THE REGULATOR OF MAT2 (ROM2) PROTEIN BINDS TO EARLY MATURATION PROMOTERS AND REPRESSES PVALF-ACTIVATED TRANSCRIPTION

The regulation of maturation (MAT)- and late embryogenesis (LEA)-speci fic gene expression in dicots involves factors related to ABI3, a seed -specific component of the abscisic acid signal transduction pathways from Arabidopsis, In French bean (Phaseolus vulgaris), the ABI3-like f actor, PvALF, activates transcription from MAT promoters of phytohemag glutinin (DLEC2) and beta-phaseolin (PHS beta) genes, We describe the regulator of MAT2 (ROM2) as a basic leucine zipper (bZIP) DNA binding protein that recognizes motifs with symmetric (ACGT) and asymmetric (A CCT) core elements present in both MAT promoters, ROM2 antagonizes tra ns-activation of the DLEC2 promoter by PvALF in transient expression a ssays. Repression was abolished by mutations that prevented binding of ROM2 to the DLEC2 seed enhancer region, Moreover, a hybrid protein co mposed of a PvALF activation domain and the DNA binding and dimerizati on domain of ROM2 activated gene expression, indicating that ROM2 reco gnizes the DLEC2 enhancer in vivo; consequently, ROM2 functions as a D NA binding site-dependent repressor. Supershift analysis of nuclear pr oteins, using a ROM2-specific antibody, revealed an increase in ROM2 D NA binding activity during seed desiccation. A corresponding increase in ROM2 mRNA coincided with the period when DLEC2 mRNA levels declined in embryos. These results demonstrate developmental regulation of the ROM2 repressor and point to a role for this factor in silencing DLEC2 transcription during late embryogenesis.