Characterization of a cyanobacterial ethylene-binding protein homologous to Arabidopsis ETR1

The gaseous plant hormone ethylene regulates a variety of development processes and environmental responses. Plants perceive and respond to ethylene at nanomolar concentrations via the ethylene receptor family. Arabidopsis ETR1 is a proto-typical ethylene receptor having a N-terminal hydrophobic ethylene-binding domain and a C-terminus that shares homology with bacterial two-component signaling proteins. The identification of a homologous ethylene-binding protein from a prokaryotic cyanobacteria, Synechocystis, has facilitated the analysis of the ethylene-signaling pathway from an evolutionary perspective. Focusing on the amino acid conservation between the ethylene-binding domains from plants and Synechocystis, we analyzed the effects of directed mutations within Arabidopsis ETR1 upon ethylene binding in yeast and ethylene signaling in plants. By determining the roles for these conserved residues upon ethylene binding and signaling, a global perspective for ethylene-binding domain function in ethylene perception was formed.; The identification of a cyanobacterial ethylene-binding domain, slr1212, sharing similarity with the plant ethylene receptors, provided insight into the origin of ethylene binding; however, the function of this ethylene-binding protein was unknown. By generating Synechocystis slr1212 knockout lines, we determined that slr1212 is associated with ethylene binding. However, we were unable to identify a role for Synechocystis slr1212 based upon strategies derived from the characteristics of the plant ethylene-binding domain.; To explore the origin of the ethylene-binding domain, a radioactive ethylene-binding survey was performed within the prokaryotic and eukaryotic kingdoms. Ethylene-binding activity was detected only in the plant lineage and a clade of cyanobacteria. This result is consistent with the ethylene-binding domain being transferred from a cyanobacterial ancestor to the plant lineage in conjunction with the origin of the chloroplast. The evolutionary points at which the plant ethylene receptor acquired its protein architecture and the gene encoding the ethylene receptor transferred to the nucleus are still unknown. To address these evolutionary questions, additional ethylene-binding proteins from cyanobacteria and less-divergent members of the plant lineage need to be characterized. The characterization of an Anabaena ethylene-binding protein, EBP1, revealed that ethylene-binding domain shared significant similarity to Synechocystis slr1212, and that the EBP1 protein, unlike the slr1212 protein, contains a histidine kinase domain.