Antibody-independent activation of C1. II. Evidence for two classes of nonimmune activators of the classical pathway of complement

Nonimmune activation of the first component of complement (C1) by cardiolipin (CL) vesicles present specific features which were not demonstrated on immune complexes. CL vesicles which activate C1 in the presence of C1-inhibitor (C1-INH) were found to bind C1s in the absence of C1r, and to induce a specific C1r-independent cleavage of C1q-bound C1s. Therefore, several known natural nonimmune activators were analyzed by comparing their ability to activate C1 in the presence of C1-INH and to mediate a C1r-independent cleavage of C1s. Freshly isolated human heart mitochondria (HHM) activated C1 only in the absence of C1-INH. However, mitoplasts derived from HHM (HHMP) activated C1 regardless of the presence of C1-INH, and induced a specific cleavage of C1q-bound C1s. The same pattern was observed in the case of smooth E. coli and a semi-rough E. coli strain. DNA, known to activate C1 only in the absence of C1-INH, does not induce C1s cleavage in the absence of C1r. Thus, nonimmune activators can be classified into two distinct categories. "Strong" activators, such as CL vesicles, HHMP, or the semi-rough E. coli strain J5 can activate C1 in the presence of C1-INH. By using C1qs2 as a probe, they exhibit a specific, C1r-independent cleavage of C1s. C1s-binding to C1q is a critical factor for the activation process in this group. In the case of "weak" activators, such as E. coli smooth strains, DNA, or HHM, no C1s-binding to activator-bound C1q was detected, and C1r-independent C1s cleavage and C1 activation in the presence of C1-INH were not observed. As in the case of immune complexes, C1r activation appears to play a key role in the C1 activation by "weak" activators.     

The role of immune complexes in the activation of the first component of human complement

Immune complex-induced C1 activation and fluid phase C1 autoactivation have been compared in order to elucidate the immune complex role in the C1 activation process. Kinetic analyses revealed that immune complex-bound C1 activates seven times faster than fluid phase C1 spontaneously activates. The rate of spontaneous C1 activation increased after decreasing the solution ionic strength. In fact at one-half physiologic ionic strength (i.e., 0.08 M), the kinetics of spontaneous C1 activation were indistinguishable from the kinetics of activation of immune complex-bound C1 at physiologic ionic strength. The enhanced fluid phase C1 activation at low ionic strength resulted neither from C1 nor C1q aggregation, nor from selective effects on the C1r2S2 subunit; however, at the reduced ionic strength, the C1 association constant (defined for C1q + C1r2S2 in equilibrium C1qr2S2) did increase to 2.3 X 10(8) M-1, which is equal to that for C1 bound to an immune complex at physiologic ionic strength. Therefore, C1 can spontaneously activate in the fluid phase as rapidly as C1 on an immune complex when the strength of interaction between C1q and C1r2S2 is the same in both systems. In conclusion, under physiologic conditions, C1q and C1r2S2 are two weakly interacting proteins. Immune complexes provide a site for the assembly of a stable C1 complex, in which C1q and C1r2S2 remain associated long enough for C1q to activate C1r2S2. Thus, immune complexes enhance the intrinsic C1 autoactivation process by strengthening the association of C1q with C1r2S2.     

几类异质小麦雄性不育系育性恢复性的细胞遗传学研究

系统调查了4类异质(粘果、易变、偏凸、二角山羊草细胞质)1BL/lRS、非1BL/1RS小麦雄性不育系与其恢复系杂种F减数分裂中期Ⅰ出现单价体细胞频率,以及后期Ⅰ出现落后染色体和染色体桥细胞频率,并对中、后期染色体变异率与杂种F自交结实率进行了相关分析.结果表明(1)1BL/1RS型杂种在中期Ⅰ、后期Ⅰ染色体变异率要明显高于非1BL/1RS杂种;(2)4类异源细胞质在非1BL/1RS杂种中有着明显提高单价体细胞频率的作用;(3)在1BL/1RS杂种中,1B@1BL/1RS杂合核型染色体联会松弛,对单价体频率的影响远大于异源细胞质的影响;(4)1BL/1RS型杂种自交结实率与中期出现单价体细胞频率不直接相关,而与后期出现落后染色体和染色体桥细胞的频率呈高度负相关;(5)非1BL/1RS型杂种在减数分裂中、后期染色体行为相对稳定,易恢复且恢复度高,很有实际利用价值.     

Regulation of the extent of splicing of influenza virus NS1 mRNA: role of the rates of splicing and of the nucleocytoplasmic transport of NS1 mRNA.

Influenza virus NS1 mRNA is spliced by host nuclear enzymes to form NS2 mRNA, and this splicing is regulated in infected cells such that the steady-state amount of spliced NS2 mRNA is only about 10% of that of unspliced NS1 mRNA. This regulation would be expected to result from a suppression in the rate of splicing coupled with the efficient transport of unspliced NS1 mRNA from the nucleus. To determine whether the rate of splicing of NS1 mRNA was controlled by trans factors in influenza virus-infected cells, the NS1 gene was inserted into an adenovirus vector. The rates of splicing of NS1 mRNA in cells infected with this vector and in influenza virus-infected cells were measured by pulse-labeling with [3H]uridine. The rates of splicing of NS1 mRNA in the two systems were not significantly different, strongly suggesting that the rate of splicing of NS1 mRNA in influenza virus-infected cells is controlled solely by cis-acting sequences in NS1 mRNA itself. In contrast to the rate of splicing, the extent of splicing of NS1 mRNA in the cells infected by the adenovirus recombinant was dramatically increased relative to that occurring in influenza virus-infected cells. This could be attributed largely, if not totally, to a block in the nucleocytoplasmic transport of unspliced NS1 mRNA in the recombinant-infected cells. Most of the unspliced NS1 mRNA was in the nuclear fraction, and no detectable NS1 protein was synthesized. When the 3' splice site of NS1 mRNA was inactivated by mutation, NS1 mRNA was transported and translated, indicating that the transport block occurred because NS1 rRNA was committed to the splicing pathway. This transport block is apparently obviated in influenza virus-infected cells. These experiments demonstrate the important role of the nucleocytoplasmic transport of unspliced NS1 mRNA in regulating the extent of splicing of NS1 mRNA.     

Structures of the O-linked oligosaccharides of the major cell surface sialoglycoprotein of MAT-B1 and MAT-C1 ascites sublines of the 13762 rat mammary adenocarcinoma

Structures of the principal O-glycosides from the major cell surface sialoglycoprotein (ASGP-1) of the MAT-B1 and MAT-C1 ascites sublines of the 13762 rat mammary adenocarcinoma have been determined. Oligosaccharitols were released by alkaline borohydride treatments of ASGP-1 and purified by gel filtration, DEAE-Sephadex ion exchange chromatography, and high performance liquid chromatography. On the basis of carbohydrate composition, methylation analysis, periodate oxidation, and exoglycosidase digestion, the five major oligosaccharides released by mild alkaline borohydride were assigned the following structures: Component II-3: (NeuAc alpha 2----3Gal beta 1----4GlcNAc beta 1----6)Ga 1 NAcOH(3----1 betaGa 1 3----2 alpha NeuAc) III-2a: (Ga 1 beta 1----4G1cNAc beta 1----6)Ga 1 NAcOH(3----1 beta Ga 1 3----2 alpha NeuAc) III-2c: (Ga 1 alpha 1----3Ga 1 beta 1----4G1cNAc beta 1----6) Ga 1 NAcOH(3----1 beta Ga 1 3----2 alpha NeuAc) IV-1a: (Ga 1 beta 1----4G 1 cNAc beta 1----6)Ga 1 NAcOH(3----1 beta Ga 1) IV-1c: (Ga 1 alpha 1----3Ga 1 beta 1----4G 1 cNAc beta 1----6) Ga 1 NAcOH(3----1 beta Ga 1) Fucosylated derivatives of III-2a, IV-1a, and IV-1c were found in smaller amounts with the fucose tentatively assigned to the 2-position of the lactosamine galactose. Components II-3, III-2a, and the fucosylated derivative of III-2A were found in both MAT-B1 and MAT-C1 sublines. The alpha-galactosides were found in detectable quantities only in subline MAT-B1. Oligosaccharides from MAT-C1 cells were enriched in sialic acid when compared to those from MAT-B1 cells. These results suggest that the 13762 ascites sublines, which bear different oligosaccharides, will provide models useful for the investigation of mechanisms regulating the expression of structures of the larger O-linked oligosaccharides.     

Comparison of the evolution of recent and late phase of old influenza A (H1N1) viruses

A comparison of the evolutionary tree of new influenza A (H1N1) viruses to that of old H1N1 viruses which disappeared in 1957 was performed. The evolutionary trees of the hemagglutinin (HA) molecule based on amino acid sequences of the HA1 polypeptide were constructed with old and new H1N1 viruses isolated from 1947 to 1957 and 1986 to 2000, respectively. The evolutionary history of recent H1N1 viruses was similar to that of old H1N1 viruses just before the disappearance in two respects. Firstly, both viruses did not originate from the viruses of the previous H1N1 epidemic season but originated from the viruses branched off at the same point on the mainstream stem as the viruses of two H1N1 epidemic seasons earlier. Secondly, recent H1N1 viruses mainly circulating in Japan have a deletion at amino acid residue 134, located close to residue 131, which was deleted in old H1N1 viruses at the time of the disappearance. However, different from the evolutionary history of old H1N1 viruses, in the 1999/2000 H1N1 epidemic season, the H1N1 viruses which were located on the same lineage as the previous epidemic viruses were also isolated sporadically in Japan.     

嗜热四膜虫异染色质蛋白Tcd1磷酸化位点的突变分析

四膜虫异染色质蛋白Tcd1在有性生殖时期特异表达,在大核基因组重排以及修复过程中发挥作用。磷酸化蛋白质组学分析表明,Tcd1存在3个磷酸化位点：S301,S303和S535。然而,Tcd1磷酸化修饰与其功能的关系并不清楚。本研究对TCD1基因的3个磷酸化位点进行了模拟磷酸化和模拟去磷酸化定点突变,获得模拟磷酸化突变基因TCD1S301D (TCD1S1D)、TCD1S301DS303D (TCD1S2D)与TCD1S301DS303DS535D (TCD1S3D) 和模拟去磷酸化的突变基因TCD1S301A (TCD1S1A)、TCD1S301AS303A (TCD1S2A)与TCD1S301AS303AS535A (TCD1S3A)。分别构建了不同突变体的过表达载体,转化四膜虫细胞并筛选获得不同突变体细胞株。Western印迹分析表明,Tcd1S1D、Tcd1S2D、Tcd1S3D与Tcd1S1A、Tcd1S2A和Tcd1S3A在四膜虫有性生殖期表达。免疫荧光定位分析发现,Tcd1S1D点状定位于细胞质中,Tcd1S2D在有性生殖初期点状定位于细胞质中,在新大核上形成均匀的定位,Tcd1S3D无法定位于亲本大核上,只是均匀定位于新大核上。Tcd1S2A和Tcd1S3A在新大核形成异常的块状定位,并且与异染色质蛋白Pdd1不能共定位。结果表明,Tcd1不同位点的磷酸化和去磷酸化修饰的动态变化决定了其在四膜虫细胞中的定位模式。     

Role of MAPK phosphatase-1 in the induction of monocyte chemoattractant protein-1 during the course of adipocyte hypertrophy

Monocyte chemoattractant protein-1 (MCP-1), an important chemokine whose expression is increased during the course of obesity, plays a role in macrophage infiltration into obese adipose tissue. This study was designed to elucidate the role of mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1) in the induction of MCP-1 during the course of adipocyte hypertrophy. We examined the time course of MKP-1 and MCP-1 mRNA expression and extracellular signal-regulated kinase (ERK) phosphorylation in the adipose tissue from mice rendered mildly obese by a short term high fat diet. We also studied the role of MKP-1 in the induction of MCP-1 in 3T3-L1 adipocytes during the course of adipocyte hypertrophy. MCP-1 mRNA expression was increased, followed by ERK activation and down-regulation of MKP-1, an inducible dual specificity phosphatase to inactivate ERK, in the adipose tissue at the early stage of obesity induced by a short term high fat diet, when macrophages are not infiltrated. Down-regulation of MKP-1 preceded ERK activation and increased production of MCP-1 in 3T3-L1 adipocytes in vitro during the course of adipocyte hypertrophy. Adenovirus-mediated restoration of MKP-1 in hypertrophied 3T3-L1 adipocytes reduced the otherwise increased ERK phosphorylation, thereby leading to the significant reduction of MCP-1 mRNA expression. This study provides evidence that the down-regulation of MKP-1 is critical for increased production of MCP-1 during the course of adipocyte hypertrophy.     

Regulation of subcellular localization of alpha1-adrenoceptor subtypes

Alpha1-adrenergic receptors (AR) are members of the superfamily of G protein-coupled receptors (GPCRs) which mediate the effects of the sympathetic nervous system. Alpha1-AR comprise a heterogeneous family of three distinct isoforms of alpha1A, alpha1B and alpha1D; however, very little is known about their difference in physiological role or regulation. We have recently observed a subtype-specific differences in subcellular localization of alpha1-ARs; thus, alpha1A-AR predominantly localize intracellularly, while alpha1B-AR on the cell surface. To examine the molecular mechanism for the subtype-specific differences in subcellular localization, we conducted a search for novel proteins that interact with the alpha1B-AR, specifically focusing on the carboxyl-terminal cytoplasmic domain. Using interaction cloning and biochemical techniques, we demonstrate that gC1q-R interacts with alpha1B-AR in vitro and in vivo through the specific site, and that in cells which co-express alpha1B-AR and gC1q-R, the subcellular localization of alpha1B-AR is markedly altered and its expression is down-regulated. These results suggest that gC1q-R plays a role in the regulation of the subcellular localization as well as the function of alpha1B-ARs.     

Activation of platelet-transforming growth factor beta-1 in the absence of thrombospondin-1

Thrombospondin-1 (TSP-1) has been shown to bind and activate transforming growth factor-beta1 (TGF-beta1). This observation raises the possibility that TSP-1 helps to sequester TGF-beta1 in platelet alpha granules and activates TGF-beta1 once both proteins are secreted. Herein, we evaluated the level of active and latent TGF-beta1 in the plasma and in the supernatant of thrombin-treated platelets from TSP-1 null and wild-type mice on two genetic backgrounds (C57BL/6 and 129Sv). The plasminogen activator inhibitor-1/luciferase bioassay and an immunological assay were used to determine active and latent TGF-beta1. No significant differences were observed in the levels of active and latent TGF-beta1 in the supernatant of thrombin-treated platelets from TSP-1 null and wild-type mice. Active and latent TGF-beta1 were significantly increased in the plasma and platelets of C57BL/6 mice as compared with 129Sv mice. In addition, there was an increase of plasma level of latent TGF-beta1 in TSP-1 null mice as compared with wild-type mice on the C57BL/6 background but not on the 129Sv background. No active TGF-beta1 was observed in the plasma of either TSP-1 null and wild-type mice. These data indicate that TSP-1 does not function as a chaperon for TGF-beta1 during platelet production and does not activate significant quantities of secreted TGF-beta1 despite a vast excess in the number of TSP-1 molecules as compared with TGF-beta1 molecules. Because platelet releasates from TSP-1 null mice contain active TGF-beta1, we suggest that other important mechanisms of physiological activation of TGF-beta1 probably exist in platelets.     

Mechanism of cytotoxicity of aflatoxin B1: role of cytochrome P1-450

A mouse hepatoma cell line, Hepa-1, is highly sensitive to the toxic effects of Aflatoxin B1 (AFB1). Half maximal survival (LD50) of cells occurs at 0.068 ug AFB1/ml. Benzo(a)anthracene, which induces aryl hydrocarbon hydroxylase and cytochrome P1-450 in Hepa-1, causes a slight increase in the toxicity of AFB1 (LD50 = 0.034 ug/ml). An aryl hydrocarbon hydroxylase- and cytochrome P1-450-deficient mutant of Hepa-1 is, however, over 100 times more resistant to AFB1 than Hepa-1. Almost no decline in survival is observed at 5 ug AFB1/ml. Cytochrome P1-450 thus effects strongly on the cytotoxicity of AFB1 in these cells. The basal activity in Hepa-1 is enough to elicit an almost full toxic effect. AFB1, although a substrate for cytochrome P1-450, does not act as an inducer of aryl hydrocarbon hydroxylase.     

Requirement of a specific group of sphingolipid-metabolizing enzyme for growth of yeast Saccharomyces cerevisiae under impaired metabolism of glycerophospholipids

Sphingolipids play critical roles in many physiologically important events in yeast Saccharomyces cerevisiae. In this study, we screened for yeast mutants showing high sensitivity to Aureobasidin A, an inhibitor of inositol phosphorylceramide synthase, and found that a lack of SAC1 encoding phosphoinositides phosphatase causes high sensitivity to the inhibitor. Double mutation analysis involving the SAC1 and non-essential sphingolipid-metabolizing enzyme genes revealed that csg1Δ, csg2Δ, ipt1Δ or scs7Δ causes synthetic lethality with deletion of SAC1. As previously reported, SAC1-repressed cells exhibited a reduced cellular phosphatidylserine (PS) level, and overexpression of PSS1 encoding PS synthase complemented the growth defects of scs7Δ, csg1Δ and ipt1Δ cells under SAC1-repressive conditions. Furthermore, repression of PSS1 expression resulted in synthetic growth defect with the deletion of CSG1, IPT1 or SCS7. The growth defects of scs7Δ, csg1Δ and ipt1Δ cells under SAC1- or PSS1-repressive conditions were also complemented by overexpression of Arf-GAP AGE1, which encodes a protein related to membrane trafficking. Under SAC1-repressive conditions, scs7Δ, csg1Δ and ipt1Δ cells showed defects in vacuolar morphology, which were complemented by overexpression of each of PSS1 and AGE1. These results suggested that a specific group of sphingolipid-metabolizing enzyme is required for yeast cell growth under impaired metabolism of glycerophospholipids.     

Further evidence for the mandatory nature of polysaccharide debranching for the aggregation of semicrystalline starch and for overlapping functions of debranching enzymes in Arabidopsis leaves

Four isoforms of debranching enzymes are found in the genome of Arabidopsis (Arabidopsis thaliana): three isoamylases (ISA1, ISA2, and ISA3) and a pullulanase (PU1). Each isoform has a specific function in the starch pathway: synthesis and/or degradation. In this work we have determined the levels of functional redundancy existing between these isoforms by producing and analyzing different combinations of mutations: isa3-1 pu1-1, isa1-1 isa3-1, and isa1-1 isa3-1 pu1-1. While the starch content strongly increased in the isa3-1 pu1-1 double mutant, the latter decreased by over 98% in the isa1-1 isa3-1 genotype and almost vanished in triple mutant combination. In addition, whereas the isa3-1 pu1-1 double mutant synthesizes starch very similar to that of the wild type, the structure of the residual starch present either in isa1-1 isa3-1 or in isa1-1 isa3-1 pu1-1 combination is deeply affected. In the same way, water-soluble polysaccharides that accumulate in the isa1-1 isa3-1 and isa1-1 isa3-1 pu1-1 genotypes display strongly modified structure compared to those found in isa1-1. Taken together, these results show that in addition to its established function in polysaccharide degradation, the activity of ISA3 is partially redundant to that of ISA1 for starch synthesis. Our results also reveal the dual function of pullulanase since it is partially redundant to ISA3 for degradation and to ISA1 for synthesis. Finally, x-ray diffraction analyses suggest that the crystallinity and the presence of the 9- to 10-nm repetition pattern in starch precisely depend on the level of debranching enzyme activity.     

Cell expression of MMP-1 and TIMP-1 in co-cultures of human gingival fibroblasts and monocytes: the involvement of ICAM-1

Matrix metalloproteinase-1 (MMP-1) plays an important role in the degradation of collagen in inflammatory diseases. The aim of this study was to investigate the cellular expression of MMP-1 and its inhibitor, tissue inhibitor of metalloproteinase-1 (TIMP-1), in gingival fibroblasts co-cultured with monocytes and the possible mediating role of intercellular adhesion molecule-1 (ICAM-1). In co-cultures, the expression of MMP-1 and TIMP-1 increased in fibroblasts, but not in monocytes, although the number of MMP-1+ and TIMP-1+ adhered monocytes increased. Moreover, ICAM-1 expression in both fibroblasts and adhered monocytes increased. In the presence of an anti-ICAM-1 antibody, the expression of MMP-1 in fibroblasts decreased whereas the number of TIMP-1+ adhered monocytes increased. The p38 MAPK inhibitor SB203580 reduced MMP-1 expression in fibroblasts, as well as ICAM-1 expression in both fibroblasts and adhered monocytes. The results suggest that co-culture with monocytes enhances cellular expression of MMP-1 and TIMP-1 in gingival fibroblasts, and that the increased MMP-1 expression, in contrast to TIMP-1, is partly mediated by the adhesion molecule ICAM-1 and the p38 MAPK signal pathway.     

Inhibition of biological actions of big endothelin-1 by phosphoramidon

Endothelin (ET)-1 and big ET-1 both caused contraction of isolated porcine coronary arteries, but the potency of big ET-1 was 1/100-1/200 that of ET-1. These responses were independent of the vascular endothelium. Phosphoramidon blocked the vasoconstriction caused by 30 nM big ET-1, but was ineffective on the action of 0.3 nM ET-1. Also in vivo, phosphoramidon had no effect on the ET-1-induced pressor actions, but blocked the pressor and airway-contractile responses to big ET-1 in rats and/or guinea pigs. Thus, it is likely that the vascular responses to exogenous big ET-1 are at least in part due to its conversion to ET-1 by a phosphoramidon-sensitive ET converting enzyme(s) in the vascular smooth muscle in vitro and in vivo.     

Effect of Ppd-1 on the expression of flowering-time genes in vegetative and reproductive growth stages of wheat

The photoperiod sensitivity gene Ppd-1 influences the timing of flowering in temperate cereals such as wheat and barley. The effect of Ppd-1 on the expression of flowering-time genes was assessed by examining the expression levels of the vernalization genes VRN1 and VRN3/WFT and of two CONSTANS-like genes, WCO1 and TaHd1, during vegetative and reproductive growth stages. Two near-isogenic lines (NILs) were used: the first carried a photoperiod-insensitive allele of Ppd-1 (Ppd-1a-NIL), the other, a photoperiod-sensitive allele (Ppd-1b-NIL). We found that the expression pattern of VRN1 was similar in Ppd-1a-NIL and Ppd-1b-NIL plants, suggesting that VRN1 is not regulated by Ppd-1. Under long day conditions, VRN3/WFT showed similar expression patterns in Ppd-1a-NIL and Ppd-1b-NIL plants. However, expression differed greatly under short day conditions: VRN3/WFT expression was detected in Ppd-1a-NIL plants at the 5-leaf stage when they transited from vegetative to reproductive growth; very low expression was present in Ppd-1b-NIL throughout all growth stages. Thus, the Ppd-1b allele acts to down-regulate VRN3/WFT under short day conditions. WCO1 showed high levels of expression at the vegetative stage, which decreased during the phase transition and reproductive growth stages in both Ppd-1a-NIL and Ppd-1b-NIL plants under short day conditions. By contrast to WCO1, TaHd1 was up-regulated during the reproductive stage. The level of TaHd1 expression was much higher in Ppd-1a-NIL than the Ppd-1b-NIL plants, suggesting that the Ppd-1b allele down-regulates TaHd1 under short day conditions. The present study indicates that down-regulation of VRN3/WFT together with TaHd1 is the cause of late flowering in the Ppd-1b-NIL plants under short day conditions.     

Antagonistic interplay of Swi1 and Tup1 on filamentous growth of Candida albicans

Candida albicans is a polymorphic human opportunistic pathogen in which the Swi-Snf complex functions as an activator whereas Tup1 acts as a general repressor during the yeast-hyphae transition. In Saccharomyces cerevisiae, the interplay between the Swi-Snf complex and the Tup1-Ssn6 repressive complex regulates the balance between active and repressed chromatin structures of a number of genes. To study the interplay between Candida albicans Swi1 and Tup1 and their effects on morphogenesis, we analyzed phenotypes of swi1/swi1, tup1/tup1 and swi1/swi1 tup1/tup1 mutants under various growth conditions. The swi1/swi1 mutant failed to form true hyphae, whereas the tup1/tup1 mutant exhibited constitutive filamentous growth. Deletion of SWI1 in the tup1/tup1 mutant completely blocked hyphal growth under all the conditions examined. Under aerobic conditions, the swi1/swi1 tup1/tup1 mutant most resembled the swi1/swi1 mutant in phenotype, actin polarization and gene expression pattern. In invaded agar, the double mutant showed similar phenotypes as the swi1/swi1 mutant, while under embedded conditions, it grew as a pseudohypha-like form different from that of the wild-type strain, swi1/swi1 or tup1/tup1 mutants. These results suggest that Swi1 may play a dominant role by antagonizing the repressive effect of the Tup1 on hyphal development in C. albicans.