Structure and expression analysis of three subtypes of Arabidopsis MBF1 genes Kenichi Tsuda * , Ken-ichi Yamazaki Laboratory of Environmental Molecular Biology, Graduate School of Environmental Earth Science, Hokkaido University, Kita 10, Nishi 5, Kita-ku, Sapporo 060-0810, Japan Received 1 June 2004; received in revised form 5 August 2004; accepted 10 August 2004 Available online 1 September 2004 Abstract Multiprotein bridging factor 1 (MBF1) is a transcriptional co-activator that mediates transcriptional activation by bridging between an activator and a TATA-box binding protein (TBP). Recently, we have reported that three Arabidopsis MBF1s play roles as transcriptional co- activators. This study shows that AtMBF1c is totally different from the other two in its structure and expression pattern, and that MBF1c genes also occur in other plant species, including monocots. We performed histochemical analysis of these genes using h-glucuronidase (GUS) assays to characterize the expression profile of each AtMBF1 gene extensively. In pAtMBF1a DGUS transformants, GUS staining was observed only in anthers and seeds, whereas strong GUS activity in pAtMBF1b DGUS transformants was detected in leaf veins, stems, anthers, and seeds. In mature pAtMBF1c DGUS transformants, GUS staining was observed in almost all tissues. It is noteworthy that intense GUS staining was observed in anthers of all transformants. We also found that AtMBF1c expression was up-regulated upon diverse stress treatments including exposure to heat, hydrogen peroxide, dehydration, and high concentrations of salt. These findings suggest that AtMBF1c may be involved in stress response pathway. D 2004 Elsevier B.V. All rights reserved. Keywords: Abiotic stress; Arabidopsis thaliana; Co-activator; Heat stress; Hydrogen peroxide; Multiprotein bridging factor 1 (MBF1) 1. Introduction Transcriptional regulation plays a central role in exertion of genomic information during complex biological pro- cesses in all organisms. Effects of binding of transcription factors to cis -acting elements must be carried to RNA polymerase complex to initiate and maintain transcription. In animals and yeasts, transcriptional co-activators can reportedly enhance gene expression by linking a tran- scription factor and components of basal transcriptional machinery [1–3], and by modification of chromatin struc- tures [4,5] and transcription factors [6]. The Arabidopsis genome contains greater numbers of transcription factor genes in proportion to all genes than other organisms: S. cerevisiae , C. elegans , and D. melanogaster [7]. These facts suggest that transcription processes may be more compli- cated in plants than in these organisms. However, molecular mechanisms of transcription in plants, particularly the functions of co-activators, have remained unclear. One co-activator, multiprotein bridging factor 1 (MBF1), was first purified from posterior silk gland extracts of the silkworm, Bombyx mori [1]. The MBF1 mainly enhances transcriptional activation by bridging between a basic region/leucine zipper (bZIP) type transcriptional activator and TATA-box binding protein (TBP) in yeast [3], human [8], and Drosophila [9]. We have reported recently that three Arabidopsis MBF1s can all play roles as transcrip- tional co-activators in yeast [10]. Several reports have specifically addressed expression analyses of MBF1 genes in plant. Tomato MBF1 (ER24 ) is induced immediately and transiently in ethylene-treated late immature fruit [11]. In addition, potato MBF1 is up- regulated during fungal attack, upon wounding, and by 0167-4781/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.bbaexp.2004.08.004 * Corresponding author. Fax: +81 11 706 4522. E-mail address: nahoken@ees.hokudai.ac.jp (K. Tsuda). Biochimica et Biophysica Acta 1680 (2004) 1 – 10 http://www.elsevier.com/locate/bba