Yeast silencers can act as orientation-dependent gene inactivation centers that respond to environmental signals.

The mating-type loci located at the ends of chromosome III in Saccharomyces cerevisiae are transcriptionally repressed by a region-specific but sequence-nonspecific silencing apparatus, mediated by cis-acting silencer sequences. Previous deletion analyses have defined the locations and organizations of the silencers in their normal context and have shown that they are composed of various combinations of replication origins and binding sites for specific DNA-binding proteins. We have evaluated what organization of silencer sequences is sufficient for establishing silencing at a novel location, by inserting individual silencers next to the MAT locus and then assessing expression of MAT. The results of this analysis indicate that efficient silencing can be achieved by inserting either a single copy of the E silencer from HMR or multiple, tandem copies of either the E or I silencer from HML. These results indicate that while all silencers are functionally equivalent, they have different efficiencies; HMR E is more active than HML E, which itself is more active than HML I. Both HMR E and HML E exhibit orientation-dependent silencing, and the particular organization of binding elements within the silencer domain is critical for function. In some situations, silencing of MAT is conditional: complete silencing is obtained when cells are grown on glucose, and complete derepression occurs when cells are shifted to a nonfermentable carbon source, a process mediated in part by the RAS/cyclic AMP signaling pathway. Finally, the E silencer from HMR is able to reestablish repression immediately upon a shift back to glucose, while the silencers from HML exhibit a long lag in reestablishing repression, thus indicating distinctions between the two silencers in their reestablishment capacities. These results demonstrate that silencers can serve as nonspecific gene inactivation centers and that the attendant silencing can be rendered responsive to potential developmental cues.

Mutations in the HML E silencer of Saccharomyces cerevisiae yield metastable inheritance of transcriptional repression.

Mating-type genes resident in the silent cassette HML at the left arm of chromosome III are repressed by the action of four SIR gene products, mediated independently through two cis-acting sites, termed the E and I silencers. We have found that in the absence of the I silencer, deletion of any one of three distinct elements within E yields partial derepression of the mating-type genes resident at HML, whereas deletion of any two yields full derepression. These elements correspond to a binding site for the abundant DNA-binding protein RAP1, an autonomous replicating sequence (ARS), and an as yet undistinguished region. From detailed deletion analysis of the E site we conclude that the ARS element contributes to silencer function in a capacity distinct from its role as an initiator of DNA replication. In addition, we find that strains deleted for any one of these elements comprise two genetically identical but phenotypically distinct types of cells: Those with HML apparently fully derepressed, and those with HML apparently completely repressed. These results reinforce the notion that epigenetic inheritance is an intrinsic characteristic of silencer action.

The yeast HML I silencer defines a heterochromatin domain boundary by directional establishment of silencing.

The eukaryotic genome is divided into functional domains defined in part by local differences in chromatin structure and delimited in many cases by boundary elements. The HML and HMR loci in the yeast Saccharomyces cerevisiae are transcriptionally silent chromosome domains. Each locus is bracketed by two cis-acting sequences, designated E and I, that serve to establish and maintain repression of genes within each locus. We show that repression at HML is uniformly high between E and I but decreases sharply beyond I. The region of repression at HML generally correlates with the domain of histone hypoacetylation. Despite the sharp definition of the boundaries of HML, no sequence capable of blocking the spread of heterochromatin resides in the sequences flanking HML. We find, though, that inverting the orientation of I increases silencing outside of HML while weakening silencing within HML. These results indicate that the HML I silencer establishes a boundary between active and inactive chromatin at HML, but does so by organizing inactive chromatin in only one direction. This represents a different mechanism for delimiting the boundaries of a eukaryotic chromosome domain.

Identification of the FKS1 gene of Candida albicans as the essential target of 1,3-beta-D-glucan synthase inhibitors.

Pneumocandins and echinocandins are fungicidal antibiotics, currently in clinical development, that inhibit 1,3-beta-D-glucan synthase (GS) in several human fungal pathogens. We have identified a gene from the diploid organism Candida albicans that encodes a target of these inhibitors. A 2.1-kb portion of this gene, designated CaFKS1, has significant homology to the Saccharomyces cerevisiae FKS1 and FKS2 genes, which encode partially functionally redundant subunits of GS. To evaluate the role of CaFkslp in susceptibility to echinocandins, we disrupted CaFKS1 on one homolog each of the spontaneous pneumocandin-resistant C. albicans mutants CAI4R1, NR2, NR3, and NR4. These mutants had been selected previously on agar plates containing the pneumocandin L-733,560. The clones derived from this transformation were either resistant (Ech[r]) or fully sensitive (Ech[s]) to inhibition by L-733,560 in both liquid broth microdilution and in vitro GS assays. The site of plasmid insertion in the transformants was mapped by Southern blot analysis, using restriction site polymorphisms in the CaFKS1 gene to distinguish between the two alleles (designated CaFKS1h and CaFKS1b). For strains CAI4R1 and NR2, the CaFKS1b allele was disrupted in each Ech(r) transformant; for strain NR4, CaFKS1h was disrupted in each Ech(r) transformant. We conclude that (i) strains CAI4R1, NR2, and NR4 are heterozygous for a dominant or semidominant pneumocandin resistance mutation at CaFKS1, (ii) drug resistance mutations can occur in either CaFKS1 allele, and (iii) CaFks1p is a target of the echinocandins. For transformants of strain NR3, all the clones we analyzed were uniformly Ech(r), and only the CaFKS1h allele, either in disrupted or wild-type form, was detected on genomic Southern blots. We believe gene conversion at the CaFKS1 locus may have produced two Cafks1h alleles that each contain an Ech(r) mutation. Transformants derived from the mutants were analyzed for susceptibility to pneumocandin treatment in a mouse model of disseminated candidiasis. Strains heterozygous for the resistant allele (i.e., C. albicans CAI4R1, NR2, and NR4) were moderately resistant to treatment, while strains without a functional Ech(s) allele (i.e., strain NR3 and derivatives of strain CAI4R1 with the disruption plasmid integrated in the Ech[s] allele) displayed strong in vivo echinocandin resistance. Finally, we were unable to inactivate both alleles at CaFKS1 by two-step integrative disruption, suggesting that CaFks1p is likely to be an essential protein in C. albicans.

Characterization of a novel sphingosine 1-phosphate receptor, Edg-8.

Three G protein-coupled receptors (Edg-1, Edg-3, and Edg-5) for the lysolipid phosphoric acid mediator sphingosine 1-phosphate have been described by molecular cloning. Using a similar sequence that we found in the expressed sequence tag data base, we cloned and characterized of a fourth, high affinity, rat brain sphingosine 1-phosphate receptor, Edg-8. When HEK293T cells were co-transfected with Edg-8 and G protein DNAs, prepared membranes showed sphingosine 1- phosphate-dependent increases in [(35)S]guanosine 5'-(3-O-thio)triphosphate binding with an EC(50) of 90 nm. In a rat hepatoma Rh7777 cell line that exhibits modest endogenous responses to sphingosine 1-phosphate, this lipid mediator inhibited forskolin-driven rises in cAMP by greater than 90% when the cells were transfected with Edg-8 DNA (IC(50) 0.7 nm). This response is blocked fully by prior treatment of cultures with pertussis toxin, thus implicating signaling through G(i/o)alpha proteins. Furthermore, Xenopus oocytes exhibit a calcium response to sphingosine 1-phosphate after injection of Edg-8 mRNA, but only when oocytes are co-injected with chimeric G(q/i)alpha protein mRNA. Membranes from HEK293T and Rh7777 cell cultures expressing Edg-8 exhibited high affinity (K(D) = 2 nm) binding for radiolabeled sphingosine 1-phosphate. Rat Edg-8 RNA is expressed in spleen and throughout adult rat brain where in situ hybridization revealed it to be associated with white matter. Together our data demonstrate that Edg-8 is a high affinity sphingosine 1-phosphate receptor that couples to G(i/o)alpha proteins and is expressed predominantly by oligodendrocytes and/or fibrous astrocytes in the rat brain.

Immune cell regulation and cardiovascular effects of sphingosine 1-phosphate receptor agonists in rodents are mediated via distinct receptor subtypes.

Sphingosine 1-phosphate (S1P) is a bioactive lysolipid with pleiotropic functions mediated through a family of G protein-coupled receptors, S1P(1,2,3,4,5). Physiological effects of S1P receptor agonists include regulation of cardiovascular function and immunosuppression via redistribution of lymphocytes from blood to secondary lymphoid organs. The phosphorylated metabolite of the immunosuppressant agent FTY720 (2-amino-2-(2-[4-octylphenyl]ethyl)-1,3-propanediol) and other phosphonate analogs with differential receptor selectivity were investigated. No significant species differences in compound potency or rank order of activity on receptors cloned from human, murine, and rat sources were observed. All synthetic analogs were high-affinity agonists on S1P(1), with IC(50) values for ligand binding between 0.3 and 14 nM. The correlation between S1P(1) receptor activation and the ED(50) for lymphocyte reduction was highly significant (p < 0.001) and lower for the other receptors. In contrast to S1P(1)-mediated effects on lymphocyte recirculation, three lines of evidence link S1P(3) receptor activity with acute toxicity and cardiovascular regulation: compound potency on S1P(3) correlated with toxicity and bradycardia; the shift in potency of phosphorylated-FTY720 for inducing lymphopenia versus bradycardia and hypertension was consistent with affinity for S1P(1) relative to S1P(3); and toxicity, bradycardia, and hypertension were absent in S1P(3)(-/-) mice. Blood pressure effects of agonists in anesthetized rats were complex, whereas hypertension was the predominant effect in conscious rats and mice. Immunolocalization of S1P(3) in rodent heart revealed abundant expression on myocytes and perivascular smooth muscle cells consistent with regulation of bradycardia and hypertension, whereas S1P(1) expression was restricted to the vascular endothelium.