The 6bgene in the T-DNA from Agrobacterium tumefacienshas oncogenic activity in plant cells, inducing tumor formation, the phytohormone-independent division of cells and altered leaf morphology.  The product of the 6bgene appears to promote some aspects of the proliferation of plant cells but the molecular mechanism of its action remains unknown.  We have recently showed that the 6b protein associates with nuclear-localized protein NtSIP1 (Nicotiana tabacum 6b-interacting protein 1) of tobacco.  NtSIP1 is supposed to be a transcription factor since its predicted amino acid sequence includes two regions that resemble, respectively, a nuclear-localization signal and a putative DNA-binding motif, which is similar in terms of amino acid sequence to the tri-helix motif of rice transcription factor GT-2.  A fusion protein composed of the DNA-binding domain of yeast GAL4 protein and 6b protein activated the transcription of a reporter gene under the control of the chimeric promoter that contained the GAL4 upstream-activating sequence and the CaMV 35S minimal promoter in tobacco cells.  Furthermore, nuclear localization of green fluorescent protein-fused 6b protein was enhanced by NtSIP1.  A cluster of acidic residues in 6b protein appeared to be essential, for nuclear localization and for transactivation, as well as for the hormone-independent growth of tobacco cells.  We propose that 6b protein might act in the proliferation of the plant cells, at least in part, through an association with NtSIP1.
 

Figure  Malformed morphology of leaves of a transgenic tobacco plant that harbored the 35S promoter-fused 6b gene. A, a leaf of non-transformed tobacco; B, a leaf of the transgenic plant exhibiting a weak phenotype; C, a magnified picture of B; D, a leaf of the transgenic plant exhibiting a strong phenotype; E, northern blot analysis of transcripts of 6b, NtSIP1, NTH15, NACK1, and ubiquitin gene. Lane 1, mature leaves; lane 2, shoot apices; lane 3, leaf in panel B; lane 4, leaf in panel D.  Transcripts of NtSIP1 (gene for 6b-interacting protein from tobacco; Kitakura et al., 2002), NTH15 class 1 knox homeobox gene, and NACK1 kinesin-like protein gene, which are normally expressed in meristematic cells, are detected in the leaves of the transgenic plants.