This article describes the design and synthesis of quinoxaline-based semiconducting polymer dots (Pdots) that exhibit near-infrared fluorescence, ultrahigh brightness, large Stokes shifts, and excellent cellular targeting capability. We also introduced fluorine atoms and long alkyl chains into polymer backbones and systematically investigated their effect on the fluorescence quantum yields of Pdots. These new series of quinoxaline-based Pdots have a fluorescence quantum yield as high as 47% with a Stokes shift larger than 150 nm. Single-particle analysis reveals that the average per-particle brightness of the Pdots is at least 6 times higher than that of the commercially available quantum dots. We further demonstrated the use of this new class of quinoxaline-based Pdots for effective and specific cellular and subcellular labeling without any noticeable nonspecific binding. Moreover, the cytotoxicity of Pdots were evaluated on HeLa cells and zebrafish embryos to demonstrate their great biocompatibility. By taking advantage of their extreme brightness and minimal cytotoxicity, we performed, for the first time, in vivo microangiography imaging on living zebrafish embryos using Pdots. These quinoxaline-based NIR-fluorescent Pdots are anticipated to find broad use in a variety of in vitro and in vivo biological research.

Background: The Zap1 transcription factor is a central player in the response of yeast to changes in zinc status. We previously used transcriptome profiling with DNA microarrays to identify 46 potential Zap1 target genes in the yeast genome. In this new study, we used complementary methods to identify additional Zap1 target genes.

Zinc deficiency is a potential risk factor for disease in humans because it leads to increased oxidative stress and DNA damage. We show here that the yeast Saccharomyces cerevisiae also experiences oxidative stress when zinc-deficient, and we have identified one mechanism yeast cells use to defend themselves against this stress. The Zap1p transcription factor is a central player in the response of yeast to zinc deficiency. To identify genes important for growth in low zinc, DNA microarrays were used to identify genes directly regulated by Zap1p. We found that the TSA1 gene is one such Zap1p target whose expression is increased under zinc deficiency. TSA1 encodes a cytosolic thioredoxin-dependent peroxidase responsible for degrading hydrogen peroxide and organic hydroperoxides. Consistent with its regulation by Zap1p, we showed that tsa1⌬ mutants have a growth defect in low zinc that can be suppressed by zinc but not by other metals. Anaerobic conditions also suppressed the tsa1⌬ low zinc growth defect indicating that oxidative stress is the likely cause of the poor growth. Consistent with this hypothesis, we demonstrated that zinc deficiency causes increased reactive oxygen species in wild type cells and that this increase is further exacerbated in tsa1⌬ mutants. The role of this regulation by Zap1p in limiting oxidative stress in low zinc was confirmed when the Zap1p-binding site was specifically mutated in the chromosomal TSA1 promoter. Thus, we conclude that TSA1 induction by Zap1p is an adaptive response to deal with the increased oxidative stress caused by zinc deficiency.Zinc is an essential nutrient for all organisms because it is required as a structural and/or catalytic cofactor by hundreds of proteins. It was recently estimated that almost 3000 different proteins encoded by the human genome, i.e. ϳ10% of all human proteins, bind zinc (1). Because of the many functions of zinc, nutritional deficiency of this metal perturbs a wide variety of processes. In mammals, zinc deficiency is associated with retarded growth, immune system dysfunction, and impaired reproduction (2, 3). In addition, studies both in vitro and in vivo have established that zinc deficiency leads to increased oxidative stress in mammalian cells (for review see Ref. 4). Zinc deficiency is associated with increased levels of lipid and protein oxidation (5, 6). In addition, the oxidative stress associated with zinc deficiency leads to increased levels of DNA damage (6 -8). For these reasons, zinc deficiency has been proposed to be an important risk factor for cancer and other human diseases (9, 10).Because of the important functions of zinc, organisms have evolved with regulatory circuits to maintain intracellular zinc at optimal levels. In the yeast Saccharomyces cerevisiae, zinc homeostasis is maintained through both transcriptional and post-transcriptional mechanisms (11). At the level of transcription, expression of many genes is induced in zinc-limited cells through the action of the Zap1p transcription factor (12). Under low zinc con...

The Zap1 transcription factor is a central player in the response of yeast to changes in zinc status. Previous studies identified over 80 genes activated by Zap1 in zinc-limited cells. In this report, we identified 36 genes repressed in a zinc-and Zap1-responsive manner. As a result, we have identified a new mechanism of Zap1-mediated gene repression whereby transcription of the MET3, MET14, and MET16 genes is repressed in zinc-limited cells. These genes encode the first three enzymes of the sulfate assimilation pathway. Zinc is an essential nutrient for all organisms because it is required as a structural or catalytic cofactor by many proteins. It was recently estimated that about 10% of the ϳ30,000 proteins encoded by the human genome need zinc for their function (1). Zinc deficiency perturbs a wide variety of processes and is associated with many disease symptoms in mammals (2). Excess zinc can also be toxic to cells (3). Thus, organisms have evolved with mechanisms to tightly control intracellular zinc levels. We have examined cellular responses to zinc deficiency in the yeast Saccharomyces cerevisiae (4, 5). In this yeast, the Zap1 transcription factor is a central player in their response to zinc deficiency (6). For many of its target genes, Zap1 acts as an activator of transcription and increases gene expression when zinc levels are low. To perform this function, Zap1 binds to one or more zinc-responsive elements (ZREs) 2 in the promoters of its target genes. The consensus sequence for a ZRE is ACCTT-NAAGGT (4, 7). Previous studies have identified a large number of Zap1 target genes in the yeast genome (4,5,8,9). Many activated Zap1 targets contribute to zinc homeostasis. For example, Zap1 induces its own expression by positive autoregulation (10). In addition, ZRT1, ZRT2, and FET4 encode transporter proteins responsible for zinc uptake and are targets of Zap1 activation (11-13). Zap1 also induces expression of ZRT3 and ZRC1, which encode vacuolar zinc transporters involved in controlling zinc storage in the vacuole (14). In addition to maintaining zinc homeostasis, Zap1 regulates the expression of genes that play a more adaptive role in zinc-limited cells. For example, Zap1 controls the level of several lipid biosynthetic enzymes (DPP1, PIS1, EKI1, and CKI1) to maintain the levels of some membrane phospholipids and to alter the levels of others (15). Thus, Zap1 mediates both homeostatic and adaptive responses to zinc limitation.Zap1 also activates expression of an antioxidant gene, TSA1, which encodes the major cytosolic thioredoxin-dependent peroxidase, to combat the oxidative stress of zinc deficiency (16). Studies of mammalian cells have shown that zinc deficiency causes the increased accumulation of reactive oxygen species (17). We have observed that yeast also experience increased oxidative stress when grown under low zinc conditions (16). The source of this oxidative stress is unknown. Reactive oxygen species (ROS), including the superoxide anion, hydrogen peroxide (H 2 O 2 ), and hydroxyl radica...

This article presents the first work to fundamentally study the effect of steric hindrance in Pdot systems.