Dof proteins are known as plant specific transcription factors involved in many physiological processes. Although tobacco species are likely to have multiple Dof proteins (termed as BBF), only BBF1 has been characterized. In this study, we cloned cDNAs of BBF2 and BBF3, which were assumed to be BBF1 homologues, and investigated biochemical and biological functions of BBF2 and BBF3. In gel shift assays, similarly to BBF1, recombinant BBF2 and BBF3 were shown to bind preferentially to DNA with a TAAAG Dof-binding core sequence. Fluorescently labeled BBF2 and BBF3, which were expressed transiently in tobacco epidermal cells, were shown to localize at the nucleus. In addition, as a result of quantitative gene expression analysis, we found that transient expression of each of the three BBF proteins enhanced the expression levels of the virus resistance gene N and multiple defense related genes. Based on these results, we discuss the possibility that BBF2 and BBF3 as well as BBF1 may play roles as transcription factors in the regulation of genes involved in defense responses.
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