Image of an x-ray crystal structure showing how regulatory proteins of a key class known as zinc 
finger bind to DNA. The protein contains three "fingers" ( shown in orange, yellow, and purple) 
that bind to the double-helical DNA (blue). Each finger has an a-helix that fits into the major 
groove of the DNA and makes critical contacts that help the protein recognize its proper bind- 
ing site. 
From Pavletich, N.P., and Pabo, CO. 1991. Science 252:809-817. Copyright© 1991 by the 
AAAS. 
311 
