The Dm-Myb oncoprotein coordinates higher-order chromatin structure to potentiate expression of target genes as well as stabilize facultative heterochromatin

Myb is a proto-oncogene that when mutated causes leukemias and lymphomas in birds and mammals. Vertebrates contain three representatives of the Myb gene family consisting of A-, B- and c-Myb, all of which encode DNA-binding factors that are important for the proper expression of genes. Several studies have described Myb’s primary function as a factor that upregulates transcription by binding to promoter regions, thus controlling the expression level of genes adjacent to these sequences. In flies, this regulation has been shown to be accomplished epigenetically such that Myb is only required after initial activation of a gene in order to potentiate this distinct transcriptional state. Here, we further characterize the potentiator role of Myb and show that its absence leads to a reduction in H3K4me3 along promoters and increase RNA polymerase pausing at promoters leading to decrease occupancy across gene bodies, resulting in downregulation of transcription. However, up to now, no other mechanisms have been proposed that account for the thousands of genes whose expression is altered in the absence of Myb, including those that appear to be “repressed” by Myb. Here we uncover a novel and critical role of Myb in demarcating and maintaining silent chromatin domains. We observe that Myb demarcates and stabilizes H3K27me3 domains associated with silent genomic regions, and in its absence, these domains become reduced in length and less enriched for this chromatin mark, promoting an enrichment of H3K4me3 and subsequent derepression of the genes within these domains. We also identify the nucleosome remodeling factor, NURF, to be genetically and physically interacting with Myb. We show that they work in concert to regulate a subset of tissue specific genes. More specifically, they are both needed for proper maintenance of H3K4me3 and RNA polymerase levels of active genes. Finally, we describe a novel function of Myb essential for silencing retrotransposable elements. Myb binds to both 5' and 3' end long terminal repeats (LTRs) and its absence leads to reduction of H3K9me3 levels along these repeats. As a result, derepression and ectopic transcription occurs in different tissues of Myb mutant animals such as brain, salivary glands and wing discs. We observed a significant increase in copy number of retrotransposons in all these tissues indicative of retrotransposition events. In conclusion, we find that genes whose expression levels change in the absence of Myb (~2,000 genes) are directly influenced or regulated by Myb, with virtually no genes showing altered expression through secondary consequences of aberrant transcription factor upregulation, thus providing strong evidence for a chromatin “buffering” effect in cells that helps prevent misprogramming.