Abstract
The phloem-specific promoter of rice tungro bacilliform virus (RTBV) is regulated in part by sequence-specific DNA-binding proteins that bind to Box II, an essential cis element. Previous studies demonstrated that the bZIP protein RF2a is involved in transcriptional regulation of the RTBV promoter. Here we report the identification and functional characterization of a second bZIP protein, RF2b. RF2b, identified by its interaction with RF2a, binds to Box II in in vitro assays as a homodimer and as RF2a/RF2b heterodimers. Like RF2a, RF2b activates the RTBV promoter in transient assays and in transgenic tobacco plants. Both RF2a and RF2b are predominantly expressed in vascular tissues. However, RF2a and RF2b have different DNA-binding affinities to Box II, show distinctive expression patterns in different rice organs, and exhibit different patterns of subcellular localization. Furthermore, transgenic rice plants with reduced levels of RF2b exhibit a disease-like phenotype. We propose that the regulation of phloem-specific expression of the RTBV promoter and potentially the control of RTBV replication are mainly achieved via interactions of the Box II cis element with multiple host factors, including RF2a and RF2b. We also propose that quenching/titration of these and perhaps other transcription factors by RTBV is involved in the development of the symptoms of rice tungro disease.