Identification of a DNA-binding factor that recognizes an alpha-coixin promoter and interacts with a Coix Opaque-2 like protein

The maize Dof protein PBF activates transcription of gamma-zein during maize seed development

Maize PBF (prolamin-box binding factor) belongs to the Dof class of plant specific transcription factors containing one highly conserved zinc finger DNA-binding domain, called Dof (DNA binding with one finger) domain. Maize PBF trans-activates the gamma-zein gene (gammaZ) promoter in developing maize seeds as shown by transient expression in maize endosperms. Co-transfection of a gammaZ:GUS construct with 35S:PBF resulted in a sevenfold increase in GUS expression, however, PBF mutation in Cys residues within the Dof domain abolishes both, binding to DNA and the capacity to activate gammaZ promoter. We present two pieces of evidence that PBF transactivates gammaZ promoter by binding to the Pb3 motif (TGTAAAG). First, recombinant Dof domain of PBF (bdPBF) specifically recognized Pb3 site as shown by gel mobility shift assays and second, co-expression of PBF with gammaZ promoter mutated in Pb3 motif suppressed PBF trans-activation capacity. Immunocytochemical analysis on developing endosperm sections shows that PBF is localized in the nuclei of the peripheral layer cells of starchy endosperm, the tissue in which the initial accumulation of gamma-zein protein occurs. By contrast, PBF is detected in the cytosol of the starchy endosperm cells newly differentiated from aleurone daughter cells, where gamma-zein was absent. Taken together these data indicate that maize PBF plays an essential role in the regulation of the temporal and spatial expression of gammaZ gene.

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.