Eukarya and, more recently, some bacteria have been shown to rely on a cytoskeleton-based apparatus to drive chromosome segregation. In contrast, the factors and mechanisms underpinning this fundamental process are underexplored in archaea, the third domain of life. Here we establish that the archaeon Sulfolobus solfataricus harbors a hybrid segrosome consisting of two interacting proteins, SegA and SegB, that play a key role in genome segregation in this organism. SegA is an ortholog of bacterial, Walker-type ParA proteins, whereas SegB is an archaea-specific factor lacking sequence identity to either eukaryotic or bacterial proteins, but sharing homology with a cluster of uncharacterized factors conserved in both crenarchaea and euryarchaea, the two major archaeal sub-phyla. We show that SegA is an ATPase that polymerizes in vitro and that SegB is a site-specific DNA-binding protein contacting palindromic sequences located upstream of the segAB cassette. SegB interacts with SegA in the presence of nucleotides and dramatically affects its polymerization dynamics. Our data demonstrate that SegB strongly stimulates SegA polymerization, possibly by promoting SegA nucleation and accelerating polymer growth. Increased expression levels of segAB resulted in severe growth and chromosome segregation defects, including formation of anucleate cells, compact nucleoids confined to one half of the cell compartment and fragmented nucleoids. The overall picture emerging from our findings indicates that the SegAB complex fulfills a crucial function in chromosome segregation and is the prototype of a DNA partition machine widespread across archaea.

Gene expression programs determine cell fate in embryonic development and their dysregulation results in disease. Transcription factors (TFs) control gene expression by binding to enhancers, but how TFs select and activate their target enhancers is still unclear. HOX TFs share conserved homeodomains with highly similar sequence recognition properties, yet they impart the identity of different animal body parts. To understand how HOX TFs control their specific transcriptional programsin vivo, we compared HOXA2 and HOXA3 binding profiles in the mouse embryo. HOXA2 and HOXA3 directly cooperate with TALE TFs and selectively target different subsets of a broad TALE chromatin platform. Binding of HOX and tissue-specific TFs convert low affinity TALE binding into high confidence, tissue-specific binding events, which bear the mark of active enhancers. We propose that HOX paralogs, alone and in combination with tissue-specific TFs, generate tissue-specific transcriptional outputs by modulating the activity of TALE TFs at selected enhancers.

We have previously shown that the glucocorticoid receptor (GR) is required for skin homeostasis and epidermal barrier competence. To understand the transcriptional program by which GR regulates skin development, we performed a microarray analysis using the skin of GR^−/− and GR^+/+ mice of embryonic d 18.5 and identified 442 differentially expressed genes. Functional clustering demonstrated overrepresentation of genes involved in ectoderm/epidermis development. We found strong repression of genes encoding proteins associated with the later stages of epidermal differentiation, such as several small proline-rich proteins (Sprrs) and corneodesmosin (Cdsn). This, together with the up-regulation of genes induced earlier during epidermal development, including the epithelial-specific gene transcripts E74-like factor 5 (Elf5) and keratin 77 (Krt77), fits with the phenotype of defective epidermal differentiation observed in the GR^−/− mice. We also found down-regulation of the antimicrobial peptide defensin β 1 (Defb1) and FK506-binding protein 51 (Fkbp51). Skin developmental expression profiling of these genes and studies in cultured keratinocytes from GR^−/− and wild type embryos demonstrated that gene regulation occurred in a cell-autonomous manner. To investigate the consequences of GR loss in adult epidermis, we generated mice with inducible inactivation of GR restricted to keratinocytes (K14-cre-ER^T2//GR^loxP/loxP mice). K14-cre-ER^T2//GR^loxP/loxP mice featured thickened skin with increased keratinocyte proliferation and impaired differentiation. Whereas Krt77 and Elf5 expression remained unaffected by loss of GR in adult epidermis, Fkbp51, Sprr2d, and Defb1 were strongly repressed. Importantly, we have identified both Fkbp51 and Defb1 as direct transcriptional targets of GR, and we have shown that GR-mediated regulation of these genes occurs in both developing and adult epidermis. We conclude that both overlapping and differential GR targets are regulated in developing vs. adult skin.

The glucocorticoid (GC) receptor (GR) and Kruppel-like factor Klf4 are transcription factors that play major roles in skin homeostasis. However, whether these transcription factors cooperate in binding genomic regulatory regions in epidermal keratinocytes was not known. Here, we show that in dexamethasone-treated keratinocytes GR and Klf4 are recruited to genomic regions containing adjacent GR and KLF binding motifs to control transcription of the anti-inflammatory genes Tsc22d3 and Zfp36. GR- and Klf4 loss of function experiments showed total GR but partial Klf4 requirement for full gene induction in response to dexamethasone. In wild type keratinocytes induced to differentiate, GR and Klf4 protein expression increased concomitant with Tsc22d3 and Zfp36 up-regulation. In contrast, GR-deficient cells failed to differentiate or fully induce Klf4, Tsc22d3 and Zfp36 correlating with increased expression of the epithelium-specific Trp63, a known transcriptional repressor of Klf4. The identified transcriptional cooperation between GR and Klf4 may determine cell-type specific regulation and have implications for developing therapies for skin diseases.