In Arabidopsis thaliana, the activation process of the A1 EF-lc~ gene depends on several elements. Using the GUS reporter gene, transient expression experiments have shown that mutations of upstream cisacting elements of the A1 promoter, or the deletion of an intron located within the 5' non-coding region, similarly affect expression in dicot or monocot protoplasts. The results reported here strongly suggest that this 5' intron is properly spliced in Zea mays. We show that two trans-acting factors, specifically interacting with an upstream activating sequence (the TEF 1 box), are present in nuclear extracts prepared from A. thaliana, Brassica rapa, Nicotiana tabacum and Z. mays. In addition, a DNA sequence homologous to the TEF 1 box, found at approximately the same location within a Lycopersicon esculenturn EF-1 ~ promoter, interacts with the same trans-acting factors. Homologies found between the A. thaliana and L. esculentum TEF 1 box sequences have allowed us to define mutations of this upstream element which affect the interaction with the corresponding trans-acting factors. These results support the notion that the activation processes of A. thaliana EF-I~ genes have been conserved among angiosperms and provide interesting data on the functional structure of the TEF 1 box.