Promoter analysis of tbzF, a gene encoding a bZIP-type transcription factor, reveals distinct variation in cis-regions responsible for transcriptional activation between senescing leaves and flower buds in tobacco plants

The tbzF gene of tobacco (Nicotiana tabacum) encodes a basic region leucine zipper protein (bZIP) that belongs to the LIP19 subfamily. It was previously shown that tbzF transcripts accumulate on cold, abscisic acid (ABA) or ethylene treatment. They were also abundant in senescing leaves and flower buds, suggesting tbzF to possess multiple functions. In order to analyze the transcript induction profile, a 1740 bp promoter region was isolated, fused to the β-glucuronidase (GUS) gene, and introduced into tobacco plants. Resulting transgenic plants exhibited GUS activity in the stomatal guard cells of senescing leaves and in flower buds, and also in ABA- and ethylene-treated young leaves. To identify promotion responsive regions, four deletion constructs were transformed into tobacco plants. As a result no response was observed with the -720 and -220 constructs in senescing leaves, or in ABA- and ethylene-treated young leaves. In contrast, response was only reduced with the -220 construct in flower buds. These observations indicate the 500 bp between -1220 and -720 to contain cis-element(s) for senescence-associated expression, and the 500 bp between -720 and -220 to be responsible for flower-specific expression. It is concluded that, although the promoter region within -220 is essential for tbzF expression in both senescing leaves and flower buds, additional but independent elements are necessary for full activation in each organ.