Positive and Negative Determinants of Target Gene Specificity in Myb Transcription Factors

Abstract

The A-Myb and c-Myb transcription factors share a highly conserved DNA-binding domain and activate the same promoters in reporter gene assays. However, the two proteins have distinct biological activities, and expressing them individually in human cells leads to the activation of distinct sets of endogenous genes, suggesting that each protein has a unique transcriptional specificity. Here, the structural and functional features of the Myb proteins were compared, using assays of endogenous gene expression to measure changes in specificity. When the Myb proteins were tested in different cell types, they activated unique and nearly nonoverlapping sets of genes in each cellular context. Deletion and domain swap experiments identified small, discreet positive and negative elements in A-Myb and c-Myb that were required for the regulation of specific genes, such as DHRS2, DSIPI, and mim-1. The results suggest that individual functional elements in the transcriptional activation domains are responsible for activating specific cellular genes in a context-specific manner. The results also have important implications for interpreting results from reporter gene assays, which fail to detect the differences in activity identified through endogenous gene assays, and fusion protein constructs that alter the transcriptional activation domains and the activities of the Myb proteins.

Fra-1/AP-1 induces EMT in mammary epithelial cells by modulating Zeb1/2 and TGFβ expression

“…19 GSEA revealed a significant overlap between the Fra-1-derived gene sets and the genes upregulated in EpRas cells upon TGF-β-induced EMT 21 (Table 1a). Genes associated with c-Myb 29 or with promoters occupied by Smad2/3, 30 two TFs connected to TGF-β and EMT, 31,32 were also enriched (Table 1b) Figure S6a, Figures 5a and b). Slug, encoded by snai2, was also increased (Figures 5a and b), whereas mRNA of snai1/Snail was moderately elevated and the protein undetectable (Figure 5a and data not shown).…”

c‐Maf interacts with c‐Myb to down‐regulate Bcl‐2 expression and increase apoptosis in peripheral CD4 cells

“…Use of different assay systems may also be expected to yield different results since complex chromatin structure is largely maintained in ChIP assays whereas a relatively small probe devoid of histones was used for gel shift assays assessing the CD13 promoter. This may be particularly relevant in the case of c-Myb where complex structural elements appear to influence its activity [60].…”

MYB IS Required for B-Selection During Thymopoises

“…Little is known about the requirements of this domain but point mutations suggest that acidity of the residues is not essential and conformation is of critical importance [135]. In addition, recent studies demonstrated that undefined regulatory elements within the transactivation domain control promoter specific target gene expression [154].…”

“…Upon examination of the lists of genes that were up or down regulated in c-Myb deficient and sufficient thymocytes, we identified a number of genes previously reported as c-Myb transcriptional targets in other cell types and model systems including Nras [326], Vcl [154], CD53 [256], Cdkn1a [154], Cbx4 [256], Ets2 [327], Bcl2 [232,233,256,328], and Itga4 [304]. However, only Cbx4 and Bcl2 were reported to be direct c-Myb targets as demonstrated by a ChIP of endogenous c-Myb from promoters in a multipotent hematopoietic cell line FDCP-mix [256].…”

“…However, many of these genes were identified by testing the ability of exogenously expressed c-Myb to activate promoter-reporter gene constructs in cells that do not normally express c-Myb [138,253] and it has recently been reported that the ability of Myb proteins to activate test constructs varied greatly from their ability to activate endogenous genes [154].…”