Molecular characterization of transparent testa (tt) mutants of Arabidopsis thaliana (ecotype Estland) impaired in flavonoid biosynthetic pathway

Detailed analysis of four transparent testa (tt) mutants of Arabidopsis thaliana (ecotype Estland) that lack anthocyanin pigments indicated that three are allelic to known mutants tt3, tt4 and ttg1 (mutants of DFR, CHS and TTG1 genes, respectively) while the fourth represents a new tt mutant (tt17). It is known through 3-D crystal structure analysis of CHS2 in Medicago [Nat. Struct. Biol. 6 (1999) 775] that Cys164 (key active site residue) is activated by His303 (corresponds to His309 in Arabidopsis). The substitution of His309 by Tyr309 in tt4 (Est) mutant analyzed in this study causes instability of CHS protein, thus providing evidence for the functional significance of this histidine residue. The ttg1 (Est) mutant harbors a change from Ser101 to Phe101 in the region preceding the WD-repeats, indicating a critical role of Ser101 in the function of transcriptional regulator TTG1. In tt3 (Est) mutant, 7 bp deletion generates pre-mature stop codon. The nature and function of TT17 in anthocyanin biosynthesis is yet to be defined. This study also revealed reduced transcript abundance of ACCase in all four tt mutants examined, suggesting it to be a control point for flux of supply products from primary to secondary metabolism. (C) 2003 Elsevier Ireland Ltd. All rights reserved.