The ETR1 receptor from Arabidopsis binds the gaseous hormone ethylene, A copper ion associated with the ethylene-binding domain is required for high-affinity ethylene-binding activity. A missense mutation in the domain that renders the plant insensitive to ethylene eliminates both ethylene binding and the interaction of topper with the receptor. A sequence from the genome of the cyanobacterium Synechocystis sp, strain 6803 that shows homology to the ethylene-binding domain of ETR1 encodes a functional ethylene-binding protein. On the basis of sequence conservation between the Arabidopsis and the cyanobacterial ethylene-binding domains and on in vitro mutagenesis of ETR1, a structural model for this copper-based ethylene sensor domain is presented.