Inducible expression of a gene encoding an L41 ribosomal protein responsible for the cycloheximide-resistant phenotype in the yeast Candida maltosa

Segmentation of yeast cell's bright-field image with an edge-tracing algorithm

Phenotype analysis of yeast cell requires high-throughput imaging and automatic analysis of abundant image data. At first, each cell needs to be segmented and labeled in the bright-field images. However, the ambiguous boundary of bright-field yeast cell images leads to the failure of traditional segmentation algorithms. We propose a segmentation algorithm based on the morphological characteristics of yeast cells. Seed points are first identified along the cell contour and then connected by an edge tracing approach. In this way, "ill-detected" noise points are removed so that edges of yeast cells can be successfully extracted in bright-field images with sparsely distributed cells. In densely packed images, yeast cells with normal morphology can also be correctly segmented and labeled.

A Gcn4p homolog is essential for the induction of a ribosomal protein L41 variant responsible for cycloheximide resistance in the yeast Candida maltosa

Cycloheximide (CYH) resistance in the yeast Candida maltosa is based on the inducible expression of genes encoding a variant of ribosomal protein L41-Q, with glutamine at position 56 instead of the proline found in normal L41. The promoter of L41-Q2a, one of the L41-Q gene alleles encoding L41-Q, has an element similar to the Gcn4p-responsive element of Saccharomyces cerevisiae. In a previous study, this element was shown to be essential for the induction of L41-Q by CYH. In the present study, a C. maltosa GCN4 homolog, C-GCN4, was cloned. It had a long 5'-leader region with three upstream open reading frames. Enhanced expression of the C-GCN4 reporter fusion gene upon the addition of 3-aminotriazole or by mutations in start codons of all three upstream open reading frames indicates that C-GCN4 expression is under translation repression as was seen with GCN4. The C-GCN4-depleted mutant was unable to grow in a nutrient medium containing CYH and did not express L41-Q genes. Recombinant C-Gcn4p bound to the consensus DNA element for Gcn4p, 5'-(G/A)TGACTCAT-3', located upstream of L41-Q2a. Thus, C-Gcn4p, which likely functions in the general control of amino acid biosynthesis, is essential for the expression of L41-Q genes.

Two membrane proteins located in the Nag regulon of Candida albicans confer multidrug resistance

Pathogenic fungus Candida albicans can efficiently utilize the aminosugar N-acetylglucosamine (GlcNAc) as energy source. Since the mucosal membrane, the site of infection is rich in amino sugars, this specific adaptation is important for the establishment of infection. The genes encoding for the enzymes of the GlcNAc catabolic pathway, GlcNAc kinase (HXK1), GlcNAc-6-phosphate deacetylase (DAC1), and glucosamine-6-phosphate deaminase (NAG1), are present in a cluster, the Nag regulon, which is associated with virulence. In this study, we have characterized two genes, TMP1 and TMP2, present within the Nag regulon, upstream to DAC1. They encode two membrane associated sugar transporters of the major facilitator superfamily (MFS). The null mutant of TMP1 and TMP2 is able to grow in GlcNAc, implying that they are not involved in GlcNAc transport. However, it shows increased susceptibility to a number of unrelated antifungal compounds such as cycloheximide, 4-nitroquinoline-N-oxide, and 1-10 phenanthroline. Northern blot analysis revealed that TMP1 and TMP2 are upregulated in response to these drugs, suggesting that they function as multiple drug efflux pumps.

The SCR1 gene from Schwanniomyces occidentalis encodes a highly hydrophobic polypeptide, which confers ribosomal resistance to cycloheximide

In Saccharomyces cerevisiae, the SCR1 gene from Schwanniomyces occidentalis is known to induce ribosomal resistance to cycloheximide (cyh). A 2.8 kb DNA fragment encoding this gene was sequenced. Its EMBL Accession No. is AJ419770. It disclosed a putative tRNA(Asn) (GUU) sequence located downstream of an open reading frame (ORF) of 1641 nucleotides. This ORF was shown to correspond to SCR1. It would encode a highly hydrophobic polypeptide (SCR1) with 12 transmembrane domains. SCR1 is highly similar to a variety of yeast proteins of the multidrug-resistance (MDR) family. However, SCR1 only conferred resistance to cyh but not to benomyl or methotrexate. The cyh-resistance phenotype induced by SCR1 was confirmed in several S. cerevisiae strains that expressed this gene to reside at the ribosomal level. In contrast, a beta-galacosidase-tagged SCR1 was found to be integrated in the endoplasmic reticulum (ER). It is proposed that the ribosomes of yeast cells expressing SCR1 undergo a conformational change during their interaction with the ER, which lowers their affinity for cyh-binding. If so, these findings would disclose a novel ribosomal resistance mechanism.

Ray38p, a homolog of a purine motif triple-helical DNA-binding protein, Stm1p, is a ribosome-associated protein and dissociated from ribosomes prior to the induction of cycloheximide resistance in Candida maltosa

Cycloheximide (CYH) resistance in Candida maltosa is dependent on the induction of a ribosomal protein, Q-type L41, the 56th residue of which is glutamine, not proline as in ordinary P-type L41. We found that a 38-kDa protein in a wild-type C. maltosa ribosomal fraction became undetectable upon CYH treatment but detectable again with the establishment of CYH resistance by the induction of Q-type L41. We cloned a gene coding for this protein and named it RAY38 (ribosome-associated protein of yeast). Ray38p is a homolog of a purine motif triple-helical DNA-binding protein, Stm1p, and has a putative RNA-binding motif RGG. The ribosome-associated Ray38p was phosphorylated at serine and threonine residues, and Ray38p that was dissociated from ribosome by CYH treatment was highly phosphorylated in threonine residues. A ray38 null mutant recovered faster from CYH-caused growth stasis than the wild-type strain, suggesting that the dissociation of Ray38p from ribosome facilitates the induction of CYH resistance in C. maltosa.

A novel multidrug efflux transporter gene of the major facilitator superfamily from Candida albicans (FLU1) conferring resistance to fluconazole

Azole resistance in Candida albicans can be mediated by several resistance mechanisms. Among these, alterations of the azole target enzyme and the overexpression of multidrug efflux transporter genes are the most frequent. To identify additional putative azole resistance genes in C. albicans, a genomic library from this organism was screened for complementation of fluconazole hypersusceptibility in Saccharomyces cerevisiae YKKB-13 lacking the ABC (ATP-binding cassette) transporter gene PDR5. Among the C. albicans genes obtained, a new gene was isolated and named FLU1 (fluconazole resistance). The deduced amino acid sequence of FLU1 showed similarity to CaMDR1 (formerly BEN(r)), a member of the major facilitator superfamily of multidrug efflux transporters. The expression of FLU1 in YKKB-13 mediated not only resistance to fluconazole but also to cycloheximide among the different drugs tested. The disruption of FLU1 in C. albicans had only a slight effect on fluconazole susceptibility; however, it resulted in hypersusceptibility to mycophenolic acid, thus suggesting that this compound could be a substrate for the protein encoded by FLU1. Disruption of FLU1 in a background of C. albicans mutants with deletions in several multidrug efflux transporter genes, including CDR1, CDR2 and CaMDR1, resulted in enhanced susceptibility to several azole derivatives. FLU1 expression did not vary significantly between several pairs of azole-susceptible and azole-resistant C. albicans clinical isolates. Therefore, FLU1 seems not to be required for the development of azole resistance in clinical isolates.

Nucleotide sequences of genes for ribosomal protein L41 and tRNAThr(AGU) from Coprinus cinereus

The nucleotide sequences of genes for the homolog in Coprinus cinereus of the eukaryotic ribosomal protein L41 and for tRNAThr(AGU) are reported. The gene for tRNAThr(AGU) was located upstream of the gene for the L41 ribosomal protein, and these genes were adjacent to each other but in opposite orientations. The deduced amino acid sequence of ribosomal protein L41 exhibited strong homology to those of L41 proteins of several yeasts. The 56th amino acid of the deduced protein was proline, as it is in the L41 protein of a cycloheximide-sensitive strain of yeast. The putative secondary structure of the tRNA gene resembled the characteristic cloverleaf structure of tRNAs. Elements resembling an A-box and a B-box were found in the gene for tRNAThr(AGU). These boxes are known as internal promoter elements in genes for eukaryotic tRNAs.

A gene coding for a ribosomal protein L41 in cycloheximide-resistant ribosomes has a promoter which is upregulated under the growth-inhibitory conditions in yeast, Candida maltosa

We previously found by using yeast, Candida maltosa, that cycloheximide (CYH) sensitivity of ribosomes is dependent on the 56th amino acid residues of a ribosomal protein, L413 (proline in sensitive and glutamine in resistant ribosomes). We also revealed that in this yeast, which has both L41-P type and L41-Q type genes, the expression of the latter type genes is induced by the addition of CYH in the medium to make the cells inducibly resistant to CYH. In this paper, we analyzed the promoter region of L41-Q2a, one of the CYH-inducible L41-Q type genes and found two elements required for the induction of expression: one was a GCRE (Gcn4p-responsive element of Saccharomyces cerevisiae)-like element and the other was a GT-rich element. This promoter region was also required for its expression under some other growth inhibitory conditions. Furthermore, it was suggested that Q-type ribosomes synthesized under these conditions are more resistant to these inhibitory conditions.

Studies on cycloheximide-sensitive and cycloheximide-resistant ribosomes in the yeast Candida maltosa

Cycloheximide sensitivity or resistance in yeast is under the control of genes encoding different forms of ribosomal protein L41. In our previous studies, we have shown by isolating L41-Q1a, L41-P1a and their respective allelic genes, L41-Q1b and L41-P1b, from the partial diploid genome of C. maltosa, that this species, which is inducibly resistant to CYH, has both types of the L41 genes and that the expression of at least one of the L41-Q genes is induced by CYH, whereas L41-P genes are constitutively expressed. Here, we have identified another L41 (L41-Q2a), its allelic gene (L41-Q2b) and a third gene (L41-Q3) from the genome of C. maltosa. By gene disruption experiments, we now show that L41-Q1a and L41-Q1b are not responsible for the resistance to CYH and that the DeltaL41-Ps strain, which has only functional L41-Q genes, shows constitutive resistance to CYH, but grows more slowly than the DeltaL41-Qs strain, which has only functional L41-P genes, in the absence of CYH. Our results also show that in vitro, ribosomes containing L41-Q-type are less active in translation than those containing L41-P-type, although only the former ribosomes are active in the presence of CYH. These data suggest that ribosomes containing L41-Q-type are less active under normal growth conditions, but that this activity is not affected in the presence of CYH. We discuss the possible multi-step evolutionary event(s) by which C. maltosa has acquired the property of inducible resistance to CYH.

Stimulation of Sendai virus C' protein synthesis by cycloheximide

The polycistronic Sendai virus P/C mRNA is translated into five proteins (P, C', C, Y1 and Y2) from distinct start sites in virus-infected cells. The translation mechanism(s) of these proteins from two overlapping open reading frames in the P/C mRNA are poorly understood [Gupta, Ono and Xu (1996) Biochemistry 35, 1223-1231]. While investigating the initiation mechanism of C' from an ACG start site, we found that C' synthesis was resistant to inhibitors of peptide chain elongation such as cycloheximide (CHX) and anisomycin, but not to pactamycin (an inhibitor of chain initiation) or puromycin (a peptide chain terminator). Moreover, low levels (less than 30 microg/ml) of CHX significantly stimulated C' synthesis. Whereas C' synthesis was stimulated, synthesis of the P and C proteins, which are translated from the same mRNA, decreased by more than 95%. Stimulation of C' synthesis by CHX is not related to its initiation at an ACG codon. Mutation of ACG to alternative start sites had no effect on the CHX-stimulated C' synthesis. Similarly, C' synthesis was preferentially stimulated when Sendai virus-infected cells were exposed to hypotonic growth medium. These results suggest that the P/C mRNA may exist in at least two reversible conformations: whereas one conformation allows synthesis of the P and C proteins, the alternative conformation allows synthesis of the C' protein. It might be that low concentrations of CHX somehow increase the alternative conformation, which increases C' synthesis. The C' protein synthesis is reminiscent of the synthesis of stress-related proteins. Perhaps Sendai virus has evolved a novel mechanism to express both non-stress-related and stress-related proteins from the same mRNA.

Galactose-inducible expression systems in Candida maltosa using promoters of newly-isolated GAL1 and GAL10 genes

The GAL1 and GAL10 gene cluster encoding the enzymes of galactose utilization was isolated from an asporogenic yeast, Candida maltosa. The structure of the gene cluster in which both genes were divergently transcribed from the central promoter region resembled those of some other yeasts. The expression of both genes was strongly induced by galactose and repressed by glucose in the medium. Galactose-inducible expression vectors in C. maltosa were constructed on low- and high-copy number plasmids using the promoter regions of both genes. With these vectors and the beta-galactosidase gene from Kluyveromyces lactis as a reporter, galactose-inducible expression was confirmed. Homologous overexpression of members of the cytochrome P-450 gene family in C. maltosa was also successful by using a high-copy-number vector under the control of these promoters.

Susceptibilities of Candida albicans multidrug transporter mutants to various antifungal agents and other metabolic inhibitors

Some Candida albicans isolates from AIDS patients with oropharyngeal candidiasis are becoming resistant to the azole antifungal agent fluconazole after prolonged treatment with this compound. Most of the C. albicans isolates resistant to fluconazole fail to accumulate this antifungal agent, and this has been considered a cause of resistance. This phenomenon was shown to be linked to an increase in the amounts of mRNA of a C. albicans ABC (ATP-binding cassette) transporter gene called CDR1 and of a gene conferring benomyl resistance (BENr), the product of which belongs to the class of major facilitator multidrug efflux transporters (D. Sanglard, K. Kuchler, F. Ischer, J. L. Pagani, M. Monod, and J. Bille, Antimicrob. Agents Chemother. 39:2378-2386, 1995). To analyze the roles of these multidrug transporters in the efflux of azole antifungal agents, we constructed C. albicans mutants with single and double deletion mutations of the corresponding genes. The mutants were tested for their susceptibilities to these antifungal agents. Our results indicated that the delta cdr1 C. albicans mutant was hypersusceptible to the azole derivatives fluconazole, itraconazole, and ketoconazole, thus showing that the ABC transporter Cdr1 can use these compounds as substrates. The delta cdr1 mutant was also hypersusceptible to other antifungal agents (terbinafine and amorolfine) and to different metabolic inhibitors (cycloheximide, brefeldin A, and fluphenazine). The same mutant was slightly more susceptible than the wild type to nocodazole, cerulenin, and crystal violet but not to amphotericin B, nikkomycin Z, flucytosine, or pradimicin. In contrast, the delta ben mutant was rendered more susceptible only to the mutagen 4-nitroquinoline-N-oxide. However, this mutation increased the susceptibilities of the cells to cycloheximide and cerulenin when the mutation was constructed in a delta cdr1 background. The assay used in the present study could be implemented with new antifungal agents and is a powerful tool for assigning these substances as putative substrates of multidrug transporters.