Plant and Cell Physiology, 2002, Vol. 43, No. 9 1006-1016
© 2002 Oxford University Press
A Novel HMG A-Like Protein Binds Differentially to the AT-Rich Regions Located in the Far Distal and Proximal Parts of a Soybean Glutamine Synthetase Gene (GS15) Promoter
1 Laboratoire de la Nutrition Azotée des Plantes, INRA, centre de Versailles, Route de Saint Cyr, F-78026 Versailles Cedex, France
2 Université de Versailles-Saint Quentin-en-Yvelines, Département de Biologie, 45 avenue des Etats Unis, F-78035 Versailles Cedex, France
In soybean (Glycine max L.) ammonium provided externally or as the result of symbiotic nitrogen fixation stimulates the transcription of GS15, a gene encoding cytosolic glutamine synthetase. Strong constitutive positive expression (SCPE), silencer-like and organ-specific elements, located respectively in the distal, the central and the proximal region of the promoter are required to control the ammonium responsiveness of the gene expression [Tercé-Laforgue et al. (1999) Plant Mol. Biol. 39: 551]. It was hypothesized that the correct spatial conformation of the promoter permitted the cooperative action of these three cis-acting elements. Further investigations were therefore required to ascertain this hypothesis. A nodule nuclear protein, binding to a 66 bp AT-rich DNA fragment containing a 13 bp AT-rich repeated sequence (AT-1) and located just downstream of the SCPE element, was identified using a gel retardation assay. A cDNA clone likely to code for this protein was isolated using the yeast one-hybrid system. It encodes a novel DNA binding protein (AT-1SNBP) similar to HMG A proteins but exhibiting a higher molecular weight. AT-1SNBP appears to be encoded by a single gene that is expressed in roots, root nodules and leaves of soybean. Since two other 13 bp AT-rich repeated sequences (AT-2 and AT-3) were localized in the organ-specific element, we have quantified the binding affinity of AT-1SNBP to these sequences. We demonstrate that AT-1SNBP binds differentially to DNA fragments containing AT-1, AT-2 and AT-3 and that its binding affinity depends on the presence of adjacent sequences. This result suggests that AT-1SNBP may be an architectural protein involved in maintaining the spatial conformation of the GS15 promoter, thus facilitating the interaction between the distal and proximal regulatory elements.
3 Corresponding author: E-mail, cren@versailles.inra.fr; Fax, + 33-1-3083-3096.
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