Cloning in Saccharomyces cerevisiae of a cycloheximide resistance gene from the Candida maltosa genome which modifies ribosomes

Strong and widespread cycloheximide resistance in Stichococcus-like eukaryotic algal taxa

This study was initiated following the serendipitous discovery of a unialgal culture of a Stichococcus-like green alga (Chlorophyta) newly isolated from soil collected on Signy Island (maritime Antarctica) in growth medium supplemented with 100 µg/mL cycloheximide (CHX, a widely used antibiotic active against most eukaryotes). In order to test the generality of CHX resistance in taxa originally identified as members of Stichococcus (the detailed taxonomic relationships within this group of algae have been updated since our study took place), six strains were studied: two strains isolated from recent substrate collections from Signy Island (maritime Antarctica) ("Antarctica" 1 and "Antarctica" 2), one isolated from this island about 50 years ago ("Antarctica" 3) and single Arctic ("Arctic"), temperate ("Temperate") and tropical ("Tropical") strains. The sensitivity of each strain towards CHX was compared by determining the minimum inhibitory concentration (MIC), and growth rate and lag time when exposed to different CHX concentrations. All strains except "Temperate" were highly resistant to CHX (MIC > 1000 µg/mL), while "Temperate" was resistant to 62.5 µg/mL (a concentration still considerably greater than any previously reported for algae). All highly resistant strains showed no significant differences in growth rate between control and treatment (1000 µg/mL CHX) conditions. Morphological examination suggested that four strains were consistent with the description of the species Stichococcus bacillaris while the remaining two conformed to S. mirabilis. However, based on sequence analyses and the recently available phylogeny, only one strain, "Temperate", was confirmed to be S. bacillaris, while "Tropical" represents the newly erected genus Tetratostichococcus, "Antarctica 1" Tritostichococcus, and "Antarctica 2", "Antarctica 3" and "Arctic" Deuterostichococcus. Both phylogenetic and CHX sensitivity analyses suggest that CHX resistance is potentially widespread within this group of algae.

Metabolic protein patterns and monascorubrin production revealed through proteomic approach for Monascus pilosus treated with cycloheximide

Monascus species have the unique ability to economically produce many secondary metabolites. However, most metabolic regulation processes in the production of secondary metabolites in Monascus remain unclear. We found that the translational inhibitor cycloheximide induced different expression patterns between the monascorubrin pigment production and the growth in Monascus pilosus. Here, we used the proteomic approach of two-dimensional gel electrophoresis, matrix-assisted laser desorption ionization time-of-flight/time-of-flight liquid chromatography-mass spectrometry (MALDI-TOF/TOF LC-MS), and tandem mass spectrometry (MS/MS) to identify the intracellular and mitochondrial proteins of M. pilosus between the cycloheximide treatment and the control. These results revealed that the cycloheximide-induced down-regulated proteins were involved in transcriptional regulation, peptide synthesis, and other metabolic processes, such as methylation of secondary metabolites. In contrast, the energy-related proteins, such as the transcriptional regulator rosAr and 1,4-alpha-glucan branching enzyme, were up-regulated as compared to the control.

Transfer and expression of heterologous genes in yeasts other than Saccharomyces cerevisiae

In the past few years, yeasts other than those belonging to the genus Saccharomyces have become increasingly important for industrial applications. Species such as Pichia pastoris, Hansenula polymorpha, Schizosaccharomyces pombe, Yarrowia lipolytica and Kluyveromyces lactis have been modified genetically and used for the production of heterologous proteins. For a number of additional yeasts such as Schwanniomyces occidentalis, Zygosaccharomyces rouxii, Trichosporon cutaneum, Pachysolen tannophilus, Pichia guilliermondii and members of the genus Candida genetic transformation systems have been worked out. Transformation was achieved using either dominant selection markers based on antibiotic resistance genes or auxotrophic markers in conjunction with cloned biosynthetic genes involved in amino acid or nucleotide metabolism.

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.

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.

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

In a previous paper (S. Kawai, S. Murao, M. Mochizuki, I. Shibuya, K. Yano, and M. Takagi, J. Bacteriol. 174:254-262, 1992), we showed that in each genome of several yeast species, there is one of two types of L41 gene, one for an L41 (Q-type) protein which confers cycloheximide (CYH) resistance or one for an L41 (P-type) protein which does not. These genes have been suggested to be responsible for the CYH response used in taxonomy. For example, Saccharomyces cerevisiae, which is CYH sensitive, has a P-type L41 gene, while Kluyveromyces fragilis and Candida maltosa, which are CYH resistant, have Q-type L41 genes. However, in contrast to K. fragilis, which is constitutively resistant to CYH, C. maltosa is inducibly resistant to CYH. Here, we show that C. maltosa has both types of the L41 gene in its genome and that expression of the Q-type L41 gene is induced by CYH while the P-type L41 gene is constitutively expressed.

Natural cycloheximide resistance in yeast. The role of ribosomal protein L41

The yeast Kluyveromyces lactis is resistant to high concentrations (1 mg/ml) of the antibiotic cycloheximide. Using in vitro translation studies it was confirmed that this extreme resistance is a property of ribosomes. The resistance determinant from K. lactis was cloned into Saccharomyces cerevisiae. Nucleotide sequence analysis of the determinant demonstrated that resistance was conferred by the K. lactis ribosomal protein L41. K. lactis was shown to contain only one copy of the gene that encodes this protein and the gene was located to chromosome III. In contrast, S. cerevisiae was found to contain multiple copies of the gene for the corresponding ribosomal protein L41 which mapped to two of the three chromosomes V, XIV and VIII. Since the cycloheximide-resistance gene of K. lactis causes essentially complete protection against inhibition by the drug, it is likely to be particularly useful as a selective marker in eukaryotic gene transfer studies.

Two different genes from Schwanniomyces occidentalis determine ribosomal resistance to cycloheximide

Two genes (SCR1 and SCR2) encoding natural cycloheximide resistance in the budding yeast Schwanniomyces occidentalis have been cloned by expression in Saccharomyces cerevisiae. Both genes determine resistance to the inhibitory action of cycloheximide on the ribosome, SCR1 and SCR2 are present as single copies in Schwanniomyces occidentalis, where they map on chromosomes II and V, respectively. The nucleotide sequence of SCR2 contains an open reading frame of 321 nucleotides which is interrupted by an intron of 452 nucleotides. It encodes a polypeptide of 106 amino acids of molecular mass 12.25 kDa and pI 11.19. The deduced amino acid sequence shows a high degree of similarity to the L41 protein of the 60S ribosomal subunit from several eukaryotic organisms. The intron and the 5' non-coding region of SCR2 possess conserved elements which are typical of yeast ribosomal protein genes. A single amino acid change determines the resistance or sensitive phenotype to cycloheximide of the 80S ribosome since replacement of Gln56 in L41 from Schwanniomyces with Pro, by site-directed mutagenesis, confers cycloheximide sensitivity. SCR2 may serve as a practical yeast cloning marker if integrated in a multicopy plasmid.

Cloning and analysis of a Candida maltosa gene which confers resistance to formaldehyde in Saccharomyces cerevisiae

A gene (FDH1) of Candida maltosa which confers resistance to formaldehyde in Saccharomyces cerevisiae was cloned and its nucleotide sequence determined. The gene has a single intron which possesses the highly conserved splicing signals found in S. cerevisiae introns. We demonstrated that processing of the pre-mRNA of the cloned gene occurred identically in both S. cerevisiae and C. maltosa. The predicted amino acid sequence from the cloned gene showed 65.5% identity to human alcohol dehydrogenase (ADH) class III and 23.9% identity to S. cerevisiae ADH1. The most probable mechanism of resistance to formaldehyde is thought to be the glutathione-dependent oxidation of formaldehyde which is characteristic for ADH class III. The cloned FDH1 gene was successfully employed as a dominant selectable marker in the transformation of S. cerevisiae.

Cloning and sequence analysis of a Candida maltosa gene which confers resistance to cycloheximide

A CYHR gene from Candida maltosa, which confers resistance to cycloheximide, was cloned in Saccharomyces cerevisiae. A 2.3-kb DNA fragment carrying this gene was sequenced, and an open reading frame able to encode 553 amino acids (aa) was found in the sequence. Computer searches of the GenBank, EMBL, SWIS-PROT and Gen-Pept databases using the FASTA program failed to detect any proteins with extensive similarities to the deduced aa sequence for CYHR. The cloned gene transforms S. cerevisiae at a frequency similar to auxotrophic markers and can be used as a dominant selectable marker for introducing recombinant plasmids into wild-type strains of S. cerevisiae, as well as for gene disruption experiments.

Drastic alteration of cycloheximide sensitivity by substitution of one amino acid in the L41 ribosomal protein of yeasts

Cycloheximide is one of the antibiotics that inhibit protein synthesis in most eukaryotic cells. We have found that a yeast, Candida maltosa, is resistant to the drug because it possesses a cycloheximide-resistant ribosome, and we have isolated the gene responsible for this. In this study, we sequenced this gene and found that the gene encodes a protein homologous to the L41 ribosomal protein of Saccharomyces cerevisiae, whose amino acid sequence has already been reported. Two genes for L41 protein, named L41a and L41b, independently present in the genome of S. cerevisiae, were isolated. L41-related genes were also isolated from a few other yeast species. Each of these genes has an intron at the same site of the open reading frame. Comparison of their deduced amino acid sequences and their ability to confer cycloheximide resistance to S. cerevisiae, when introduced in a high-copy-number plasmid, suggested that the 56th amino acid residue of the L41 protein determines the sensitivity of the ribosome to cycloheximide; the amino acid is glutamine in the resistant ribosome, whereas that in the sensitive ribosome is proline. This was confirmed by constructing a cycloheximide-resistant strain of S. cerevisiae having a disrupted L41a gene and an L41b gene with a substitution of the glutamine codon for the proline codon.

Cycloheximide resistance as a yeast cloning marker

In CYH2/cyh2 heterozygous diploids of the yeast Saccharomyces cerevisiae resistance is dominant over sensitivity at low (0.5-5 micrograms/ml) cycloheximide (cyh) concentrations. The cyh-resistant haploid strain MMY1 confers relatively high (10 micrograms/ml) cyh-resistance to heterozygous diploids constructed by mating this strain with cyh-sensitive haploid strains. We present here a genetic and biochemical study of strain MMY1. Analysis of tetrads obtained from a MMY1 heterozygous diploid showed that two unlinked nuclear mutations, determining high- and low-cycloheximide resistance, were present in MMY1. From a genomic library of this strain, constructed in vector YCp50, two plasmids (pRC1 and pRC13) have been isolated which, respectively, confer high- and low-resistance phenotypes to cyh-sensitive S. cerevisiae strains. The restriction maps of pRC1 and pRC13 are totally unrelated. This finding suggests that the genes harboring the two mutations encoding cyh-resistance from MMY1 were cloned in plasmids pRC1 and pRC13, respectively. Pulse field gel electrophoresis showed that the DNA insert of pRC1 maps at either chromosome VII or XV, whereas that from pRC13 maps at chromosome XI. This latter gene appears to define a previously unreported locus and has been named cyh5. By restriction and nucleotide sequencing analysis, the cyh gene present in pRC1 has been shown to correspond to cyh2, which maps at chromosome VII. These results suggest that the dominant cyh-resistance phenotype conferred by MMY1 in heterozygous diploids is promoted by the presence of both cyh2 and cyh5.(ABSTRACT TRUNCATED AT 250 WORDS)

Nonconventional yeasts: their genetics and biotechnological applications

To date, more than 500 species of yeasts have been described. Most of the genetic and biochemical studies have, however, been carried out with Saccharomyces cerevisiae. Although a considerable amount of knowledge has been accumulated on fundamental processes and biotechnological applications of this industrially important yeast, the large variety of other yeast genera and species may offer various advantages for experimental study as well as for product formation in biotechnology. The genetic investigation of these so-called unconventional yeasts is poorly developed and information about corresponding data is dispersed. It is the aim of this review to summarize and discuss the main results of genetic studies and biotechnological applications of unconventional yeasts and to serve as a guide for scientists who wish to enter this field or are interested in only some aspects of these yeasts.