Sequence selectivity of c-Myb in vivo. Resolution of a DNA target specificity paradox

MYB regulates the SUMO protease SENP1 and its novel interaction partner UXT, modulating MYB target genes and the SUMO landscape

SUMOylation is a post-translational modification frequently found on nuclear proteins, including transcription factors (TFs) and coactivators. By controlling the activity of several TFs, SUMOylation may have far-reaching effects. MYB is an example of a developmental TF subjected to SUMO-mediated regulation, through both SUMO conjugation and SUMO binding. How SUMO affects MYB target genes is unknown. Here, we explored the global effect of reduced SUMOylation of MYB on its downstream gene programs. RNA-Seq in K562 cells after MYB knockdown and rescue with mutants having an altered SUMO status revealed a number of differentially regulated genes and distinct gene ontology term enrichments. Clearly, the SUMO status of MYB both quantitatively and qualitatively affects its regulome. The transcriptome data further revealed that MYB upregulates the SUMO protease SENP1, a key enzyme that removes SUMO conjugation from SUMOylated proteins. Given this role of SENP1 in the MYB regulome, we expanded the analysis, mapped interaction partners of SENP1, and identified UXT as a novel player affecting the SUMO system by acting as a repressor of SENP1. MYB inhibits the expression of UXT suggesting that MYB is able not only to control a specific gene program directly but also indirectly by affecting the SUMO landscape through SENP1 and UXT. These findings suggest an autoactivation loop whereby MYB, through enhancing SENP1 and reducing UXT, is itself being activated by a reduced level of repressive SUMOylation. We propose that overexpressed MYB, seen in multiple cancers, may drive this autoactivation loop and contribute to oncogenic activation of MYB.

Endoplasmic reticulum-localized UBC34 interaction with lignin repressors MYB221 and MYB156 regulates the transactivity of the transcription factors in Populus tomentosa

BACKGROUND

Regulation of lignin biosynthesis is known to occur at the level of transcription factors (TFs), of which R2R3-MYB family members have been proposed to play a central role via the AC cis-elements. Despite the important roles of TFs in lignin biosynthesis, the post-translational regulation of these TFs, particularly their ubiquitination regulation, has not been thoroughly explored.

RESULTS

We describe the discovery of a Populus tomentosa E2 ubiquitin-conjugating enzyme 34 (PtoUBC34), which is involved in the post-translational regulation of transactivation activity of lignin-associated transcriptional repressors PtoMYB221 and PtoMYB156. PtoUBC34 is localized at the endoplasmic reticulum (ER) membrane where it interacts with transcriptional repressors PtoMYB221 and PtoMYB156. This specific interaction allows for the translocation of TFs PtoMYB221 and PtoMYB156 to the ER and reduces their repression activity in a PtoUBC34 abundance-dependent manner. By taking a molecular biology approach with quantitative real-time polymerase chain reaction (qRT-PCR) analysis, we found that PtoUBC34 is expressed in all aboveground tissues of trees in P. tomentosa, and in particular, it is ubiquitous in all distinct differentiation stages across wood formation, including phloem differentiation, cambium maintaining, early and developing xylem differentiation, secondary cell wall thickening, and programmed cell death. Additionally, we discovered that PtoUBC34 is induced by treatment with sodium chloride and heat shock.

CONCLUSIONS

Our data suggest a possible mechanism by which lignin biosynthesis is regulated by ER-localized PtoUBC34 in poplar, probably through the ER-associated degradation (ERAD) of lignin-associated repressors PtoMYB221 and PtoMYB156.

SlMYB12 Regulates Flavonol Synthesis in Three Different Cherry Tomato Varieties

Cherry tomato (Lycopersicon esculentum M.) is considered a healthy fruit worldwide due to its wide range of nutrients. Flavonol, one of the major nutrients in cherry tomato, has antioxidant and cell-modulating properties. In this study, we showed a correlation between the expression of SlMYB12 and flavonol content (R² = 0.922). To characterize the function of SlMYB12, SlMYB12-overexpressing transgenic tomato plants were generated in three different cherry tomato varieties. Significant increases in flavonol content and flavonol biosynthetic gene expression were identified in SlMYB12-overexpressing plants. Therefore, we suggest that SlMYB12 plays a positive role in the flavonol biosynthesis pathway in cherry tomatoes, which further indicates a potential role as a marker in analyzing flavonol content in different cherry tomato varieties.

Development of Marker-Free Transgenic Potato Tubers Enriched in Caffeoylquinic Acids and Flavonols

Potato (Solanum tuberosum L.) is a major crop worldwide that meets human economic and nutritional requirements. Potato has several advantages over other crops: easy to cultivate and store, cheap to consume, and rich in a variety of secondary metabolites. In this study, we generated three marker-free transgenic potato lines that expressed the Arabidopsis thaliana flavonol-specific transcriptional activator AtMYB12 driven by the tuber-specific promoter Patatin. Marker-free potato tubers displayed increased amounts of caffeoylquinic acids (CQAs) (3.35-fold increases on average) and flavonols (4.50-fold increase on average). Concentrations of these metabolites were associated with the enhanced expression of genes in the CQA and flavonol biosynthesis pathways. Accumulation of CQAs and flavonols resulted in 2-fold higher antioxidant capacity compared to wild-type potatoes. Tubers from these marker-free transgenic potatoes have therefore improved antioxidant properties.

c-Myb Enhances Breast Cancer Invasion and Metastasis through the Wnt/β-Catenin/Axin2 Pathway

The molecular underpinnings of aggressive breast cancers remain mainly obscure. Here we demonstrate that activation of the transcription factor c-Myb is required for the prometastatic character of basal breast cancers. An analysis of breast cancer patients led us to identify c-Myb as an activator of Wnt/β-catenin signaling. c-Myb interacted with the intracellular Wnt effector β-catenin and coactivated the Wnt/β-catenin target genes Cyclin D1 and Axin2 Moreover, c-Myb controlled metastasis in an Axin2-dependent manner. Expression microarray analyses revealed a positive association between Axin2 and c-Myb, a target of the proinflammatory cytokine IL1β that was found to be required for IL1β-induced breast cancer cell invasion. Overall, our results identified c-Myb as a promoter of breast cancer invasion and metastasis through its ability to activate Wnt/β-catenin/Axin2 signaling. Cancer Res; 76(11); 3364-75. ©2016 AACR.

PIAS1 binds p300 and behaves as a coactivator or corepressor of the transcription factor c-Myb dependent on SUMO-status

The PIAS proteins (Protein Inhibitor of Activated STATs) constitute a family of multifunctional nuclear proteins operating as SUMO E3 ligases and being involved in a multitude of interactions. They participate in a range of biological processes, also beyond their well-established role in the immune system and cytokine signalling. They act both as transcriptional corepressors and coactivators depending on the context. In the present work, we investigated mechanisms by which PIAS1 causes activation or repression of c-Myb dependent target genes. Analysis of global expression data shows that c-Myb and PIAS1 knockdowns affect a subset of common targets, but with a dual outcome consistent with a role of PIAS1 as either a corepressor or coactivator. Our mechanistic studies show that PIAS1 engages in a novel interaction with the acetyltransferase and coactivator p300. Interaction and ChIP analysis suggest a bridging function where PIAS1 enhances p300 recruitment to c-Myb-bound sites through interaction with both proteins. In addition, the E3 activity of PIAS1 enhances further its coactivation. Remarkably, the SUMO status of c-Myb had a decisive role, indicating a SUMO-dependent switch in the way PIAS1 affects c-Myb, either as a coactivator or corepressor. Removal of the two major SUMO-conjugation sites in c-Myb (2KR mutant), which enhances its activity significantly, turned PIAS1 into a corepressor. Also, p300 was less efficiently recruited to chromatin by c-Myb-2KR. We propose that PIAS1 acts as a "protein inhibitor of activated c-Myb" in the absence of SUMOylation while, in its presence, PIAS behaves as a "protein activator of repressed c-Myb".

Evolutionary analysis of the MIXTA gene family highlights potential targets for the study of cellular differentiation

Differentiated epidermal cells such as trichomes and conical cells perform numerous essential functions in plant biology and are important for our understanding of developmental patterning and cell shape regulation. Many are also commercially significant, such as cotton fibers and trichomes that secrete pharmaceutically useful or herbivore-deterring compounds. Here, we focus on the phylogeny and evolution of the subgroup 9 R2R3 MYB gene transcription factors, which include the MIXTA gene, and that are important for the specification and regulation of plant cellular differentiation. We have sequenced 49 subgroup 9 R2R3 MYB genes from key experimental taxa and combined these sequences with those identified by an exhaustive bioinformatic search, to compile a data set of 223 subgroup 9 R2R3 MYB genes. Our phylogenetic analyses demonstrate, for the first time, the complex evolutionary history of the subgroup 9 R2R3 MYB genes. A duplication event is inferred before the origin of seed plants giving rise to two major gene lineages, here termed SBG9-A and SBG9-B. The evolutionary conservation of the SBG9-B gene lineage has not been previously recognized and its role in cellular differentiation is unknown, thus an entire clade of potential candidate genes for epidermal cell regulation remains to be explored. Using a heterologous transformation bioassay, we provide functional data that implicate members of the SBG9-B lineage in the specification of epidermal projections. Furthermore, we reveal numerous putative duplication events in both SBG9-A and SBG9-B lineages, resolving uncertainty about orthology and paralogy among the subgroup 9 R2R3 MYB genes. Finally, we provide a robust framework over which to interpret existing functional data and to direct ongoing comparative genetic research into the evolution of plant cellular diversity.

Biochemical and molecular analysis of pink tomatoes: deregulated expression of the gene encoding transcription factor SlMYB12 leads to pink tomato fruit color

The color of tomato fruit is mainly determined by carotenoids and flavonoids. Phenotypic analysis of an introgression line (IL) population derived from a cross between Solanum lycopersicum 'Moneyberg' and the wild species Solanum chmielewskii revealed three ILs with a pink fruit color. These lines had a homozygous S. chmielewskii introgression on the short arm of chromosome 1, consistent with the position of the y (yellow) mutation known to result in colorless epidermis, and hence pink-colored fruit, when combined with a red flesh. Metabolic analysis showed that pink fruit lack the ripening-dependent accumulation of the yellow-colored flavonoid naringenin chalcone in the fruit peel, while carotenoid levels are not affected. The expression of all genes encoding biosynthetic enzymes involved in the production of the flavonol rutin from naringenin chalcone was down-regulated in pink fruit, suggesting that the candidate gene underlying the pink phenotype encodes a regulatory protein such as a transcription factor rather than a biosynthetic enzyme. Of 26 MYB and basic helix-loop-helix transcription factors putatively involved in regulating transcription of genes in the phenylpropanoid and/or flavonoid pathway, only the expression level of the MYB12 gene correlated well with the decrease in the expression of structural flavonoid genes in peel samples of pink- and red-fruited genotypes during ripening. Genetic mapping and segregation analysis showed that MYB12 is located on chromosome 1 and segregates perfectly with the characteristic pink fruit color. Virus-induced gene silencing of SlMYB12 resulted in a decrease in the accumulation of naringenin chalcone, a phenotype consistent with the pink-colored tomato fruit of IL1b. In conclusion, biochemical and molecular data, gene mapping, segregation analysis, and virus-induced gene silencing experiments demonstrate that the MYB12 transcription factor plays an important role in regulating the flavonoid pathway in tomato fruit and suggest strongly that SlMYB12 is a likely candidate for the y mutation.

MYB58 and MYB63 are transcriptional activators of the lignin biosynthetic pathway during secondary cell wall formation in Arabidopsis

It has previously been shown that SECONDARY WALL-ASSOCIATED NAC DOMAIN PROTEIN1 (SND1) is a key transcription factor regulating secondary cell wall formation, including the biosynthesis of cellulose, xylan, and lignin. In this study, we show that two closely related SND1-regulated MYB transcription factors, MYB58 and MYB63, are transcriptional regulators specifically activating lignin biosynthetic genes during secondary wall formation in Arabidopsis thaliana. MYB58 and MYB63 are phylogenetically distinct from previously characterized MYBs shown to be associated with secondary wall formation or phenylpropanoid metabolism. Expression studies showed that MYB58 and MYB63 are specifically expressed in fibers and vessels undergoing secondary wall thickening. Dominant repression of their functions led to a reduction in secondary wall thickening and lignin content. Overexpression of MYB58 and MYB63 resulted in specific activation of lignin biosynthetic genes and concomitant ectopic deposition of lignin in cells that are normally unlignified. MYB58 was able to activate directly the expression of lignin biosynthetic genes and a secondary wall-associated laccase (LAC4) gene. Furthermore, the expression of MYB58 and MYB63 was shown to be regulated by the SND1 close homologs NST1, NST2, VND6, and VND7 and their downstream target MYB46. Together, our results indicate that MYB58 and MYB63 are specific transcriptional activators of lignin biosynthesis in the SND1-mediated transcriptional network regulating secondary wall formation.

Functional analysis of the epidermal-specific MYB genes CAPRICE and WEREWOLF in Arabidopsis

Epidermis cell differentiation in Arabidopsis thaliana is a model system for understanding the developmental end state of plant cells. Two types of MYB transcription factors, R2R3-MYB and R3-MYB, are involved in cell fate determination. To examine the molecular basis of this process, we analyzed the functional relationship of the R2R3-type MYB gene WEREWOLF (WER) and the R3-type MYB gene CAPRICE (CPC). Chimeric constructs made from the R3 MYB regions of WER and CPC used in reciprocal complementation experiments showed that the CPC R3 region cannot functionally substitute for the WER R3 region in the differentiation of hairless cells. However, WER R3 can substantially substitute for CPC R3. There are no differences in yeast interaction assays of WER or WER chimera proteins with GLABRA3 (GL3) or ENHANCER OF GLABRA3 (EGL3). CPC and CPC chimera proteins also have similar activity in preventing GL3 WER and EGL3 WER interactions. Furthermore, we showed by gel mobility shift assays that WER chimera proteins do not bind to the GL2 promoter region. However, a CPC chimera protein, which harbors the WER R3 motif, still binds to the GL2 promoter region.

Ovol1 represses its own transcription by competing with transcription activator c-Myb and by recruiting histone deacetylase activity

Ovol1 belongs to a family of evolutionarily conserved zinc finger proteins that act downstream of key developmental signaling pathways such as Wnt and TGF-β/BMP. It plays important roles in epithelial and germ cell development, particularly by repressing c-Myc and Id2 genes and modulating the balance between proliferation and differentiation of progenitor cells. In this study, we show that Ovol1 negatively regulates its own expression by binding to and repressing the activity of its promoter. We further demonstrate that Ovol1 uses both passive and active repression mechanisms to auto-repress: (1) it antagonizes transcriptional activation of c-Myb, a known positive regulator of proliferation, by competing for DNA binding; (2) it recruits histone deacetylase activity to the promoter via an N-terminal SNAG repressor domain. At Ovol1 cognate sites in the endogenous Ovol1 promoter, c-Myb binding correlates with increased histone acetylation, whereas the expression of Ovol1 correlates with a displacement of c-Myb from the DNA and decreased histone acetylation. Collectively, our data suggest that Ovol1 restricts its own expression by counteracting c-Myb activation and histone acetylation of the Ovol1 promoter.

The Arabidopsis transcription factor MYB12 is a flavonol-specific regulator of phenylpropanoid biosynthesis

Comprehensive functional data on plant R2R3-MYB transcription factors is still scarce compared to the manifold of their occurrence. Here, we identified the Arabidopsis (Arabidopsis thaliana) R2R3-MYB transcription factor MYB12 as a flavonol-specific activator of flavonoid biosynthesis. Transient expression in Arabidopsis protoplasts revealed a high degree of functional similarity between MYB12 and the structurally closely related factor P from maize (Zea mays). Both displayed similar target gene specificity, and both activated target gene promoters only in the presence of a functional MYB recognition element. The genes encoding the flavonoid biosynthesis enzymes chalcone synthase, chalcone flavanone isomerase, flavanone 3-hydroxylase, and flavonol synthase were identified as target genes. Hence, our observations further add to the general notion of a close relationship between structure and function of R2R3-MYB factors. High-performance liquid chromatography analyses of myb12 mutant plants and MYB12 overexpression plants demonstrate a tight linkage between the expression level of functional MYB12 and the flavonol content of young seedlings. Quantitative real time reverse transcription-PCR using these mutant plants showed MYB12 to be a transcriptional regulator of CHALCONE SYNTHASE and FLAVONOL SYNTHASE in planta, the gene products of which are indispensable for the biosynthesis of flavonols.

Involvement of multiple DNA elements in iron-inducible transcription of the ap65-1 gene in the protozoan parasite Trichomonas vaginalis

A reputed iron-responsive region, which contains multiple nuclear protein-binding DNA sequences, was shown previously to regulate iron-inducible transcription of the ap65-1 gene in the protozoan pathogen, Trichomonas vaginalis. These DNA sequences include two overlapping MYB recognition elements (MRE-1/MRE-2r) and three abutted T-tract elements. Additional nuclear protein-binding DNA sequences flanking the 5' (AGTGAAGTGA) and 3' (MRE-2f) of the iron-responsive region were identified in the present study. A stable promoter assay and primer extension revealed that transcriptional activity of the ap65-1 promoter is iron inducible as well as growth related, being lowest in the early logarithmic phase and highest in the mid-logarithmic phase. Subsequent mutational analysis of individual DNA elements of the ap65-1 promoter suggests that closely spaced T-tract elements together with an intervening GAAGGAAG sequence within the iron-responsive region are most critical for regulation of overall transcriptional activity, whereas an additional AGTGAAGTGA and MRE-2f together with an upstream T-rich region are required for optimal iron-inducible activity, and the MRE-1/MRE-2r overlap is only involved in growth-related activity. These observations suggest that expression of the ap65-1 gene is dynamically regulated under various growth conditions via interactions among multiple DNA regulatory elements.

The human neuroendocrine thyrotropin-releasing hormone receptor promoter is activated by the haematopoietic transcription factor c-Myb

Thyrotropin-releasing hormone (TRH) receptor (TRHR) is a G-protein-coupled receptor playing a crucial role in the anterior pituitary where it controls the synthesis and secretion of thyroid-stimulating hormone and prolactin. Its widespread presence not only in the central nervous system, but also in peripheral tissues, including thymus, indicates other important, but unknown, functions. One hypothesis is that the neuropeptide TRH could play a role in the immune system. We report here that the human TRHR promoter contains 11 putative response elements for the haematopoietic transcription factor c-Myb and is highly Myb-responsive in transfection assays. Analysis of Myb binding to putative response elements revealed one preferred binding site in intron 1 of the receptor gene. Transfection studies of promoter deletions confirmed that this high-affinity element is necessary for efficient Myb-dependent transactivation of reporter plasmids in CV-1 cells. The Myb-dependent activation of the TRHR promoter was strongly suppressed by expression of a dominant negative Myb-Engrailed fusion. In line with these observations, reverse transcriptase PCR analysis of rat tissues showed that the TRHR gene is expressed both in thymocytes and bone marrow. Furthermore, specific, high-affinity TRH agonist binding to cell-surface receptors was demonstrated in thymocytes and a haematopoietic cell line. Our findings imply a novel functional link between the neuroendocrine and the immune systems at the level of promoter regulation.

Transactivation properties of c-Myb are critically dependent on two SUMO-1 acceptor sites that are conjugated in a PIASy enhanced manner

The transcription factor v-Myb is a potent inducer of myeloid leukemias, and its cellular homologue c-Myb plays a crucial role in the regulation of hematopoiesis. Recently, Bies and coworkers (Bies, J., Markus, J. & Wolff, L. (2002) J. Biol. Chem, 277, 8999-9009) presented evidence that murine c-Myb can be sumoylated under overexpression conditions in COS7 cells when cotransfected with FLAG-tagged SUMO-1. Here we provide independent evidence that human c-Myb is also subject to SUMO-1 conjugation under more physiological conditions as revealed by coimmunoprecipitation analysis of Jurkat cells and transfected CV-1 cells. Analysis in an in vitro conjugation system showed that modification of the two sites K503 and K527 is interdependent. A two-hybrid screening revealed that the SUMO-1 conjugase Ubc9 is one of a few major Myb-interacting proteins. The moderate basal level of sumoylation was greatly enhanced by cotransfection of PIASy, an E3 ligase for SUMO-1. The functional consequence of abolishing sumoylation was enhanced activation both of a transiently transfected reporter gene and of a resident Myb-target gene. When single and double mutants were compared, we found a clear correlation between reduction in sumoylation and increase in transcriptional activation. Enhancing sumoylation by contransfection of PIASy had a negative effect on both Myb-induced and basal level reporter activation. Furthermore, PIASy caused a shift in nuclear distribution of c-Myb towards the insoluble matrix fraction. We propose that the negative influence on transactivation properties by the negative regulatory domain region of c-Myb depends on the sumoylation sites located here.

Phosphorylation-dependent down-regulation of c-Myb DNA binding is abrogated by a point mutation in the v-myb oncogene

The viral Myb (v-Myb) oncoprotein of the avian myeloblastosis virus (AMV) is an activated form of the cellular transcription factor c-Myb causing acute monoblastic leukemia in chicken. Oncogenic v-Myb alterations include N- and C-terminal deletions as well as point mutations. Whereas truncations in Myb cause loss of various protein modifications, none of the point mutations in v-Myb has been directly linked to protein modifications. Here we show that the DNA-binding domain of c-Myb can be phosphorylated on serine 116 by the catalytic subunit of protein kinase A. Phosphorylation of Ser(116) differentially destabilizes a subtype of c-Myb-DNA complexes. The V117D mutation of the AMV v-Myb oncoprotein abolishes phosphorylation of the adjacent Ser(116) residue. Modification of Ser(116) was also detected in live cells in c-Myb, but not in AMV v-Myb. Phosphorylation-mimicking mutants of c-Myb failed to activate the resident mim-1 gene. Our data imply that protein kinase A or a kinase with similar specificity negatively regulates c-Myb function, including collaboration with C/EBP, and that the leukemogenic AMV v-Myb version evades inactivation by a point mutation that abolishes a phosphoacceptor consensus site. This suggests a novel link between Myb, a signal transduction pathway, cooperativity with C/EBP, and a point mutation in the myb oncogene.

Combinatorial regulation of the murine RAG-2 promoter by Sp1 and distinct lymphocyte-specific transcription factors

The recombination activation genes, RAG-1 and RAG-2, encode the critical components of the recombinase complex responsible for the generation of functional antigen receptor genes. In order to gain an insight into the transcription factors and cis-acting elements that regulate the lymphocyte-specific expression of RAG-2, the promoter-region of this gene was isolated and characterized. This analysis demonstrated that a relatively small promoter fragment could confer lymphocyte-restricted expression to a reporter construct. Our work and that of others subsequently revealed that RAG-2 promoter expression is positively regulated by BSAP (PAX-5) and c-Myb transcription factors in B- and T-lineage cells, respectively. Although BSAP and c-Myb were deemed necessary for lymphocyte-specific expression, our analysis also uncovered a G-rich region at the 5'-end of the core promoter that was essential for full activity in lymphocyte cell lines. Site-directed mutagenesis revealed that a GA-box within the G-rich region was required for full promoter activity and subsequent DNA binding assays demonstrated that this element was specifically recognized by Sp1. Apart from showing that Sp1 interacts within the RAG-2 promoter, we also demonstrate that the Sp1-binding site is necessary for the high-level activation of this promoter.

A novel yeast system for in vivo selection of recognition sequences: defining an optimal c-Myb-responsive element

Yeast (Saccharomyces cerevisiae) has proved to be a highly valuable tool in a range of screening methods. We present in this work the design and use of a novel yeast effector-reporter system for selection of sequences recognised by DNA-binding proteins in vivo. A dual HIS3-lacZ reporter under the control of a single randomised response element facilitates both positive growth selection of binding sequences and subsequent quantification of the strength of the selected sequence. A galactose-inducible effector allows discrimination between reporter activation caused by the protein under study and activation due to endogenous factors. The system mimics the physiological gene dosage relationship between transcription factor and target genes in vivo by using a low copy effector plasmid and a high copy reporter plasmid, favouring sequence selectivity. The utility of the novel yeast screening system was demonstrated by using it to refine the definition of an optimal recognition element for the c-Myb transcription factor (MRE). We present screening data supporting an extended MRE consensus closely mimicking known strong response elements and where a sequence of 11 nt influences activity. Novel features include a more strict sequence requirement in the second half-site of the MRE where a T-rich sequence is preferred in vivo.

The highly conserved DNA-binding domains of A-, B- and c-Myb differ with respect to DNA-binding, phosphorylation and redox properties

In the Myb family, as in other families of transcription factors sharing similar DNA-binding domains (DBDs), diversity of function is believed to rely mainly on the less conserved parts of the proteins and on their distinct patterns of expression. However, small conserved differences between DBDs of individual members could play a role in fine-tuning their function. We have compared the highly conserved DBDs of the three vertebrate Myb proteins (A-, B- and c-Myb) and found distinct functional differences. While A- and c-Myb behaved virtually identically in a variety of DNA-binding assays, B-Myb formed complexes of comparatively lower stability, rapidly dissociating under competitive conditions and showing less tolerance to binding site variations. The three protein domains also differed as substrates for protein kinases. Whereas PKA in theory should target the DBDs of A- and c-Myb, but not B-Myb, only c-Myb was phosphorylated by PKA. CK2 phosphorylated all three proteins, although on different sites in the N-terminal region. Finally, B-Myb was remarkably sensitive to cysteine-directed oxidation compared to the other Myb proteins. Our data suggest that the small differences that have evolved between individual Myb family members lead to clear differences in DBD properties even if their sequence recognition remains the same.

Transcriptional repression by AtMYB4 controls production of UV-protecting sunscreens in Arabidopsis

An Arabidopsis thaliana line that is mutant for the R2R3 MYB gene, AtMYB4, shows enhanced levels of sinapate esters in its leaves. The mutant line is more tolerant of UV-B irradiation than wild type. The increase in sinapate ester accumulation in the mutant is associated with an enhanced expression of the gene encoding cinnamate 4-hydroxylase, which appears to be the principal target of AtMYB4 and an effective rate limiting step in the synthesis of sinapate ester sunscreens. AtMYB4 expression is downregulated by exposure to UV-B light, indicating that derepression is an important mechanism for acclimation to UV-B in A.thaliana. The response of target genes to AtMYB4 repression is dose dependent, a feature that operates under physiological conditions to reinforce the silencing effect of AtMYB4 at high activity. AtMYB4 works as a repressor of target gene expression and includes a repression domain. It belongs to a novel group of plant R2R3 MYB proteins involved in transcriptional silencing. The balance between MYB activators and repressors on common target promoters may provide extra flexibility in transcriptional control.