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1.
Most Candida albicans strains are heterozygous at the MTL (mating-type-like) locus, but mating occurs in hemi- or homozygous strains. The white-opaque switch process is repressed by the heterodimer of the MTLa1 and MTLalpha2 gene products, while mating genes are induced by a2 and alpha1. Mating occurs in opaque cells and produces tetraploid progeny. A small percentage (3-7%) of clinical isolates are homozygous at the MTL locus and most are mating-competent. MTL gene expression is controlled in part by a gene which activates MTLalpha genes and represses MTLa genes in response to hemoglobin. A failure to find meiosis and the lack of evidence of mating in vivo, together with some of the properties of opaque cells, leads to the suggestion that mating may have persisted because the tightly associated switch facilitates the commensal lifestyle of this fungus. 相似文献
2.
Lockhart SR Pujol C Daniels KJ Miller MG Johnson AD Pfaller MA Soll DR 《Genetics》2002,162(2):737-745
The relationship between the configuration of the mating type locus (MTL) and white-opaque switching in Candida albicans has been examined. Seven genetically unrelated clinical isolates selected for their capacity to undergo the white-opaque transition all proved to be homozygous at the MTL locus, either MTLa or MTLalpha. In an analysis of the allelism of 220 clinical isolates representing the five major clades of C. albicans, 3.2% were homozygous and 96.8% were heterozygous at the MTL locus. Of the seven identified MTL homozygotes, five underwent the white-opaque transition. Of 20 randomly selected MTL heterozygotes, 18 did not undergo the white-opaque transition. The two that did were found to become MTL homozygous at very high frequency before undergoing white-opaque switching. Our results demonstrate that only MTL homozygotes undergo the white-opaque transition, that MTL heterozygotes that become homozygous at high frequency exist, and that the generation of MTL homozygotes and the white-opaque transition occur in isolates in different genetic clades of C. albicans. Our results demonstrate that mating-competent strains of C. albicans exist naturally in patient populations and suggest that mating may play a role in the genesis of diversity in this pernicious fungal pathogen. 相似文献
3.
Pujol C Daniels KJ Lockhart SR Srikantha T Radke JB Geiger J Soll DR 《Eukaryotic cell》2004,3(4):1015-1027
Because Candida dubliniensis is closely related to Candida albicans, we tested whether it underwent white-opaque switching and mating and whether white-opaque switching depended on MTL homozygosity and mating depended on switching, as they do in C. albicans. We also tested whether C. dubliniensis could mate with C. albicans. Sequencing revealed that the MTLalpha locus of C. dubliniensis was highly similar to that of C. albicans. Hybridization with the MTLa1, MTLa2, MTLalpha1, and MTLalpha2 open reading frames of C. albicans further revealed that, as in C. albicans, natural strains of C. dubliniensis exist as a/alpha, a/a, and alpha/alpha, but the proportion of MTL homozygotes is 33%, 10 times the frequency of natural C. albicans strains. C. dubliniensis underwent white-opaque switching, and, as in C. albicans, the switching was dependent on MTL homozygosis. C. dubliniensis a/a and alpha/alpha cells also mated, and, as in C. albicans, mating was dependent on a switch from white to opaque. However, white-opaque switching occurred at unusually high frequencies, opaque cell growth was frequently aberrant, and white-opaque switching in many strains was camouflaged by an additional switching system. Mating of C. dubliniensis was far less frequent in suspension cultures, due to the absence of mating-dependent clumping. Mating did occur, however, at higher frequencies on agar or on the skin of newborn mice. The increases in MTL homozygosity, the increase in switching frequencies, the decrease in the quality of switching, and the decrease in mating efficiency all reflected a general deterioration in the regulation of developmental processes, very probably due to the very high frequency of recombination and genomic reorganization characteristic of C. dubliniensis. Finally, interspecies mating readily occurred between opaque C. dubliniensis and C. albicans strains of opposite mating type in suspension, on agar, and on mouse skin. Remarkably, the efficiency of interspecies mating was higher than intraspecies C. dubliniensis mating, and interspecies karyogamy occurred readily with apparently the same sequence of nuclear migration, fusion, and division steps observed during intraspecies C. albicans and C. dubliniensis mating and Saccharomyces cerevisiae mating. 相似文献
4.
Candida albicans strains that are homozygous at the mating type locus (MTLa or MTLalpha) can spontaneously switch from the normal round-to-oval yeast cell morphology to an elongated, so-called opaque cell form that can mate with opaque cells of the opposite mating type. In response to environmental signals, C. albicans also undergoes a transition from yeast to filamentous growth, which is negatively regulated by the general repressor Tup1p. Therefore, C. albicans mutants in which the TUP1 gene is inactivated grow constitutively in the filamentous form. We found that tup1Delta mutants of the MTLalpha strain WO-1 are still able to undergo phenotypic switching. Although the mutants had lost the capacity to grow in the normal yeast (white) or opaque forms, they could still reversibly switch between four different cell and colony phenotypes (designated as fuzzy, frizzy, wrinkled and smooth) at a frequency of about 10(-3) to 10(-4). Deletion of TUP1 resulted in deregulated expression of phase-specific genes. While the white-specific WH11 gene was constitutively expressed in all four cell types, the opaque-specific SAP1 gene remained repressed and the opaque-specific OP4 gene was weakly induced in all phase variants. In spite of the loss of white- and opaque-specific cell morphology and gene expression, the tup1Delta mutants retained an important characteristic of their wild-type parent, the ability to switch to a mating-competent form. The three filamentous phase variants (fuzzy, frizzy and wrinkled) all were able to mate and produce recombinant progeny with opaque cells of an MTLa strain at frequencies that were somewhat lower than those of normal opaque cells, whereas the smooth phase variant was unable to do so. Therefore, although deletion of TUP1 in C. albicans MTLalpha cells affects cellular morphology and gene expression patterns, the mutants can still reversibly switch between mating-competent and -incompetent cell types and mate with a partner of the opposite mating type. 相似文献
5.
Identification and characterization of MFA1, the gene encoding Candida albicans a-factor pheromone
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In the opaque state, MTLa and MTLalpha strains of Candida albicans are able to mate, and this mating is directed by a pheromone-mediated signaling process. We have used comparisons of genome sequences to identify a C. albicans gene encoding a candidate a-specific mating factor. This gene is conserved in Candida dubliniensis and is similar to a three-gene family in the related fungus Candida parapsilosis but has extremely limited similarity to the Saccharomyces cerevisiae MFA1 (ScMFA1) and ScMFA2 genes. All these genes encode C-terminal CAAX box motifs characteristic of prenylated proteins. The C. albicans gene, designated CaMFA1, is found on chromosome 2 between ORF19.2165 and ORF19.2219. MFA1 encodes an open reading frame of 42 amino acids that is predicted to be processed to a 14-amino-acid prenylated mature pheromone. Microarray analysis shows that MFA1 is poorly expressed in opaque MTLa cells but is induced when the cells are treated with alpha-factor. Disruption of this C. albicans gene blocks the mating of MTLa cells but not MTLalpha cells, while the reintegration of the gene suppresses this cell-type-specific mating defect. 相似文献
6.
Hemoglobin regulates expression of an activator of mating-type locus alpha genes in Candida albicans
Phenotypic switching from the white to the opaque phase is a necessary step for mating in the pathogenic fungus Candida albicans. Suppressing switching during vascular dissemination of the organism may be advantageous, because opaque cells are more susceptible to host defenses. A repressor of white-opaque switching, HBR1 (hemoglobin response gene 1), was identified based on its specific induction following growth in the presence of exogenous hemoglobin. Deletion of a single HBR1 allele allowed opaque phase switching and mating competence, accompanied by a lack of detectable MTL alpha1 and alpha2 gene expression and enhanced MTLa1 gene expression. Conversely, overexpression of Hbr1p or exposure to hemoglobin increased MTLalpha gene expression. The a1/alpha2 repressed target gene CAG1 was derepressed in the same mutant in a hemoglobin-sensitive manner. Regulation of CAG1 by hemoglobin required an intact MTLa1 gene. Several additional Mtlp targets were perturbed in HBR1 mutants in a manner consistent with commitment to an a mating phenotype, including YEL007w, MFalpha, HST6, and RAM2. Therefore, Hbr1 is part of a host factor-regulated signaling pathway that controls white-opaque switching and mating in the absence of allelic deletion at the MTL locus. 相似文献
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8.
A genome sequence survey shows that the pathogenic yeast Candida parapsilosis has a defective MTLa1 allele at its mating type locus
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Candida parapsilosis is responsible for ca. 15% of Candida infections and is of particular concern in neonates and surgical intensive care patients. The related species Candida albicans has recently been shown to possess a functional mating pathway. To analyze the analogous pathway in C. parapsilosis, we carried out a genome sequence survey of the type strain. We identified ca. 3,900 genes, with an average amino acid identity of 59% with C. albicans. Of these, 23 are predicted to be predominantly involved in mating. We identified a genomic locus homologous to the MTLa mating type locus of C. albicans, but the C. parapsilosis type strain has at least two internal stop codons in the MTLa1 open reading frame, and two predicted introns are not spliced. These stop codons were present in MTLa1 of all eight C. parapsilosis isolates tested. Furthermore, we found that all isolates of C. parapsilosis tested appear to contain only the MTLa idiomorph at the presumptive mating locus, unlike C. albicans and C. dubliniensis. MTLalpha sequences are present but at a different chromosomal location. It is therefore likely that all (or at least the majority) of C. parapsilosis isolates have a mating pathway that is either defective or substantially different from that of C. albicans. 相似文献
9.
Candida orthopsilosis and Candida metapsilosis are closely related to Candida parapsilosis, a major cause of infection in premature neonates. Mating has not been observed in these species. We show that ~190 isolates of C. parapsilosis contain only an MTLa idiomorph at the mating-type-like locus. Here, we describe the isolation and characterization of the MTL loci from C. orthopsilosis and C. metapsilosis. Among 16 C. orthopsilosis isolates, 9 were homozygous for MTLa, 5 were homozygous for MTLα, and 2 were MTLa/α heterozygotes. The C. orthopsilosis isolates belonged to two divergent groups, as characterized by restriction patterns at MTL, which probably represent subspecies. We sequenced both idiomorphs from each group and showed that they are 95% identical and that the regulatory genes are intact. In contrast, 18 isolates of C. metapsilosis contain only MTLα idiomorphs. Our results suggest that the role of MTL in determining cell type is being eroded in the C. parapsilosis species complex. The population structure of C. orthopsilosis indicates that mating may occur. However, expression of genes in the mating signal transduction pathway does not respond to exposure to alpha factor. C. parapsilosis is also nonresponsive, even when the GTPase-activating protein gene SST2 is deleted. In addition, splicing of introns in MTLa1 and MTLa2 is defective in C. orthopsilosis. Mating is not detected. The alpha factor peptide, which is the same sequence in C. parapsilosis, C. orthopsilosis, and C. metapsilosis, can induce a mating response in Candida albicans. It is therefore likely either that mating of C. orthopsilosis takes place under certain unidentified conditions or that the mating pathway has been adapted for other functions, such as cross-species communication. 相似文献
10.
Candida glabrata, the second most prevalent Candida species colonizing humans, possesses three mating type-like (MTL) loci (MTL1, MTL2, and MTL3). These loci contain pairs of MTL genes with their respective coding regions on complementary Crick and Watson DNA strands. Each pair of genes is separated by a shared intergenic promoter region, the same configuration found at the mating type loci of Saccharomyces cerevisiae. Two of the MTL loci, MTL1 and MTL2, contain either the MTLa1/MTLa2 configuration or the MTLalpha1/MTLalpha2 configuration in different strains. All but one of the 38 tested C. glabrata strains were either aaalpha or aalphaalpha. One test strain was alphaalphaalpha. Based on the mating type genotype, the MTL genes at the MTL1 or MTL2 loci, and the size of the XbaI fragment harboring MTL1 or MTL2, four classes of C. glabrata strains (I, II, III, and IV) were distinguished. Northern analysis revealed that strains were either a-expressors or alpha-expressors and that expression always reflected the genotype of either the MTL1 or MTL2 locus, depending on the class. The expression pattern in each class, therefore, is similar to that observed in S. cerevisiae, which harbors two silent cassette loci, HMR and HML, and the expression locus MAT. High-frequency phenotypic switching between core phenotypes in an alpha-expressing, but not in an a-expressing, strain modulated the level of MTL expression, suggesting a possible relationship between core phenotypic switching and mating. 相似文献
11.
Soll DR 《BioEssays : news and reviews in molecular, cellular and developmental biology》2004,26(1):10-20
A small proportion of clinical strains of Candida albicans undergo white-opaque switching. Until recently it was not clear why, since most strains carry the genes differentially expressed in the unique opaque phase. The answer to this enigma lies in the mating process. The majority of C. albicans strains are heterozygous for the mating type locus MTL (a/alpha) and cannot undergo white-opaque switching. However, when these cells undergo homozygosis at the mating type locus (i.e., become a/a or alpha/alpha), they can switch, and they must switch in order to mate. Even though the newly identified stages of mating mimic those of Saccharomyces cerevisiae, the process differs in its dependency on switching, and the effects switching has on gene regulation. This unique feature of C. albicans mating appears to be intimately intertwined with its pathogenesis. The unique, newly discovered dependencies of switching on homozygosis at the MTL locus and of mating on switching are, therefore, reviewed within the context of pathogenesis. 相似文献
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13.
Cell biology of mating in Candida albicans 总被引:3,自引:0,他引:3
14.
Many of the genes required for mating in Saccharomyces cerevisiae are also required for mating in Candida albicans 总被引:2,自引:0,他引:2
Candida albicans is the single, most frequently isolated human fungal pathogen. As with most fungal pathogens, the factors which contribute to pathogenesis in C. albicans are not known, despite more than a decade of molecular genetic analysis. Candida albicans was thought to be asexual until the discovery of the MTL loci homologous to the mating type (MAT) loci in Saccharomyces cerevisiae led to the demonstration that mating is possible. Using Candida albicans mutants in genes likely to be involved in mating, we analysed the process to determine its similarity to mating in Saccharomyces cerevisiae. We examined disruptions of three of the genes in the MAPK pathway which is involved in filamentous growth in both S. cerevisiae and C. albicans and is known to control pheromone response in the former fungus. Disruptions in HST7 and CPH1 blocked mating in both MTLa and MTL(alpha) strains, whereas disruptions in STE20 had no effect. A disruption in KEX2, a gene involved in processing the S. cerevisiae pheromone Mf(alpha), prevented mating in MTL(alpha) but not MTLa cells, whereas a disruption in HST6, the orthologue of the STE6 gene which encodes an ABC transporter responsible for secretion of the Mfa pheromone, prevented mating in MTLa but not in MTL(alpha) cells. Disruption of two cell wall genes, ALS1 and INT1, had no effect on mating, even though ALS1 was identified by similarity to the S. cerevisiae sexual agglutinin, SAG1. The results reveal that these two diverged yeasts show a surprising similarity in their mating processes. 相似文献
15.
Song Yi Nidhi Sahni Claude Pujol Karla J. Daniels Thyagarajan Srikantha Ning Ma David R. Soll 《Molecular microbiology》2009,71(4):925-947
Candida albicans strains homozygous at the mating type locus can switch from white to opaque, and must do so to mate. Opaque cells then secrete mating pheromones that stimulate opaque cells of opposite mating type to undergo mating. These same pheromones stimulate mating-incompetent white cells to become cohesive and adhesive, and enhance white cell biofilm development, a pathogenic trait. Stimulation is mediated through the same receptor, G protein complex and mitogen-activated protein kinase pathway. Here we present evidence that a C. albicans -specific 55-amino-acid region of the first intracellular loop, IC1, of the α-pheromone receptor Ste2p, is required for the α-pheromone response of white cells, but not that of opaque cells. This represents a unique regulatory configuration in which activation of a common pathway by the same ligand, the same receptor and the same signal transduction pathway is dependent on a unique region of an intracellular loop of the common receptor in one of the two responding phenotypes. 相似文献
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17.
Resistance of the pathogenic yeast Candida albicans to the antifungal agent fluconazole is often caused by active drug efflux out of the cells. In clinical C. albicans strains, fluconazole resistance frequently correlates with constitutive activation of the MDR1 gene, encoding a membrane transport protein of the major facilitator superfamily that is not expressed detectably in fluconazole-susceptible isolates. However, the molecular changes causing MDR1 activation have not yet been elucidated, and direct proof for MDR1 expression being the cause of drug resistance in clinical C. albicans strains is lacking as a result of difficulties in the genetic manipulation of C. albicans wild-type strains. We have developed a new strategy for sequential gene disruption in C. albicans wild-type strains that is based on the repeated use of a dominant selection marker conferring resistance against mycophenolic acid upon transformants and its subsequent excision from the genome by FLP-mediated, site-specific recombination (MPAR-flipping). This mutagenesis strategy was used to generate homozygous mdr1/mdr1 mutants from two fluconazole-resistant clinical C. albicans isolates in which drug resistance correlated with stable, constitutive MDR1 activation. In both cases, disruption of the MDR1 gene resulted in enhanced susceptibility of the mutants against fluconazole, providing the first direct genetic proof that MDR1 mediates fluconazole resistance in clinical C. albicans strains. The new gene disruption strategy allows the generation of specific knock-out mutations in any C. albicans wild-type strain and therefore opens completely novel approaches for studying this most important human pathogenic fungus at the molecular level. 相似文献
18.
The Candida albicans MYO5 gene encodes myosin I, a protein required for the formation of germ tubes and true hyphae. Because the polarized growth of opaque-phase cells in response to pheromone results in mating projections that can resemble germ tubes, we examined the role of Myo5p in this process. We localized green fluorescent protein (GFP)-tagged Myo5p in opaque-phase cells of C. albicans during both bud and shmoo formation. In vegetatively growing opaque cells, Myo5p is found at sites of bud emergence and bud growth, while in pheromone-stimulated cells, Myo5p localizes at the growing tips of shmoos. Intriguingly, cells homozygous for MTLa in which the MYO5 gene was deleted failed to switch efficiently from the white phase to the opaque phase, although ectopic expression of WOR1 from the MET3 promoter can convert myo5 mutants into mating-competent opaque cells. However, when WOR1 expression was shut off, the myo5-defective cells rapidly lost both their opaque phenotype and mating competence, suggesting that Myo5p is involved in the maintenance of the opaque state. When MYO5 is expressed conditionally in opaque cells, the opaque phenotype, as well as the mating ability of the cells, becomes unstable under repressive conditions, and quantitative real-time PCR demonstrated that the shutoff of MYO5 expression correlates with a dramatic reduction in WOR1 expression. It appears that while myosin I is not directly required for mating in C. albicans, it is involved in WOR1 expression and the white-opaque transition and thus is indirectly implicated in mating. 相似文献
19.
Chromosome loss followed by duplication is the major mechanism of spontaneous mating-type locus homozygosis in Candida albicans
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Candida albicans, which is diploid, possesses a single mating-type (MTL) locus on chromosome 5, which is normally heterozygous (a/alpha). To mate, C. albicans must undergo MTL homozygosis to a/a or alpha/alpha. Three possible mechanisms may be used in this process, mitotic recombination, gene conversion, or loss of one chromosome 5 homolog, followed by duplication of the retained homolog. To distinguish among these mechanisms, 16 spontaneous a/a and alpha/alpha derivatives were cloned from four natural a/alpha strains, P37037, P37039, P75063, and P34048, grown on nutrient agar. Eighteen polymorphic (heterozygous) markers were identified on chromosome 5, 6 to the left and 12 to the right of the MTL locus. These markers were then analyzed in MTL-homozygous derivatives of the four natural a/alpha strains to distinguish among the three mechanisms of homozygosis. An analysis of polymorphisms on chromosomes 1, 2, and R excluded meiosis as a mechanism of MTL homozygosis. The results demonstrate that while mitotic recombination was the mechanism for homozygosis in one offspring, loss of one chromosome 5 homolog followed by duplication of the retained homolog was the mechanism in the remaining 15 offspring, indicating that the latter mechanism is the most common in the spontaneous generation of MTL homozygotes in natural strains of C. albicans in culture. 相似文献