Floral stem cells produce a defined number of floral organs before ceasing to be maintained as stem cells. Therefore, floral stem cells offer an ideal model to study the temporal control of stem cell maintenance within a developmental context. AGAMOUS (AG), a MADS domain transcription factor essential for the termination of floral stem cell fate, has long been thought to repress the stem cell maintenance gene WUSCHEL (WUS) indirectly. Here, we uncover a role of Polycomb Group (PcG) genes in the temporally precise repression of WUS expression and termination of floral stem cell fate. We show that AG directly represses WUS expression by binding to the WUS locus and recruiting, directly or indirectly, PcG that methylates histone H3 Lys-27 at WUS. We also show that PcG acts downstream of AG and probably in parallel with the known AG target KNUCKLES to terminate floral stem cell fate. Our studies identify core components of the network governing the temporal program of floral stem cells.

BACKGROUND & AIMS Chronic hepatitis C virus infection activates an intrahepatic immune response, leading to increased expression of interferon (IFN)-stimulated genes and activation of natural killer (NK) cells—the most prevalent innate immune cell in the liver. We investigated whether the elimination of HCV with direct-acting antiviral agents normalizes expression of IFN-stimulated genes and NK cell function. METHODS We used multicolor flow cytometry to analyze NK cells from liver and blood of 13 HCV-infected patients who did not respond to treatment with pegylated interferon and ribavirin. Samples were collected before and during IFN-free treatment with daclatasvir and asunaprevir therapy and compared with those from blood of 13 healthy individuals (controls). Serum levels of CXCL10 and CXCL11 were measured by ELISA. RESULTS Before treatment, all patients had increased levels of CXCL10 or CXCL11 and a different NK cell phenotype from controls, characterized by increased expression of HLA-DR, NKp46, NKG2A, CD85j, pSTAT1, STAT1, and TNF-related apoptosis-inducing ligand (TRAIL). NK cells from patients also had increased degranulation and decreased production of IFNγ and TNFα compared with NK cells from controls. Nine patients had an end-of-treatment response (undetectable virus) and 4 had virologic breakthrough between weeks 4 and 12 of therapy. A rapid decrease in viremia and level of inflammatory cytokines in all patients was associated with decreased activation of intrahepatic and blood NK cells; it was followed by restoration of a normal NK cell phenotype and function by week 8 in patients with undetectable viremia. This normalized NK cell phenotype was maintained until week 24 (EOT). CONCLUSIONS DAA-mediated clearance of HCV is associated with loss of intrahepatic immune activation by IFNα, indicated by decreased levels of CXCL10 and CXCL11 and normalization of NK cell phenotype and function.

Mediator is a conserved multi-subunit complex known to promote the transcription of protein-coding genes by RNA polymerase II (Pol II) in eukaryotes. It has been increasingly realized that Pol II transcribes a large number of intergenic loci to generate noncoding RNAs, but the role of Mediator in Pol II-mediated noncoding RNA production has been largely unexplored. The role of Mediator in noncoding RNA production in plants is particularly intriguing given that plants have evolved from Pol II two additional polymerases, Pol IV and Pol V, to specialize in noncoding RNA production and transcriptional gene silencing at heterochromatic loci. Here, we show that Mediator is required for microRNA (miRNA) biogenesis by recruiting Pol II to promoters of miRNA genes. We also show that several well-characterized heterochromatic loci are de-repressed in Mediator mutants and that Mediator promotes Pol II-mediated production of long noncoding scaffold RNAs, which serve to recruit Pol V to these loci. This study expands the function of Mediator to include Pol II-mediated intergenic transcription and implicates a role of Mediator in genome stability.

The rational construction and operation of an ideal helical spring has been investigated. The infinite helices, [Ag(Py 2 O)]X (Py 2 O ) 3,3′-oxybispyridine; X -) NO 3 -, BF 4 -, ClO 4 -, and PF 6 -), have been constructed in high yield via cooperative effects of the skewed conformer of Py 2 O and the potential linear geometry of the N-Ag(I)-N bond. Crystallographic characterization reveals that the polymer framework is an ideal cationic cylindrical helix and that its counteranions are pinched in two columns inside the helix. The four anions have been exchanged for each other in an aqueous solution without destruction of the helical skeleton. In particular, [Ag(Py 2 O)]NO 3 prepared by the counteranion exchange can be isolated as crystals suitable for X-ray crystallography in water. The helical pitch is reversibly stretched via the counteranion exchange from 7.430(2) to 9.621(2) Å, and is exactly proportional to the volume of the anion guests. This pitch-tuning is attributed to subtle change in the nonrigid dihedral angles between two pyridyl groups around O and Ag atoms that act as hinges within the helical subunit. Thermal analyses indicate that the helical compounds are stable up to 231-332 °C in the solid state.