The regulatory regions of the rice tungro bacilliform virus promoter and interacting nuclear factors in rice (Oryza sativa L.).

The Rice Tungro Bacilliform Virus (RTBV) promoter confers phloem-specific gene expression in transgenic rice plants. A series of promoter deletion mutants were fused with the Escherichia coli beta-glucuronidase A (uidA) reporter gene and introduced into transgenic rice plants. The RTBV promoter confers substantially stronger expression in shoots than in roots. A fragment of the promoter comprising nucleotides -164 to +45 relative to the transcriptional start site contains sufficient information for phloem-specific gene expression. Within this region, nucleotides -164 to -43 were essential for promoter function since deletion of this fragment dramatically reduced promoter activity. Gel-retardation assays identified two groups of rice nuclear factors (RNFG1 and RNFG2) that bind to the -164 to +45 promoter fragment. Competition and DNasel footprinting experiments indicated that RNFG1 bound to nucleotides -3 to +8 (Box I) while RNFG2 bound to nucleotides -53 to -39 (Box II). Interactions between the two groups of factors were observed. In addition, we found differences in the binding of nuclear factors from shoots versus from roots, in agreement with the different activities of the promoter in these two organs. It is proposed that binding of RNFG1 and RNFG2 between nucleotides -164 to +45 is essential for the tissue-specific expression of this promoter.