Transcription factor RF2a alters expression of the rice tungro bacilliformvirus promoter in transgenic tobacco plants

The promoter from rice tungro bacilliform badnavirus (RTBV) is expressed on ly in phloem tissues in transgenic rice plants. RF2a, a b-Zip protein from rice, is known to bind to the Box II Eis element near the TATA box of the p romoter. Here, we report that the full-length RTBV promoter and a truncated fragment E of the promoter, comprising nucleotides -164 to +45. result in phloem-specific expression of beta -glucuronidase (GUS) reporter genes in t ransgenic tobacco plants. When a fusion gene comprising the cauliflower mos aic virus 35S promoter and RF2a cDNA was coexpressed with the GUS reporter genes. GUS activity was increased by 2-20-fold. The increase in GUS activit y was positively correlated with the amount of RF2a, and the expression pat tern of the RTBV promoter was altered from phloem-specific to constitutive. Constitutive expression of RF2a did not induce morphological changes in th e transgenic plants. In contrast, constitutive overexpression of the b-ZIP domain of RF2a had a strong effect on the development of transgenic plants. These studies suggest that expression of the b-Zip domain can interfere wi th the function of homologues of RF2a that regulate development of tobacco plants.