Increased copy number of the 5' end of the SPS2 gene inhibits sporulation of Saccharomyces cerevisiae

The Glycosylphosphatidylinositol-Anchored DFG Family Is Essential for the Insertion of Galactomannan into the β-(1,3)-Glucan-Chitin Core of the Cell Wall of Aspergillus fumigatus

The fungal cell wall is a complex and dynamic entity essential for the development of fungi. It is composed mainly of polysaccharides that are synthetized by protein complexes. At the cell wall level, enzyme activities are involved in postsynthesis polysaccharide modifications such as cleavage, elongation, branching, and cross-linking. Glycosylphosphatidylinositol (GPI)-anchored proteins have been shown to participate in cell wall biosynthesis and specifically in polysaccharide remodeling. Among these proteins, the DFG family plays an essential role in controlling polar growth in yeast. In the filamentous fungus and opportunistic human pathogen Aspergillus fumigatus, the DFG gene family contains seven orthologous DFG genes among which only six are expressed under in vitro growth conditions. Deletions of single DFG genes revealed that DFG3 plays the most important morphogenetic role in this gene family. A sextuple-deletion mutant resulting from the deletion of all in vitro expressed DFG genes did not contain galactomannan in the cell wall and has severe growth defects. This study has shown that DFG members are absolutely necessary for the insertion of galactomannan into the cell wall of A. fumigatus and that the proper cell wall localization of the galactomannan is essential for correct fungal morphogenesis in A. fumigatusIMPORTANCE The fungal cell wall is a complex and dynamic entity essential for the development of fungi. It is composed mainly of polysaccharides that are synthetized by protein complexes. Enzymes involved in postsynthesis polysaccharide modifications, such as cleavage, elongation, branching, and cross-linking, are essential for fungal life. Here, we investigated in Aspergillus fumigatus the role of the members of the Dfg family, one of the 4 GPI-anchored protein families common to yeast and molds involved in cell wall remodeling. Molecular and biochemical approaches showed that DFG members are required for filamentous growth, conidiation, and cell wall organization and are essential for the life of this fungal pathogen.

Glycosylphosphatidylinositol-anchored Ecm33p influences conidial cell wall biosynthesis in Aspergillus fumigatus

ECM33 encodes a glycosylphosphatidylinositol-anchored protein whose orthologs in yeast are essential for sporulation. Aspergillus fumigatus Ecm33p is unique and has an apparent mass of 55 kDa. Disruption of A. fumigatus ECM33 results in a mutant with several morphogenetic aberrations, including the following: (i) a defect in conidial separation, (ii) an increase in the diameter of the conidia of the mutant associated with an increase in the concentration of the cell wall chitin, (iii) conidia that were sensitive to the absence of aeration during long-term storage, and (iv) conidia that were more resistant to killing by phagocytes, whereas the mycelium was more easily killed by neutrophils.

Identification of an endo-beta-1,4-D-xylanase from Magnaporthe grisea by gene knockout analysis, purification, and heterologous expression

Magnaporthe grisea, a destructive ascomycetous pathogen of rice, secretes cell wall-degrading enzymes into a culture medium containing purified rice cell walls as the sole carbon source. From M. grisea grown under the culture conditions described here, we have identified an expressed sequenced tag, XYL-6, a gene that is also expressed in M. grisea-infected rice leaves 24 h postinoculation with conidia. This gene encodes a protein about 65% similar to endo-beta-1,4-D-glycanases within glycoside hydrolase family GH10. A M. grisea knockout mutant for XYL-6 was created, and it was shown to be as virulent as the parent strain in infecting the rice host. The proteins secreted by the parent strain and by the xyl-6Delta mutant were each fractionated by liquid chromatography, and the collected fractions were assayed for endo-beta-1,4-D-glucanase or endo-beta-1,4-D-xylanase activities. Two protein-containing peaks with endo-beta-1,4-D-xylanase activity secreted by the parent strain are not detectable in the column eluant of the proteins secreted by the mutant. The two endoxylanases (XYL-6alpha and XYL-6beta) from the parent were each purified to homogeneity. N-terminal amino acid sequencing indicated that XYL-6alpha is a fragment of XYL-6beta and that XYL-6beta is identical to the deduced protein sequence encoded by the XYL-6 gene. Finally, XYL-6 was introduced into Pichia pastoris for heterologous expression, which resulted in the purification of a fusion protein, XYL-6H, from the Pichia pastoris culture filtrate. XYL-6H is active in cleaving arabinoxylan. These experiments unequivocally established that the XYL-6 gene encodes a secreted endo-beta-1,4-D-xylanase.

Dtrlp, a multidrug resistance transporter of the major facilitator superfamily, plays an essential role in spore wall maturation in Saccharomyces cerevisiae

The de novo formation of multilayered spore walls inside a diploid mother cell is a major landmark of sporulation in the yeast Saccharomyces cerevisiae. Synthesis of the dityrosine-rich outer spore wall takes place toward the end of this process. Bisformyl dityrosine, the major building block of the spore surface, is synthesized in a multistep process in the cytoplasm of the prospores, transported to the maturing wall, and polymerized into a highly cross-linked macromolecule on the spore surface. Here we present evidence that the sporulation-specific protein Dtrlp (encoded by YBR180w) plays an important role in spore wall synthesis by facilitating the translocation of bisformyl dityrosine through the prospore membrane. DTR1 was identified in a genome-wide screen for spore wall mutants. The null mutant accumulates unusually large amounts of bisformyl dityrosine in the cytoplasm and fails to efficiently incorporate this precursor into the spore surface. As a result, many mutant spores have aberrant surface structures. Dtrlp, a member of the poorly characterized DHA12 (drug:H+ antiporter with 12 predicted membrane spans) family, is localized in the prospore membrane throughout spore maturation. Transport by Dtrlp may not be restricted to its natural substrate, bisformyl dityrosine. When expressed in vegetative cells, Dtrlp renders these cells slightly more resistant against unrelated toxic compounds, such as antimalarial drugs and food-grade organic acid preservatives. Dtrlp is the first multidrug resistance protein of the major facilitator superfamily with an assigned physiological role in the yeast cell.

Comprehensive isolation of meiosis-specific genes identifies novel proteins and unusual non-coding transcripts in Schizosaccharomyces pombe

In order to isolate meiosis-specific genes in Schizosaccharomyces pombe, we have constructed a subtracted cDNA library enriched in clones whose expression is enhanced during meiosis induced by nitrogen starvation. Using northern blot analysis, we isolated 31 kinds of clones whose expression was induced in a meiosis/sporulation-specific manner. We comprehensively named them meu after meiotic expression upregulated. The transcription of 20 meu genes was found to be dependent on the mei4(+) gene, which encodes a transcription factor required for the progression of meiosis. DNA sequencing indicated that most of the meu genes encode novel proteins. Notably, five of the meu genes harbor no apparent protein coding sequences, and the transcripts form stable hairpin structures, suggesting that they may generate non-coding RNAs or antisense RNAS: The results presented here imply that RNAs are also important for the comprehensive characterization of genomic expression.

In vivo analyses of the internal control region in the 5S rRNA gene from Saccharomyces cerevisiae

The internal control region of the Saccharomyces cerevisiae 5S rRNA gene has been characterized in vivo by genomic DNase I footprinting and by mutational analyses using base substitutions, deletions or insertions. A high copy shuttle vector was used to efficiently express mutant 5S rRNA genes in vivo and isotope labelling kinetics were used to distinguish impeded gene expression from nascent RNA degradation. In contrast to mutational studies in reconstituted systems, the analyses describe promoter elements which closely resemble the three distinct sequence elements that have been observed in Xenopus laevis 5S rRNA. The results indicate a more highly conserved structure than previously reported with reconstituted systems and suggest that the saturated conditions which are used in reconstitution studies mask sequence dependence which may be physiologically significant. Footprint analyses support the extended region of protein interaction which has recently been observed in some reconstituted systems, but mutational analyses indicate that these interactions are not sequence specific. Periodicity in the footprint provides further detail regarding the in vivo topology of the interacting protein.

Inhibition of protein synthesis by an efficiently expressed mutation in the yeast 5.8S ribosomal RNA

Recent studies on the inhibition of protein synthesis by specific anti 5.8S rRNA oligonucleotides strongly suggested that this RNA plays an important role in eukaryotic ribosome function. To evaluate this possibility further, a ribosomal DNA transcription unit from Schizosaccharomyces pombe was cloned into yeast shuttle vectors with copy numbers ranging from 2 to approximately 90 per cell; to allow direct detection of expressed RNA and to disrupt the function of the 5.8S rRNA molecule, a five base insertion was made in a universally conserved GAAC sequence. The altered mobility of the mutant RNA was readily detected by gel electrophoresis and analyses indicated that mutant RNA transcription reflected the ratio of plasmid to endogenous rDNA. The highest copy number plasmid resulted in about 40-50% mutant RNA. This mutant RNA was readily integrated into the ribosome structure resulting in an in vivo ribosome population which was also about 40-50% mutant; the rates of growth and protein synthesis were equally reduced by approximately 40%. A comparable level of inhibition in protein synthesis was demonstrated in vitro and polyribosomal profiles revealed a consistent increase in size. Subsequent RNA analyses indicated a normal distribution of mutant RNA in both monoribosomes and polyribosomes, but elevated tRNA levels in mutant polyribosomes. Additional mutations in alternate GAAC sequences revealed similar but cumulative effects on both protein synthesis and polyribosome profiles. Taken together, these results suggest little or no effect on initiation but provide in vivo evidence of a functional role for the 5.8S rRNA in protein elongation.

Life cycle of the budding yeast Saccharomyces cerevisiae

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The SPS100 gene of Saccharomyces cerevisiae is activated late in the sporulation process and contributes to spore wall maturation

We previously described the use of a differential hybridization screen of a genomic DNA library of Saccharomyces cerevisiae to identify sporulation-specific (SPS) genes (A. Percival-Smith and J. Segall, Mol. Cell. Biol. 4:142-150, 1984). This initial screen identified 14 SPS genes that are first expressed 6 to 8 h after transfer of cells to sporulation medium. Accumulation of transcripts corresponding to these genes becomes maximal at 8 to 12 h of sporulation, the time at which meiotic events are nearing completion, and by 15 h of sporulation, transcript levels are beginning to decrease. In the present study two additional SPS genes, first expressed at 12 h of sporulation, were isolated. The steady-state level of transcripts corresponding to these two genes, termed SPS100 and SPS101, remains unchanged from 15 to 35 h, a time coincident with spore wall maturation. The nature of the putative 34.2-kilodalton protein encoded by the SPS100 gene is consistent with its being a component of the glycoprotein matrix of the spore wall; the protein contains a potential signal sequence and cleavage site and numerous sites for potential glycosylation. A MATa sps100/MAT alpha sps100 strain was found to be indistinguishable from the wild-type strain when assessed for efficiency of ascus formation and spore viability. However, a more detailed analysis of the mutant strain revealed that the SPS100 gene product serves a protective role during the early stages of spore wall formation. The time at which resistance to ether could first be detected in developing spores was delayed by 5 h in the mutant strain relative to the wild-type strain. This phenotype is presumably a reflection of a defect in spore wall maturation. This study has confirmed that temporally distinct classes of sporulation-specific genes are sequentially activated during the process of meiosis and spore formation and has shown that the SPS100 gene, identified on the basis of its developmental-specific expression pattern, contributes to spore development.

The SPS100 gene of Saccharomyces cerevisiae is activated late in the sporulation process and contributes to spore wall maturation

We previously described the use of a differential hybridization screen of a genomic DNA library of Saccharomyces cerevisiae to identify sporulation-specific (SPS) genes (A. Percival-Smith and J. Segall, Mol. Cell. Biol. 4:142-150, 1984). This initial screen identified 14 SPS genes that are first expressed 6 to 8 h after transfer of cells to sporulation medium. Accumulation of transcripts corresponding to these genes becomes maximal at 8 to 12 h of sporulation, the time at which meiotic events are nearing completion, and by 15 h of sporulation, transcript levels are beginning to decrease. In the present study two additional SPS genes, first expressed at 12 h of sporulation, were isolated. The steady-state level of transcripts corresponding to these two genes, termed SPS100 and SPS101, remains unchanged from 15 to 35 h, a time coincident with spore wall maturation. The nature of the putative 34.2-kilodalton protein encoded by the SPS100 gene is consistent with its being a component of the glycoprotein matrix of the spore wall; the protein contains a potential signal sequence and cleavage site and numerous sites for potential glycosylation. A MATa sps100/MAT alpha sps100 strain was found to be indistinguishable from the wild-type strain when assessed for efficiency of ascus formation and spore viability. However, a more detailed analysis of the mutant strain revealed that the SPS100 gene product serves a protective role during the early stages of spore wall formation. The time at which resistance to ether could first be detected in developing spores was delayed by 5 h in the mutant strain relative to the wild-type strain. This phenotype is presumably a reflection of a defect in spore wall maturation. This study has confirmed that temporally distinct classes of sporulation-specific genes are sequentially activated during the process of meiosis and spore formation and has shown that the SPS100 gene, identified on the basis of its developmental-specific expression pattern, contributes to spore development.