CHARACTERIZATION OF 3 RICE BASIC LEUCINE ZIPPER FACTORS, INCLUDING 2 INHIBITORS OF EMBP-1 DNA-BINDING ACTIVITY/

The promoter of the wheat Em gene contains elements with a CACGTG core sequence (G-boxes), which are recognized by EmBP-1, a wheat basic/leu cine zipper (bZIP) protein, G-boxes are required for Em expression in response to the phytohormone abscisic acid and for transactivation by the Viviparous-1 protein (VP1) using transient expression systems, In order to identify other factors that are part of the transcriptional c omplex that associates with Gr-boxes, we have screened a rice (Oryza s ativa) cDNA library with biotinylated EmBP-1. We have isolated osZIP-1 a, a homolog of EmBP-1 and other plant G-box-binding factors. me show that EmBP-1 and osZIP-1a will preferentially heterodimerize in vitro, Over-expression of osZIP-1a in rice protoplasts can enhance expression from the Em promoter only in the presence of abscisic acid. Two Other clones have been identified by screening with EmBP-1: osZIP-2a and os ZIP-2b. These osZIP-2 factors represent a novel class of bZIP proteins with an unusual DNA-binding domain that does not recognize G-boxes, T he osZIP-2 factors can heterodimerize with EmBP-1 and prevent it from binding to the Em promoter. Interestingly, osZIP-1a does not heterodim erize with the osZIP-2 factors and its DNA binding activity is unaffec ted by their presence. Thus, osZIP-2 factors may be involved in seques tering a particular group of G-box binding factors into inactive heter odimers.