Ming-Ming Zhou at Mount Sinai School of Medicine in New York determined the three-dimensional structure of the enzyme and studied how it binds to histones. They surmised that vSET has two binding areas, of which only one can bind to the substrate at a time. Once methylation has occurred at the first binding site, vSET is released at the same time that its second binding area attaches to another site. This 'walking' mechanism allows the enzyme to work efficiently across the whole genome, the authors say.