MYB Transcription Factors in Plants: * Structural characteristic common to all MYB proteins is the DNAbinding domain, the signature motif of transcription factor families * MYB proteins from animals contain 3 repeats, R1R3; the DNAbinding domain of plant MYB’s usually consist of 2 imperfect repeats, R2 & R3 * cMYB repeats fold into a variant of the helixturnhelix motif & contain 3 regularly spaced tryptophan residues A) Tryptophans play role in folding the hydrophobic core of the MYB domain & Are generally conserved in all MYB proteins B) The first tryptophan in the R3 repeat in plant MYB proteins is substituted for Another aromatic amino acid * DNA binding specificity of plant MYB proteins differs considerably between them selves & vertebrates * Mutations in residues that do not contact bases also affect sequence specific binding & may account for DNAbinding specificity A) of 8 putative base containing residues in MYB proteins, 6 are fully conserved in plants & the other 2 are conserved in at least 80% of these proteins * Pretranslational control evident from the many differences in the organ specific & tem poral patterns of RNA expression in different plant MYB genes * Posttranslational control operates through several different mechanisms: cellular redox potentials, phosphorylation, and proteinprotein interactions A) Plant MYB proteins contain several serine & threonine residues at the C Domain, which are possible substrates for kinase activity Plant MYB Functions: * 3 members of the vertebrate MYB family, all recognize similar motifs & play roles in cell proliferation * Plant MYB proteins have various functions; 1. Secondary Metabolism * Phenylpropanoid metabolism is 1 of 3 main types of secondary metabolism, involving modifications of compounds originally derived from phenylalanine * One branch of this pathway (flavanoid metabolism) produces the major plant pigment family anthocyanins * Also produces the flavones & flavanols which interact with anthocyanins, to protect the plant from UV * Several flavanoids act as signaling molecules in legumes induced gene expression * MYB proteins commonly regulate the phenylpropanoid pathway: A) maize CI protein activates several genes controlling anthocyanin biosynthesis B) MYB proteins found to both activate all genes for a pathway (maize), and also Work with other proteins to initiate a pathway * In petunias, one gene encoding chalcone synthase (an MYB), can be activated by another MYB protein, which is selectively expressed in petal epidermis where anthocyanin is made 2.Cell Shape * Very well established role for MYB genes in control of cell shape A) GL1 gene of Arabidopsis is essential for differentiation of trichomes, begins Begins the initial cell expansion, acts upstream of several other genes, of Which mutants do not develop into mature trichomes
B) Overexpression of MIXTA in tobacco results in trichome formation on petals, Suggesting that conical petal cells might be “trichoblasts” arrested at an early Stage of development 3. Responses to Hormones; * Barley MTB protein produced in response to gibberelic acid which activates after germination * Similar tests suggest other GA inducible genes can also be activated by MYB proteins in many other species during germination