Matsui Kyoko, Umemura Yoshimi, Ohme-Takagi Masaru
Research Institute of Genome-based Biofactory, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan.
Plant J. 2008 Sep;55(6):954-67. doi: 10.1111/j.1365-313X.2008.03565.x. Epub 2008 Jun 4.
In Arabidopsis, MYB transcription factors regulate flavonoid biosynthesis via the formation of protein complexes with a basic helix-loop-helix (bHLH) transcription factor and a WD40 repeat protein. Several R3-type single-MYB proteins (R3-MYB), such as CPC and TRY, act as negative regulators of the development of epidermal cells. However, such regulators of flavonoid biosynthesis have not yet been reported, to our knowledge. We show here that an R3-MYB protein, AtMYBL2, acts as a transcriptional repressor and negatively regulates the biosynthesis of anthocyanin in Arabidopsis. In an AtMYBL2 knockout line (mybl2), the expression of the DFR and TT8 genes was enhanced and resulted in the ectopic accumulation of anthocyanin, while ectopic expression of AtMYBL2 or of a chimeric repressor that is a dominant negative form of AtMYBL2 suppressed the expression of DFR and TT8, and the biosynthesis of anthocyanin. The expression of AtMYBL2 was detected in various tissues but not in those in which anthocyanin accumulated or TT8 was expressed. The minimal repression domain of AtMYBL2 was found to be the six amino acids (TLLLFR) at the carboxyl terminus, and TLLLFR appears to be a novel repression motif that is different from the ERF-associated amphiphilic repression (EAR) motif. The defective phenotype of mybl2 mutants was complemented by 35S:AtMYBL2 but enhanced by a truncated form of AtMYBL2 from which the repression domain had been deleted. AtMYBL2 bound directly to TT8 protein and this complex suppressed the expression of DFR and TT8. The repression activity of AtMYBL2 appears to play a critical role in the regulation of anthocyanin biosynthesis.
在拟南芥中，MYB转录因子通过与碱性螺旋-环-螺旋（bHLH）转录因子和WD40重复蛋白形成蛋白质复合物来调控类黄酮生物合成。几种R3型单-MYB蛋白（R3-MYB），如CPC和TRY，作为表皮细胞发育的负调控因子。然而，据我们所知，尚未报道过此类类黄酮生物合成的调控因子。我们在此表明，一种R3-MYB蛋白AtMYBL2作为转录抑制因子，对拟南芥中花青素的生物合成起负调控作用。在AtMYBL2敲除系（mybl2）中，DFR和TT8基因的表达增强，并导致花青素异位积累，而AtMYBL2或作为AtMYBL2显性负性形式的嵌合抑制因子的异位表达抑制了DFR和TT8的表达以及花青素的生物合成。在各种组织中均检测到AtMYBL2的表达，但在花青素积累或TT8表达的组织中未检测到。发现AtMYBL2的最小抑制结构域是羧基末端的六个氨基酸（TLLLFR），并且TLLLFR似乎是一种不同于ERF相关两亲性抑制（EAR）基序的新型抑制基序。mybl2突变体的缺陷表型由35S:AtMYBL2互补，但由缺失抑制结构域的AtMYBL2截短形式增强。AtMYBL2直接与TT8蛋白结合，并且该复合物抑制了DFR和TT8的表达。AtMYBL2的抑制活性似乎在花青素生物合成的调控中起关键作用。
