<p>The B3-domain transcription factor ABI3 is a major regulator of gene expression of seed maturation during Arabidopsis embryogenesis. The <i>napA</i> gene encodes for a <i>Brassica napus</i> 2S storage protein specifically expressed in the embryo during the early and mid-maturation phase (MAT program).The <i>napA</i> promoter contains two essential cis-sequences; the B-box, which functions as an Abscisic acid-responsive element (ABRE) and the RY/G cluster. ABI3 is known to target both these cis-sequences. Several bZIP factors expressed during seed maturation, bZIP12, bZIP38 and bZIP66, as well as a heterodimer of ABI5 and bZIP67, can bind the B-box ABRE in a yeast one-hybrid assay. Amongst them ABI3 and bZIP67 are able to activate synergistically the two cis-elements in a transient protoplast assay. We also show that bZIP67 interacts directly with ABI3 in a yeast two-hybrid assay. Therefore, we hypothesize that i)ABI3 is recruited indirectly to <i>napA</i> through molecular interaction with bZIP67 bound to the B-box ABRE, ii) ABI3 binds directly to the RY-element and interacts with bZIP67 targeted to the adjacent G-box found in the napA RY/G-cluster.</p><p>We also show that the RY/G cluster is responsible for repression of <i>napA</i> expression during the late maturation LEA program, and for repression of ABI3-mediated transactivation during germination. ABI3 from which the A1 activation domain had been removed, can bind to the <i>napA</i> RY-element in a yeast one-hybrid assay, in contrast to full-length ABI3, suggesting that ABI3 DNA-binding abilities are regulated by auto-inhibition. We propose that during late maturation ABI3 loses ability to bind RY, which results in repression of MAT genes but not of LEA genes that contain fewer RY-elements. In parallel, we show that the B3-domain VAL proteins bind to RY-elements and decrease ABI3-mediated transactivation of the <i>napA</i> RY/G and therefore act as active repressors maintaining silencing of MAT genes during vegetative growth.</p>
| Identifer | oai:union.ndltd.org:UPSALLA/oai:DiVA.org:uu-8713 |
| Date | January 2008 |
| Creators | Martin, Nathalie |
| Publisher | Uppsala University, Department of Medical Biochemistry and Microbiology, Uppsala : Acta Universitatis Upsaliensis |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Doctoral thesis, comprehensive summary, text |
| Relation | Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Medicine, 1651-6206 ; 347 |
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