The HMG box containing protein UBF binds to the promoter of vertebrate ribosomal repeats and is required for their transcription by RNA polymerase I in vitro. UBF can also bind in vitro to a variety of sequences found across the intergenic spacer in Xenopus and mammalian ribosomal DNA (rDNA) repeats. The high abundance of UBF, its colocalization with rDNA in vivo, and its DNA binding characteristics, suggest that it plays a more generalized structural role over the rDNA repeat. Until now this view has not been supported by any in vivo data. Here, we utilize chromatin immunoprecipitation from a highly enriched nucleolar chromatin fraction to show for the first time that UBF binding in vivo is not restricted to known regulatory sequences but extends across the entire intergenic spacer and transcribed region of Xenopus, human, and mouse rDNA repeats. These results are consistent with a structural role for UBF at active nucleolar organizer regions in addition to its recognized role in stable transcription complex formation at the promoter.

Blocking the function of Stat (signal transducer and activator of transcription) proteins, which are critical for antiviral responses, has evolved as a common mechanism for pathogen immune evasion. The poxvirus-encoded phosphatase H1 is critical for viral replication, and may play an additional role in the evasion of host defense by dephosphorylating Stat1 and blocking interferon (IFN)-stimulated innate immune responses. Vaccinia virus (VACV) H1 can inhibit the phosphorylation of the transcription factor Stat1 after IFN-γ stimulation of epithelial cells, greatly attenuating IFN-induced biological functions. In this study, we demonstrate that VACV infection is capable of inhibiting the phosphorylation of Stat1 and Stat2 after stimulation of fibroblasts or bone marrowderived macrophages with either type I or type II IFNs, but did not inhibit the activation of Stat3 or Stat5 in either cell type. By using recombinant proteins for in vitro assays, we observe that variola virus H1 is more active than VACV H1, although it has similar selectivity for Stat targets. Differential effects of VACV infection were observed on the induction of IFN-stimulated genes, with complete inhibition of some genes by VACV infection, while others were less affected. Despite the IFN-γ-induced expression of some genes in VACV-infected cells, IFN-γ was unable to rescue the VACV-mediated inhibition of MHC class II antigen presentation. Moreover, VACV infection can affect the IFN-induced expression of Stat1-dependent and Stat1-independent genes, suggesting that the virus may target additional IFN-activated pathways. Thus, VACV targets multiple signaling pathways in the evasion of antiviral immune responses.

Interleukin 1 is a pleiotropic cytokine produced from macrophages and dendritic cells, which induces several responses in T cells including increased proliferation, increased cytotoxic activity, and Th1 differentiation (1). IL-12 activates several signaling pathways that may mediate these biological activities including p38 MAPK and the Janus kinase (JAK)-signal transducer and activator of transcription (STAT) pathway (2-5). Upon binding of IL-12, both chains of the IL-12 receptor (IL-12R␤1 and IL-12R␤2) heterodimerize and activate the associated JAKs, TYK2, and JAK2. The IL-12R␤2 chain is subsequently tyrosine phosphorylated and recruits STAT4 to a specific docking site where it is itself phosphorylated (6). The phosphorylated STAT4 monomers then homodimerize and translocate into the nucleus. STAT4 is required for many of the functions of IL-12 including the induction of IFN-␥ and the differentiation of Th1 cells (7,8).Once in the nucleus, STAT4 binds to a cognate binding sequence within IL-12 responsive genes to subsequently mediate activation of transcription (9 -11). Several IL-12 responsive genes that require STAT4 for transcriptional activation have been identified including IFN-␥ (7-9, 12), IL-18R␣ (13, 14), ERM (15) and IRF-1 (10, 11). STAT4 may interact with other transcription factors at a promoter through several domains, including the C-terminal "transactivation domain." However, both STAT4 isoforms, STAT4␣ and STAT4␤ (16), the latter of which lacks the C-terminal 40 amino acids encompassing the putative transcription activation domain, are able to activate transcription and mediate many IL-12 responses. This suggests that interactions of other transcription factors with the STAT4 transactivation domain are not required for all STAT4-dependent responses. Furthermore, the events that STAT4 initiates at a promoter, and the kinetics with which they occur have not been characterized.CD25 is the ␣ chain of the high affinity IL-2 receptor complex. CD25 is only expressed upon T cell activation, and expression is further modulated by IL-2 stimulation. Transcription of CD25 is regulated by multiple transcription factors that bind to elements termed positive regulatory regions (PRR) in the CD25 promoter and intronic regions. PRRI is located around 260 nucleotides upstream of the major transcription initiation sites in the mouse gene (Ϫ276 to Ϫ244 in human) (17-19) and binds NF-B and serum response factor (20). PR-RII is around Ϫ100 in the mouse gene (Ϫ137 to Ϫ64 in human) and binds . PRRIII, also known as the IL-2 responsive element (IL-2rE), is located Ϫ1306 to Ϫ1387 in the mouse gene (Ϫ3780 to Ϫ3703 in human), and one of the tandem STAT5 binding sites overlaps a binding site for . In the mouse gene, an additional regulatory element at Ϫ576 to Ϫ667 that contains binding sites for NFAT and AP-1 proteins is also required for T cell receptor-mediated induction of IL-2R␣ chain expression (27). Recently, two additional elements in the human gene have been identified, a CD28 responsive element (CD28rE) at Ϫ8.5 kB t...

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ABSTRACT:Dulcin (DL), 4-ethoxyphenylurea, a synthetic chemical about 200 times as sweet as sucrose, has been proposed for use as an artificial sweetener. DL is excreted as a urinary ureido-N-glucuronide after oral administration to rabbits. The phenylurea N-glucuronide is the only ureido conjugate with glucuronic acid known at present; therefore, DL is interesting as a probe to search for new functions of UDP-glucuronosyltransferases (UGTs). Seven UGT isoforms (UGT1A3, UGT1A4, UGT1A6, UGT1A7, UGT2B13, UGT2B14, and UGT2B16) have been identified from rabbit liver, but these UGTs have not been investigated using DL as a substrate. In this work, the identities of UGT isoforms catalyzing the formation of DL glucuronide were investigated using rabbit liver microsomes (RabLM) and cloned/expressed as rabbit UGT isoforms. DL-Nglucuronide (DNG) production was determined quantitatively in RabLM and homogenates of COS-7 cells expressing each UGT isoform by using electrospray liquid chromatography-tandem mass spectrometry. Analysis of DNG formation using RabLM, by Eadie-Hofstee plot, gave a V max of 0.911 nmol/min/mg protein and the K m of 1.66 mM. DNG formation was catalyzed only by cloned expressed rabbit UGT1A7 and UGT2B16 (V max of 3.98 and 1.16 pmol/min/mg protein and a K m of 1.23 and 1.69 mM, respectively). Substrate inhibition of UGT1A7 by octylgallate confirmed the significant contribution of UGT1A7 to the formation of DNG. Octylgallate was further shown to competitively inhibit DNG production by RabLM (K i ‫؍‬ 0.149 mM). These results demonstrate that UGT1A7 is the major isoform catalyzing the N-glucuronidation of DL in RabLM.DL tastes about 200 times as sweet as sugar and previously has been used as an artificial sweetener (Siedler, 1916). Metabolic studies have demonstrated that it has a unique conjugation reaction in rabbits (Akagi et al., 1962). It was reported that DNG, one of the main metabolites of DL in rabbit urine, is produced by glucuronidation of the primary amino group of the ureido residue ( Fig. 1) (Akagi et al., 1963). Phenylureas, such as DL, are the only type of ureido compound that has shown evidence of glucuronidation, in vivo (Akagi et al., 1966). This fact suggests that DL might become an important probe for elucidation of substrate recognition mechanisms of UGTs. Publications related to the DL glucuronidation ceased in the 1960s when use of DL as a sweetener was terminated due to its toxicity.Rabbits are an important animal model of N-glucuronidation since rodents do not possess UGT1A4, the major N-glucuronidating UGT. Characterization of UGT(s) catalyzing the glucuronidation of DL or its analogs has not been reported. At present, seven UGT isoforms from RabL have been sequenced (Tukey et al., 1993;Bruck et al., 1997;Li et al., 1997Li et al., , 2000Nguyen and Tukey, 1997). Typical substrates of each isoform were also investigated. In this study, these UGTs were expressed in a mammalian cell system to examine the specificity of the glucuronidation of DL. The DNG produced in the enzyme reaction...