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Transcription factor paralogs orchestrate alternative gene regulatory networks by context-dependent cooperation with multiple cofactors

ABSTRACT: In eukaryotes, members of large transcription factor families often exhibit similar DNA binding properties in vitro, yet initiate paralog-specific gene regulatory networks in vivo. The serially homologous first (T1) and third (T3) thoracic legs of Drosophila, which result from alternative gene regulatory networks specified by the Hox proteins Scr and Ubx, respectively, offer a unique opportunity to address this paradox in vivo. Genome-wide analyses using epitope-tagged alleles of both Hox loci in the T1 and T3 leg imaginal discs, which are the precursors to the adult appendages and ventral body regions, show that ~8% of Hox binding is paralog-specific. Binding specificity is mediated by interactions with distinct cofactors in different domains: the known Hox cofactor Exd acts in the proximal domain and is necessary for Scr to bind ~70% of its paralog-specific targets, while in the distal leg domain, we identified the homeodomain protein Distal-less (Dll) as a novel Hox cofactor that enhances Scr binding to a different subset of genomic loci. Reporter genes confirm the in vivo roles of Scr+Dll and suggest that ~1/3 of paralog-specific Hox binding in enhancers is functional. Together, these findings provide a genome-wide view of how Hox paralogs, and perhaps paralogs of other transcription factor families, orchestrate alternative downstream gene networks and suggest the importance of multiple, context-specific cofactors.

ORGANISM(S): synthetic construct Drosophila melanogaster

PROVIDER: GSE184454 | GEO | 2022/05/23

REPOSITORIES: GEO

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Project description:Hox protein transcription factors specify segmental diversity along the anterior-posterior body axis in metazoans. Understanding the basis of Hox function has long faced the problem that, while the different members of the Hox family show clear functional specificity in vivo, they all show very similar binding specificity in vitro. Based on in vitro studies, Hox cofactors may increase Hox binding selectivity but a satisfactory understanding of in vivo Hox target selectivity is still lacking. We have carried out a systematic analysis of the in vivo genomic binding profiles of all eight Drosophila Hox proteins using transient transfection in Kc167 cells to examine Hox protein targeting. We find that Hox proteins show considerable binding selectivity in vivo in the absence of the canonical Hox cofactors Extradenticle and Homothorax. Hox binding selectivity is strongly associated with chromatin accessibility; binding sites in less accessible chromatin show the highest selectivity and the different Hox proteins exhibit different propensities to bind less accessible chromatin. High Hox binding selectivity is also associated with high affinity binding regions, leading to a model where Hox proteins derive binding selectivity through an affinity-based competition with nucleosomes. Provision of the Extradenticle/Homothorax cofactors generally leads to an increase in the number of Hox binding regions and promotes the binding to regions in less accessible chromatin, however the provision of these cofactors has little effect on the overall selectivity of Hox targeting. These studies indicate that chromatin accessibility plays a key role in Hox selectivity and we propose that relative chromatin accessibility provides a basis for subtle differences in binding specificity and affinity to generate significantly different sets of genomic targets for different Hox proteins. We suggest that this mechanism may also be relevant to other transcription factor families. This SuperSeries is composed of the SubSeries listed below.

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Transcription factor paralogs orchestrate alternative gene regulatory networks by context-dependent cooperation with multiple cofactors

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