The developmentally regulated bZIP factor ROM1 modulates transcription from lectin and storage protein genes in bean embryos.

Expression of the MAT genes DLEC2 and PHS beta, coding for phytohemagglutinin L-subunit and beta-phaseolin, respectively, is coordinately induced in bean embryos at the onset of seed maturation and regulated at the level of transcription. Previously, it was reported that expression of DLEC2 and PHS beta genes is positively regulated by the acidic transcription activator PvALF. This article describes the regulator of MAT1 (ROM1), a basic/leucine-zipper protein that binds to hybrid G box/C box motifs found on the DLEC2 and PHS beta seed enhancers. ROM1 repressed DLEC2- and PHS beta-driven expression and antagonized transactivation of either promoter by PvALF, in a dosage-dependent manner. By contrast, a PvALF/ROM1 hybrid protein activated transcription. Both repression by ROM1 and activation by the hybrid protein required the presence of intact G-box/C-box motifs. The abundance of ROM1 mRNA in embryos was highest during the cotyledon stage and decreased rapidly at the beginning of maturation, as phytohemagglutinin and phaseolin mRNAs were induced. The results demonstrate that ROM1 is a DNA-binding site-dependent repressor and point to its possible role as a negative modulator of PvALF-activated transcription of MAT genes.