The DNA binding site of the Dof protein NtBBF1 is essential for tissue-specific and auxin-regulated expression of the rolB oncogene in plants

The Dof proteins are a large family of plant transcription factors that sha re a single highly conserved zinc finger. The tobacco Dof protein NtBBF1 wa s identified by its ability to bind to regulatory domain B in the promoter of the rolB oncogene. In this study, we show that the ACTTTA target sequenc e of NtBBF1 in domain B is necessary for tissue-specific expression of rolB . P-Glucuronidase (GUS) activity of tobacco plants containing a rolB promot er-GUS fusion with a mutated NtBBF1 target sequence within domain B is almo st completely suppressed in apical meristems and is severely abated in the vascular system. The ACTTTA motif is shown here also to be one of the cia-r egulatory elements involved in auxin induction of rolB. The pattern of NtBB F1 expression in plants is remarkably similar to that of rolB, except in me sophyll cells of mature leaves, in which only NtBBF1 expression could be de tected. Ectopic expression of rolB in mesophyll cells was achieved by parti cle gun delivery if the NtBBF1 binding sequence was intact. These data prov ide evidence that in the plant, a Dof protein DNA binding sequence acts as a transcriptional regulatory motif, and they point to NtBBF1 as the protein involved in mediating tissue-specific and auxin-inducible expression of ro lB.