Identification and properties of a small protein that interacts with a tobacco bZIP-type transcription factor TBZF

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The tobacco bZIP transcription factor, TBZF, functions in leaf senescence and floral development. In order to identify TBZF-interaction partner(s), we screened a cDNA library constructed from leaves of tobacco (Nicotiana tabacum cv. Xanthi) by the yeast two hybrid method. Among 16 clones initially identified, one clone was further characterized because of its highly specific binding to the bait. The full length cDNA of 776 bp encoded a polypeptide of 12.5 kDa, which was designated NtTIP1 (Nicotiana tabacum TBZF-Interaction Protein 1). A high similarity was scored to potato CI21A, a member of the ASR/CI21 family of proteins, most of which are induced by biotic and abiotic stresses. NtTIP1 was revealed to be localized in the nucleus by fluorescence assay of NtTIP1-GFP fusion protein in onion epidermal cells. Both NtTIP1 and TBZF transcripts accumulated at high levels in flowers and senescing leaves. The expressional correlation was also observed particularly after 6 h cold stress, and after ABA, JA and ethylene treatments. The results suggest that NtTIP1 and TBZF form a complex in vivo, and they possibly function in flower development and stress response.