Kimberly J. Gerik

Characterization of the Two Small Subunits of Saccharomyces cerevisiae DNA Polymerase δ

Yeast DNA polymerase δ (Polδ) has three subunits of 125, 58, and 55 kDa. The gene for the 125-kDa catalytic subunit (POL3) has been known for several years. Here we describe the cloning of the genes for the 58- and 55-kDa subunits using peptide sequence analysis and searching of the yeast genome data base. The 58-kDa subunit, encoded by the POL31 gene, shows 23–28% sequence similarity to the 48-kDa subunit of human Polδ and to S. pombe Cdc1. POL31 is allelic toHYS2 and SDP5. The 55-kDa subunit is encoded by the POL32 gene (ORF YJR043c in the yeast data base). Very limited sequence similarity was observed between Pol32p andSchizosaccharomyces pombe Cdc27, the functionally analogous subunit in S. pombe Polδ. The POL32 gene is not essential, but a deletion mutant shows cold sensitivity for growth and is sensitive to hydroxyurea and DNA damaging agents. In addition, lethality was observed when the POL32 deletion mutation was combined with conditional mutations in either the POL3 orPOL31 gene. Pol32Δ strains are weak antimutators and are defective for damage-induced mutagenesis. ThePOL32 gene product binds proliferating cell nuclear antigen. A gel filtration analysis showed that Pol32p is a dimer in solution. When POL31 and POL32 were co-expressed in Escherichia coli, a tetrameric (Pol31p·Pol32p)2 species was detected by gel filtration, indicating that the two subunits form a complex.

STRUCTURE AND PROCESSIVITY OF TWO FORMS OF SACCHAROMYCES CEREVISIAE DNA POLYMERASE DELTA

Yeast DNA polymerase δ (Polδ) consists of three subunits encoded by the POL3, POL31, andPOL32 genes. Each of these genes was cloned under control of the galactose-inducible GAL1–10 promoter and overexpressed in various combinations. Overexpression of all three genes resulted in a 30-fold overproduction of Polδ, which was identical in enzymatic properties to Polδ isolated from a wild-type yeast strain. Whereas overproduction of POL3 together withPOL32 did not lead to an identifiable Pol3p·Pol32p complex, a chromatographically distinct and novel complex was identified upon overproduction of POL3 andPOL31. This two-subunit complex, designated Polδ*, is structurally and functionally analogous to mammalian Polδ. The properties of Polδ* and Polδ were compared. A gel filtration analysis showed that Polδ* is a heterodimer (Pol3p·Pol31p) and Polδ a dimer of a heterotrimer, (Pol3p·Pol31p·Pol32p)2. In the absence of proliferating cell nuclear antigen (PCNA), Polδ* showed a processivity of 2–3 on poly(dA)·oligo(dT) compared with 5–10 for Polδ. In the presence of PCNA, both enzymes were fully processive on this template. DNA replication by Polδ* on a natural DNA template was dependent on PCNA and on replication factor C. However, Polδ*-mediated DNA synthesis proceeded inefficiently and was characterized by frequent pause sites. Reconstitution of Polδ was achieved upon addition of Pol32p to Polδ*.

Analysis of the genome and transcriptome of Cryptococcus neoformans var. grubii reveals complex RNA expression and microevolution leading to virulence attenuation.

Cryptococcus neoformans is a pathogenic basidiomycetous yeast responsible for more than 600,000 deaths each year. It occurs as two serotypes (A and D) representing two varieties (i.e. grubii and neoformans, respectively). Here, we sequenced the genome and performed an RNA-Seq-based analysis of the C. neoformans var. grubii transcriptome structure. We determined the chromosomal locations, analyzed the sequence/structural features of the centromeres, and identified origins of replication. The genome was annotated based on automated and manual curation. More than 40,000 introns populating more than 99% of the expressed genes were identified. Although most of these introns are located in the coding DNA sequences (CDS), over 2,000 introns in the untranslated regions (UTRs) were also identified. Poly(A)-containing reads were employed to locate the polyadenylation sites of more than 80% of the genes. Examination of the sequences around these sites revealed a new poly(A)-site-associated motif (AUGHAH). In addition, 1,197 miscRNAs were identified. These miscRNAs can be spliced and/or polyadenylated, but do not appear to have obvious coding capacities. Finally, this genome sequence enabled a comparative analysis of strain H99 variants obtained after laboratory passage. The spectrum of mutations identified provides insights into the genetics underlying the micro-evolution of a laboratory strain, and identifies mutations involved in stress responses, mating efficiency, and virulence.

A Chitin Synthase and Its Regulator Protein Are Critical for Chitosan Production and Growth of the Fungal Pathogen Cryptococcus neoformans

ABSTRACT Chitin is an essential component of the cell wall of many fungi. Chitin also can be enzymatically deacetylated to chitosan, a more flexible and soluble polymer. Cryptococcus neoformans is a fungal pathogen that causes cryptococcal meningoencephalitis, particularly in immunocompromised patients. In this work, we show that both chitin and chitosan are present in the cell wall of vegetatively growing C. neoformans yeast cells and that the levels of both rise dramatically as cells grow to higher density in liquid culture. C. neoformans has eight putative chitin synthases, and strains with any one chitin synthase deleted are viable at 30°C. In addition, C. neoformans genes encode three putative regulator proteins, which are homologs of Saccharomyces cerevisiae Skt5p. None of these three is essential for viability. However, one of the chitin synthases (Chs3) and one of the regulators (Csr2) are important for growth. Cells with deletions in either CHS3 or CSR2 have several shared phenotypes, including sensitivity to growth at 37°C. The similarity of their phenotypes also suggests that Csr2 specifically regulates chitin synthesis by Chs3. Lastly, both chs3Δ and the csr2Δ mutants are defective in chitosan production, predicting that Chs3-Csr2 complex with chitin deacetylases for conversion of chitin to chitosan. These data suggest that chitin synthesis could be an excellent antifungal target.

Cell wall integrity is dependent on the PKC1 signal transduction pathway in Cryptococcus neoformans

Cell wall biogenesis and integrity are crucial for fungal growth, pathogenesis and survival, and are attractive targets for antifungal therapy. In this study, we identify, delete and analyse mutant strains for 10 genes involved in the PKC1 signal transduction pathway and its regulation in Cryptococcus neoformans. The kinases Bck1 and Mkk2 are critical for maintaining integrity, and deletion of each of these causes severe phenotypes different from each other. In stark contrast to results seen in Saccharomyces cerevisiae, a deletion in LRG1 has severe repercussions for the cell, and one in ROM2 has little effect. Also surprisingly, the phosphatase Ppg1 is crucial for cell integrity. These data indicate that the mechanisms of maintaining cell integrity differ between the two fungi. Deletions in SSD1 and PUF4, potential alternative regulators of cell integrity, also exhibit phenotypes. This is the first comprehensive analysis examining genes involved the maintenance of cell integrity in C. neoformans and sets the foundation for future biochemical and virulence studies.

PKC1 Is Essential for Protection against both Oxidative and Nitrosative Stresses, Cell Integrity, and Normal Manifestation of Virulence Factors in the Pathogenic Fungus Cryptococcus neoformans†

ABSTRACT Cell wall integrity is crucial for fungal growth, survival, and pathogenesis. Responses to environmental stresses are mediated by the highly conserved Pkc1 protein and its downstream components. In this study, we demonstrate that both oxidative and nitrosative stresses activate the PKC1 cell integrity pathway in wild-type cells, as measured by phosphorylation of Mpk1, the terminal protein in the PKC1 phosphorylation cascade. Furthermore, deletion of PKC1 shows that this gene is essential for defense against both oxidative and nitrosative stresses; however, other genes involved directly in the PKC1 pathway are dispensable for protection against these stresses. This suggests that Pkc1 may have multiple and alternative functions other than activating the mitogen-activated protein kinase cascade from a “top-down” approach. Deletion of PKC1 also causes osmotic instability, temperature sensitivity, severe sensitivity to cell wall-inhibiting agents, and alterations in capsule and melanin. Furthermore, the vital cell wall components chitin and its deacetylated form chitosan appear to be mislocalized in a pkc1Δ strain, although this mutant contains wild-type levels of both of these polymers. These data indicate that loss of Pkc1 has pleiotropic effects because it is central to many functions either dependent on or independent of PKC1 pathway activation. Notably, this is the first time that Pkc1 has been implicated in protection against nitrosative stress in any organism.

Overproduction and Affinity Purification of Saccharomyces cerevisiae Replication Factor C

Yeast replication factor C (RF-C) is a heteropentamer encoded by the RFC1-5 genes. RF-C activity in yeast extracts was overproduced about 80-fold after induction of a strain containing all five genes on a single plasmid, with expression of each gene placed under control of the galactose-inducible GAL1-10 promoter. This strongly indicates that overexpression of the five known RFC genes is sufficient for overproduction of RF-C. Overexpression of all five genes was also necessary to achieve overproduction of RF-C as omission of any single gene from the plasmid gave uninduced, i.e. normal cellular levels of RF-C. The interaction between RF-C and proliferating cell nuclear antigen (PCNA) was studied with PCNA-agarose beads. Binding of RF-C to PCNA-agarose beads is negligible in buffers containing 0.3 M NaCl. However, addition of Mg-ATP to the binding buffer caused strong binding of RF-C to the beads even at 0.8 M NaCl. Binding of ATP, but not its hydrolysis, was required for the strong binding mode as nonhydrolyzable analogs were also effective. The existence of two distinct binding modes between PCNA and RF-C was used as the key step in a greatly improved procedure for the purification of RF-C. RF-C from the overproduction strain purified by this procedure was essentially homogeneous and had a severalfold higher specific activity than RF-C preparations that had previously been purified through multicolumn procedures.

Cryptococcus neoformans Gene Involved in Mammalian Pathogenesis Identified by a Caenorhabditis elegans Progeny-Based Approach

ABSTRACT Caenorhabditis elegans can serve as a substitute host for the study of microbial pathogenesis. We found that mutations in genes of the fungal pathogen Cryptococcus neoformans involved in mammalian virulence allow C. elegans to produce greater numbers of progeny than when exposed to wild-type fungus. We used this property to screen a library of C. neoformans mutants for strains that permit larger C. elegans brood sizes. In this screen, we identified a gene homologous to Saccharomyces cerevisiae ROM2. C. neoformans rom2 mutation resulted in a defect in mating and growth defects at elevated temperature or in the presence of cell wall or hyperosmolar stresses. An effect of the C. neoformans rom2 mutation in virulence was confirmed in a murine inhalation infection model. We propose that a screen for progeny-permissive mutants of microorganisms can serve as a high-throughput method for identifying novel loci related to mammalian pathogenesis.

KRE genes are required for β‐1,6‐glucan synthesis, maintenance of capsule architecture and cell wall protein anchoring in Cryptococcus neoformans

The polysaccharide β‐1,6‐glucan is a major component of the cell wall of Cryptococcus neoformans, but its function has not been investigated in this fungal pathogen. We have identified and characterized seven genes, belonging to the KRE family, which are putatively involved in β‐1,6‐glucan synthesis. The H99 deletion mutants kre5Δ and kre6Δskn1Δ contained less cell wall β‐1,6‐glucan, grew slowly with an aberrant morphology, were highly sensitive to environmental and chemical stress and were avirulent in a mouse inhalation model of infection. These two mutants displayed alterations in cell wall chitosan and the exopolysaccharide capsule, a primary cryptococcal virulence determinant. The cell wall content of the GPI‐anchored phospholipase B1 (Plb1) enzyme, which is required for cryptococcal cell wall integrity and virulence, was reduced in kre5Δ and kre6Δskn1Δ. Our results indicate that KRE5, KRE6 and SKN1 are involved in β‐1,6‐glucan synthesis, maintenance of cell wall integrity and retention of mannoproteins and known cryptococcal virulence factors in the cell wall of C. neoformans. This study sets the stage for future investigations into the function of this abundant cell wall polymer.

Role of Cryptococcus neoformans Rho1 GTPases in the PKC1 Signaling Pathway in Response to Thermal Stress

ABSTRACT To initiate and establish infection in mammals, the opportunistic fungal pathogen Cryptococcus neoformans must survive and thrive upon subjection to host temperature. Primary maintenance of cell integrity is controlled through the protein kinase C1 (PKC1) signaling pathway, which is regulated by a Rho1 GTPase in Saccharomyces cerevisiae. We identified three C. neoformans Rho GTPases, Rho1, Rho10, and Rho11, and have begun to elucidate their role in growth and activation of the PKC1 pathway in response to thermal stress. Western blot analysis revealed that heat shock of wild-type cells resulted in phosphorylation of Mpk1 mitogen-activated protein kinase (MAPK). Constitutive activation of Rho1 caused phosphorylation of Mpk1 independent of temperature, indicating its role in pathway regulation. A strain with a deletion of RHO10 also displayed this constitutive Mpk1 phosphorylation phenotype, while one with a deletion of RHO11 yielded phosphorylation similar to that of wild type. Surprisingly, like a rho10Δ strain, a strain with a deletion of both RHO10 and RHO11 displayed temperature sensitivity but mimicked wild-type phosphorylation, which suggests that Rho10 and Rho11 have coordinately regulated functions. Heat shock-induced Mpk1 phosphorylation also required the PKC1 pathway kinases Bck1 and Mkk2. However, Pkc1, thought to be the major regulatory kinase of the cell integrity pathway, was dispensable for this response. Together, our results argue that Rho proteins likely interact via downstream components of the PKC1 pathway or by alternative pathways to activate the cell integrity pathway in C. neoformans.