THE GCN4 MOTIF IN A RICE GLUTELIN GENE IS ESSENTIAL FOR ENDOSPERM-SPECIFIC GENE-EXPRESSION AND IS ACTIVATED BY OPAQUE-2 IN TRANSGENIC RICE PLANTS

The GCN4 motif is conserved in a number of seed storage protein genes, and promoter fragments containing this motif have been shown to be in volved in controlling seed-specific expression of the genes studied. A ll genes encoding the rice seed storage protein glutelin contain the G CN4 motif at similar sites in their 5' flanking regions. Using a stabl e homologous transgenic system, we have analysed the promoter of the r ice glutelin gene GluB-1 and demonstrated that the GCN4 motif function s as an essential cis-element for endosperm-specific gene expression. Moreover, a 21 bp GluB-1 promoter fragment spanning the GCN4 motif, as a multimer, directed GUS gene expression in endosperm of transgenic r ice plants, when fused directly to the core promoter (-46) of CaMV 35S . In transiently transfected rice protoplasts, over a hundredfold tran sactivation was observed from the 21 bp sequence by the bZIP type tran scriptional activator Opaque-2 (O2) co-expressed under a CaMV 35S prom oter. The transactivation was also evident in transgenic plants contai ning both O2 and the 21 bp sequence/GUS fusion. The O2-mediated activa tion requires binding of O2 to an intact GCN4 motif. Our results sugge st that a bZIP protein functionally similar to O2 may exist in rice an d participate in controlling the endosperm-specific expression of GluB -1 through the GCN4 motif.