The maize O2 and PBF proteins act additively to promote transcription from storage protein gene promoters in rice endosperm cells

A transient expression assay system was employed to investigate the possible use of the maize Opaque 2 (O2) and prolamin box binding factor (PBF) proteins as transcriptional activators of rice and wheat storage protein gene promoters. When assayed in developing rice endosperm cells, either O2 or PBF alone could increase transcription from the promoter of the rice glutelin gene, Gt1. However, mutant forms of O2 and PBF that are defective in DNA binding could not. Co-transfection with both transcriptional activators resulted in an additive increase in transactivation of the Gt1 promoter. Co-bombardment of a Gt1:: GUS construct with plasmids expressing the DNA binding domains of O2 and PBF in antisense orientation resulted in a decrease of GUS expression below background levels. Similar stimulatory and additive effects of O2 and PBF could be observed on the promoters from other storage protein genes including rice globulin (Glb), prolamins (RP6 and PG5a) and a wheat glutenin (Bx7). However, responsiveness of the promoters from non-storage protein genes like rice actin and CaMV 35S to O2 and PBF was insignificant. Our results indicate that the maize O2 and PBF proteins can act singly or additively as effective stimulators of heterologous storage protein promoters in developing rice endosperm cells. These data support the use of well-characterized transcription factors from maize as an effective means of increasing the expression level of recombinant proteins in developing rice seeds.