Melon EIN3-like transcription factors (CmEIL1 and CmEIL2) are positive regulators of an ethylene- and ripening-induced 1-aminocyclopropane-1-carboxylic acid oxidase gene (CM-ACO1)

The melon 1-aminocyclopropane-1-carboxylic acid oxidase gene (CM-ACO1) is an ethylene responsive gene and plays a role in ethylene synthesis in ripening fruits. To elucidate the regulatory mechanism of the CM-ACO1 gene during the ripening stage, we isolated two cDNAs encoding melon Ethylene Insensitive 3 (EIN3)-like genes (CmEIL1 and CmEIL2) from ripening melon (Cucumis melo L. cv. Andes) fruits, and studied their properties. The predicted amino acid sequences of CmEIL1 and CmEIL2 were highly homologous to that of EIN3 (60 and 52% similarity for CmEIL1 and CmEIL2, respectively). CmEIL1 and CmEIL2 proteins interacted with a CM-ACO1 promoter in the yeast one-hybrid system, and activated transcription of CM-ACO1 promoter in melon leaf discs by a transient experiment with the luciferase reporter gene. The mRNA levels of CM-ACO1, CmEIL1 and CmEIL2 genes were very low in immature melon fruits, but increased markedly in fruits during the ripening stage. Ethylene did not affect the expression levels of CmEIL1 and CmEIL2 genes in melon leaf tissues. These results suggest that CmEIL1 and CmEIL2 were ripening-related genes, and that their products acted as transcriptional activators of the CM-ACO1 gene in ripening melon fruits.