Folding free energies of 5'-UTRs impact post-transcriptional regulation on a genomic scale in yeast

Using high-throughput technologies, abundances and other features of genes and proteins have been measured on a genome-wide scale in Saccharomyces cerevisiae. In contrast, secondary structure in 5′–untranslated regions (UTRs) of mRNA has only been investigated for a limited number of genes. Here, the aim is to study genome-wide regulatory effects of mRNA 5′-UTR folding free energies. We performed computations of secondary structures in 5′-UTRs and their folding free energies for all verified genes in S. cerevisiae. We found significant correlations between folding free energies of 5′-UTRs and various transcript features measured in genome-wide studies of yeast. In particular, mRNAs with weakly folded 5′-UTRs have higher translation rates, higher abundances of the corresponding proteins, longer half-lives, and higher numbers of transcripts, and are upregulated after heat shock. Furthermore, 5′-UTRs have significantly higher folding free energies than other genomic regions and randomized sequences. We also found a positive correlation between transcript half-life and ribosome occupancy that is more pronounced for short-lived transcripts, which supports a picture of competition between translation and degradation. Among the genes with strongly folded 5′-UTRs, there is a huge overrepresentation of uncharacterized open reading frames. Based on our analysis, we conclude that (i) there is a widespread bias for 5′-UTRs to be weakly folded, (ii) folding free energies of 5′-UTRs are correlated with mRNA translation and turnover on a genomic scale, and (iii) transcripts with strongly folded 5′-UTRs are often rare and hard to find experimentally.

In cells, proteins are made from messenger RNA copied from genes in the DNA. The amount of each protein needs to be controlled by cells. For this purpose, cells use a strategy that includes decomposing RNA and varying the number of proteins made from each RNA. One part of the RNA molecule is called the 5′–untranslated region (UTR), and it is known that this region can fold into a three-dimensional structure. For some genes, such structures are important for protein production. In this article, structures in 5′-UTRs are calculated for all genes in the yeast Saccharomyces cerevisiae. The authors show that structures in 5′-UTRs likely play a role in RNA decomposition and protein production for many genes in the genome: RNA molecules with weakly folded 5′-UTRs live relatively longer and produce more proteins. This study provides an example of how genome-wide computational analysis complements experimental results.

PhyloGibbs: a Gibbs sampling motif finder that incorporates phylogeny

A central problem in the bioinformatics of gene regulation is to find the binding sites for regulatory proteins. One of the most promising approaches toward identifying these short and fuzzy sequence patterns is the comparative analysis of orthologous intergenic regions of related species. This analysis is complicated by various factors. First, one needs to take the phylogenetic relationship between the species into account in order to distinguish conservation that is due to the occurrence of functional sites from spurious conservation that is due to evolutionary proximity. Second, one has to deal with the complexities of multiple alignments of orthologous intergenic regions, and one has to consider the possibility that functional sites may occur outside of conserved segments. Here we present a new motif sampling algorithm, PhyloGibbs, that runs on arbitrary collections of multiple local sequence alignments of orthologous sequences. The algorithm searches over all ways in which an arbitrary number of binding sites for an arbitrary number of transcription factors (TFs) can be assigned to the multiple sequence alignments. These binding site configurations are scored by a Bayesian probabilistic model that treats aligned sequences by a model for the evolution of binding sites and "background" intergenic DNA. This model takes the phylogenetic relationship between the species in the alignment explicitly into account. The algorithm uses simulated annealing and Monte Carlo Markov-chain sampling to rigorously assign posterior probabilities to all the binding sites that it reports. In tests on synthetic data and real data from five Saccharomyces species our algorithm performs significantly better than four other motif-finding algorithms, including algorithms that also take phylogeny into account. Our results also show that, in contrast to the other algorithms, PhyloGibbs can make realistic estimates of the reliability of its predictions. Our tests suggest that, running on the five-species multiple alignment of a single gene's upstream region, PhyloGibbs on average recovers over 50% of all binding sites in S. cerevisiae at a specificity of about 50%, and 33% of all binding sites at a specificity of about 85%. We also tested PhyloGibbs on collections of multiple alignments of intergenic regions that were recently annotated, based on ChIP-on-chip data, to contain binding sites for the same TF. We compared PhyloGibbs's results with the previous analysis of these data using six other motif-finding algorithms. For 16 of 21 TFs for which all other motif-finding methods failed to find a significant motif, PhyloGibbs did recover a motif that matches the literature consensus. In 11 cases where there was disagreement in the results we compiled lists of known target genes from the literature, and found that running PhyloGibbs on their regulatory regions yielded a binding motif matching the literature consensus in all but one of the cases. Interestingly, these literature gene lists had little overlap with the targets annotated based on the ChIP-on-chip data. The PhyloGibbs code can be downloaded from http://www.biozentrum.unibas.ch/~nimwegen/cgi-bin/phylogibbs.cgi or http://www.imsc.res.in/~rsidd/phylogibbs. The full set of predicted sites from our tests on yeast are available at http://www.swissregulon.unibas.ch.

Histone acetylation and transcriptional regulation in the genome of Saccharomyces cerevisiae

MOTIVATION

In eukaryotic genomes, histone acetylation and thereafter departure from the chromatin are essential for gene transcription initiation. Because gene transcription is tightly regulated by transcription factors, there are some speculations on the cooperation of histone acetylation and transcription factor binding. However, systematic statistical analyses of this relationship on a genomic scale have not been reported.

RESULTS

We apply several statistical methods to explore this relationship on two recent genomic datasets: acetylation levels on 11 histone lysines and binding activities of 203 transcription factors, both in promoter regions across the yeast genome. By canonical correlation analysis, we find that a histone acetylation pattern is correlated with a certain profile of transcription factor binding in the genome. Furthermore, after clustering the genes by their acetylation levels on the 11 histone lysines, the genes within clusters show distinct transcription factor binding profiles, as discovered by principle component analysis. Even after applying fairly stringent statistical measurement, most of these clusters have transcription factors with binding activities significantly deviated from the overall genome. We conclude that in the yeast genome, there is a strong correlation between histone acetylation and transcription factor binding in the promoter regions.

Sequencing of Aspergillus nidulans and comparative analysis with A. fumigatus and A. oryzae

The aspergilli comprise a diverse group of filamentous fungi spanning over 200 million years of evolution. Here we report the genome sequence of the model organism Aspergillus nidulans, and a comparative study with Aspergillus fumigatus, a serious human pathogen, and Aspergillus oryzae, used in the production of sake, miso and soy sauce. Our analysis of genome structure provided a quantitative evaluation of forces driving long-term eukaryotic genome evolution. It also led to an experimentally validated model of mating-type locus evolution, suggesting the potential for sexual reproduction in A. fumigatus and A. oryzae. Our analysis of sequence conservation revealed over 5,000 non-coding regions actively conserved across all three species. Within these regions, we identified potential functional elements including a previously uncharacterized TPP riboswitch and motifs suggesting regulation in filamentous fungi by Puf family genes. We further obtained comparative and experimental evidence indicating widespread translational regulation by upstream open reading frames. These results enhance our understanding of these widely studied fungi as well as provide new insight into eukaryotic genome evolution and gene regulation.

Profiling condition-specific, genome-wide regulation of mRNA stability in yeast

The steady-state abundance of an mRNA is determined by the balance between transcription and decay. Although regulation of transcription has been well studied both experimentally and computationally, regulation of transcript stability has received little attention. We developed an algorithm, MatrixREDUCE, that discovers the position-specific affinity matrices for unknown RNA-binding factors and infers their condition-specific activities, using only genomic sequence data and steady-state mRNA expression data as input. We identified and computationally characterized the binding sites for six mRNA stability regulators in Saccharomyces cerevisiae, which include two members of the Pumilio-homology domain (Puf) family of RNA-binding proteins, Puf3p and Puf4p. We provide computational and experimental evidence that regulation of mRNA stability by these factors is modulated in response to a variety of environmental stimuli.

Reconstructing the pathways of a cellular system from genome-scale signals by using matrix and tensor computations

We describe the use of the matrix eigenvalue decomposition (EVD) and pseudoinverse projection and a tensor higher-order EVD (HOEVD) in reconstructing the pathways that compose a cellular system from genome-scale nondirectional networks of correlations among the genes of the system. The EVD formulates a genes x genes network as a linear superposition of genes x genes decorrelated and decoupled rank-1 subnetworks, which can be associated with functionally independent pathways. The integrative pseudoinverse projection of a network computed from a "data" signal onto a designated "basis" signal approximates the network as a linear superposition of only the subnetworks that are common to both signals and simulates observation of only the pathways that are manifest in both experiments. We define a comparative HOEVD that formulates a series of networks as linear superpositions of decorrelated rank-1 subnetworks and the rank-2 couplings among these subnetworks, which can be associated with independent pathways and the transitions among them common to all networks in the series or exclusive to a subset of the networks. Boolean functions of the discretized subnetworks and couplings highlight differential, i.e., pathway-dependent, relations among genes. We illustrate the EVD, pseudoinverse projection, and HOEVD of genome-scale networks with analyses of yeast DNA microarray data.

Characterization of the CaENG1 Gene Encoding an Endo-1,3-β-Glucanase Involved in Cell Separation in Candida albicans

The Candida albicans CaENG1 gene encoding an endo-1,3-beta-glucanase was cloned by screening a genomic library with a DNA probe obtained by polymerase chain reaction using synthetic oligonucleotides designed according to conserved regions found between two Saccharomyces cerevisiae endo-1,3-beta-glucanases (Eng1p and Eng2p). The gene contains a 3435-bp open reading frame (ORF), capable of encoding a protein of 1145 amino acids (124,157 Da), that contains no introns. Comparison of the ScEng1p sequence with partial C. albicans genomic sequences revealed the presence of a second protein with sequence similarity (the product of the Ca20C1.22c ORF, which was named CaENG2). Disruption of the CaENG1 gene in C. albicans had no dramatic effects on the growth rate of the strains, but it resulted in the formation of chains of cells, suggesting that the protein is involved in cell separation. Expression of CaENG1 in S. cerevisiae cells afforded a 12-fold increase in the 1,3-beta-glucanase activity detected in culture supernatants, showing that the protein has similar enzymatic activity to that of the S. cerevisiae Eng1p. In addition, when the C. albicans protein was expressed under its native promoter in S. cerevisiae eng1 mutant cells, it was able to complement the separation defect of this mutant, indicating that these two proteins are true functional homologues.

A genomewide screen for petite-negative yeast strains yields a new subunit of the i-AAA protease complex

Unlike many other organisms, the yeast Saccharomyces cerevisiae can tolerate the loss of mitochondrial DNA (mtDNA). Although a few proteins have been identified that are required for yeast cell viability without mtDNA, the mechanism of mtDNA-independent growth is not completely understood. To probe the relationship between the mitochondrial genome and cell viability, we conducted a microarray-based, genomewide screen for mitochondrial DNA-dependent yeast mutants. Among the several genes that we discovered is MGR1, which encodes a novel subunit of the i-AAA protease complex located in the mitochondrial inner membrane. mgr1Delta mutants retain some i-AAA protease activity, yet mitochondria lacking Mgr1p contain a misassembled i-AAA protease and are defective for turnover of mitochondrial inner membrane proteins. Our results highlight the importance of the i-AAA complex and proteolysis at the inner membrane in cells lacking mitochondrial DNA.

Genetic mosaics in the massive persisting rhizosphere colony "shiro" of the ectomycorrhizal basidiomycete Tricholoma matsutake

The ectomycorrhizal basidiomycete Tricholoma matsutake produces commercially valuable fruit bodies "matsutake" on a massive persisting rhizosphere aggregate of mycelia and mycorrhizas called "shiro." Using inter-retrotransposon amplified polymorphism analysis, we attempted to explore the potential diversity within the population of T. matsutake isolated from small Pinus densiflora woodlands located in various parts of Japan. In general, random phylogenetic relationship was noted among T. matsutake tested. The population from each limited sampling area was highly heterogeneous. Even some isolates from fruit bodies produced in the same shiro and those from spores in the same fruit bodies were found to be genetically diverse, indicating the occurrence of genetic mosaics in shiro. In a mosaic shiro, heterologous genets produced their fruit bodies concurrently. Data suggested that the dispersal of spores through sexual reproduction may have been more prevalent than generally accepted in T. matsutake to bring mosaicism and coordination of heterologous genets within the shiro. Implementation of management taking such diversity into consideration is urgently needed for the restoration of devastated matsutake fields in Japan. Exploration of individual clones in mosaic fungal resources that promote colonization and fruit body production is necessary for it.

Variant histone H2A.Z is globally localized to the promoters of inactive yeast genes and regulates nucleosome positioning

H2A.Z is an evolutionary conserved histone variant involved in transcriptional regulation, antisilencing, silencing, and genome stability. The mechanism(s) by which H2A.Z regulates these various biological functions remains poorly defined, in part due to the lack of knowledge regarding its physical location along chromosomes and the bearing it has in regulating chromatin structure. Here we mapped H2A.Z across the yeast genome at an approximately 300-bp resolution, using chromatin immunoprecipitation combined with tiling microarrays. We have identified 4,862 small regions--typically one or two nucleosomes wide--decorated with H2A.Z. Those "Z loci" are predominantly found within specific nucleosomes in the promoter of inactive genes all across the genome. Furthermore, we have shown that H2A.Z can regulate nucleosome positioning at the GAL1 promoter. Within HZAD domains, the regions where H2A.Z shows an antisilencing function, H2A.Z is localized in a wider pattern, suggesting that the variant histone regulates a silencing and transcriptional activation via different mechanisms. Our data suggest that the incorporation of H2A.Z into specific promoter-bound nucleosomes configures chromatin structure to poise genes for transcriptional activation. The relevance of these findings to higher eukaryotes is discussed.

A genomewide screen reveals a role of mitochondria in anaerobic uptake of sterols in yeast

The mechanisms that govern intracellular transport of sterols in eukaryotic cells are not well understood. Saccharomyces cerevisiae is a facultative anaerobic organism that becomes auxotroph for sterols and unsaturated fatty acids in the absence of oxygen. To identify pathways that are required for uptake and transport of sterols, we performed a systematic screen of the yeast deletion mutant collection for genes that are required for growth under anaerobic conditions. Of the approximately 4800 nonessential genes represented in the deletion collection, 37 were essential for growth under anaerobic conditions. These affect a wide range of cellular functions, including biosynthetic pathways for certain amino acids and cofactors, reprogramming of transcription and translation, mitochondrial function and biogenesis, and membrane trafficking. Thirty-three of these mutants failed to grow on lipid-supplemented media when combined with a mutation in HEM1, which mimics anaerobic conditions in the presence of oxygen. Uptake assays with radio- and fluorescently labeled cholesterol revealed that 17 of the 33 mutants strongly affect uptake and/or esterification of exogenously supplied cholesterol. Examination of the subcellular distribution of sterols in these uptake mutants by cell fractionation and fluorescence microscopy indicates that some of the mutants block incorporation of cholesterol into the plasma membrane, a presumably early step in sterol uptake. Unexpectedly, the largest class of uptake mutants is affected in mitochondrial functions, and many of the uptake mutants show electron-dense mitochondrial inclusions. These results indicate that a hitherto uncharacterized mitochondrial function is required for sterol uptake and/or transport under anaerobic conditions and are discussed in light of the fact that mitochondrial import of cholesterol is required for steroidogenesis in vertebrate cells.

DNA replication origins fire stochastically in fission yeast

DNA replication initiates at discrete origins along eukaryotic chromosomes. However, in most organisms, origin firing is not efficient; a specific origin will fire in some but not all cell cycles. This observation raises the question of how individual origins are selected to fire and whether origin firing is globally coordinated to ensure an even distribution of replication initiation across the genome. We have addressed these questions by determining the location of firing origins on individual fission yeast DNA molecules using DNA combing. We show that the firing of replication origins is stochastic, leading to a random distribution of replication initiation. Furthermore, origin firing is independent between cell cycles; there is no epigenetic mechanism causing an origin that fires in one cell cycle to preferentially fire in the next. Thus, the fission yeast strategy for the initiation of replication is different from models of eukaryotic replication that propose coordinated origin firing.

Retrotransposon-microsatellite amplified polymorphism (REMAP) markers for genetic diversity assessment of the rice blast pathogen (Magnaporthe grisea)

This present study is the first report of the application of the retrotransposon-microsatellite amplified polymorphism (REMAP) technique in fungi. Genome fingerprinting has a major role in the characterization of population structure and in the analysis of the variability in fungi. Retrotransposon-microsatellite amplified polymorphism assay was used in virulent isolates of a rice blast pathogen (Magnaporthe grisea) as a new assay system for genetic variability studies that overcomes the limitations of previous techniques. The high polymorphism observed in REMAP could be due to past or recent actions of retrotransposon in M. grisea. Retrotransposon-microsatellite amplified polymorphism, with its superior marker utility, was concluded to be the marker of choice for characterizing M. grisea isolates.Key words: Retrotransposon-microsatellite amplified polymorphism (REMAP), MAGGY, rice blast, Magnaporthe grisea, genetic diversity, retrotransposons.

Do disparate mechanisms of duplication add similar genes to the genome?

Gene duplication is the fundamental source of new genes. Biases in duplication have profound implications for the dynamics of gene content during evolution. In this article, we compare genes arising from whole gene duplication (WGD), smaller scale duplication (SSD) and singletons in Saccharomyces cerevisiae. Our results demonstrate that genes duplicated by WGD and SSD are similarly biased with respect to codon bias and evolutionary rate, although differing significantly in their functional constituency.

Molecular evolution of minisatellites in hemiascomycetous yeasts

Minisatellites are DNA tandem repeats exhibiting size polymorphism among individuals of a population. This polymorphism is generated by two different mechanisms, both in human and yeast cells, "replication slippage" during S-phase DNA synthesis and "repair slippage" associated to meiotic gene conversion. The Saccharomyces cerevisiae genome contains numerous natural minisatellites. They are located on all chromosomes without any obvious distribution bias. Minisatellites found in protein-coding genes have longer repeat units and on the average more repeat units than minisatellites in noncoding regions. They show an excess of cytosines on the coding strand, as compared to guanines (negative GC skew). They are always multiples of three, encode serine- and threonine-rich amino acid repeats, and are found preferably within genes encoding cell wall proteins, suggesting that they are positively selected in this particular class of genes. Genome-wide, there is no statistically significant association between minisatellites and meiotic recombination hot spots. In addition, minisatellites that are located in the vicinity of a meiotic hot spot are not more polymorphic than minisatellites located far from any hot spot. This suggests that minisatellites, in S. cerevisiae, evolve probably by strand slippage during replication or mitotic recombination. Finally, evolution of minisatellites among hemiascomycetous yeasts shows that even though many minisatellite-containing genes are conserved, most of the time the minisatellite itself is not conserved. The diversity of minisatellite sequences found in orthologous genes of different species suggests that minisatellites are differentially acquired and lost during evolution of hemiascomycetous yeasts at a pace faster than the genes containing them.

Higher duplicability of less important genes in yeast genomes

Gene duplication plays an important role in evolution because it is the primary source of new genes. Many recent studies showed that gene duplicability varies considerably among genes. Several considerations led us to hypothesize that less important genes have higher rates of successful duplications, where gene importance is measured by the fitness reduction caused by the deletion of the gene. Here, we test this hypothesis by comparing the importance of two groups of singleton genes in the yeast Saccharomyces cerevisiae (Sce). Group S genes did not duplicate in four other yeast species examined, whereas group D experienced duplication in these species. Consistent with our hypothesis, we found group D genes to be less important than group S genes. Specifically, 17% of group D genes are essential in Sce, compared to 28% for group S. Furthermore, deleting a group D gene in Sce reduces the fitness by 24% on average, compared to 38% for group S. Our subsequent analysis showed that less important genes have more cis-regulatory motifs, which could lead to a higher chance of subfunctionalization of duplicate genes and result in an enhanced rate of gene retention. Less important genes may also have weaker dosage imbalance effects and cause fewer genetic perturbations when duplicated. Regardless of the cause, our observation indicates that the previous finding of a less severe fitness consequence of deleting a duplicate gene than deleting a singleton gene is at least in part due to the fact that duplicate genes are intrinsically less important than singleton genes and suggests that the contribution of duplicate genes to genetic robustness has been overestimated.

Capture of extranuclear DNA at fission yeast double-strand breaks

Proper repair of DNA double-strand breaks (DSBs) is necessary for the maintenance of genomic integrity. Here, a new simple assay was used to study extrachromosomal DSB repair in Schizosaccharomyces pombe. Strikingly, DSB repair was associated with the capture of fission yeast mitochondrial DNA (mtDNA) at high frequency. Capture of mtDNA fragments required the Lig4p/Pku70p nonhomologous end-joining (NHEJ) machinery and its frequency was highly increased in fission yeast cells grown to stationary phase. The fission yeast Mre11 complex Rad32p/Rad50p/Nbs1p was also required for efficient capture of mtDNA at DSBs, supporting a role for the complex in promoting intermolecular ligation. Competition assays further revealed that microsatellite DNA from higher eukaryotes was preferentially captured at yeast DSBs. Finally, cotransformation experiments indicated that, in NHEJ-deficient cells, capture of extranuclear DNA at DSBs was observed if homologies--as short as 8 bp--were present between DNA substrate and DSB ends. Hence, whether driven by NHEJ, microhomology-mediated end-joining, or homologous recombination, DNA capture associated with DSB repair is a mutagenic process threatening genomic stability.

Agrobacterium tumefaciens-mediated transformation ofMucor circinelloides

The Agrobacterium tumefaciens-mediated transformation of the zygomycetous fungus Mucor circinelloides is described. A method was also developed for the hygromycin B-based selection of Mucor transformants. Transformation with the hygromycin B phosphotransferase gene of Escherichia coli controlled by the heterologous Aspergillus nidulans trpC promoter resulted in hygromycin B-resistant clones. The presence of the hygromycin resistance gene in the genome of the transformants was verified by polymerase chain reaction and Southern hybridization: the latter analyses revealed integrations in the host genome at different sites in different transformants. The stability of transformants remained questionable during the latter analyses.

Characterization of the yeast ionome: a genome-wide analysis of nutrient mineral and trace element homeostasis in Saccharomyces cerevisiae

BACKGROUND

Nutrient minerals are essential yet potentially toxic, and homeostatic mechanisms are required to regulate their intracellular levels. We describe here a genome-wide screen for genes involved in the homeostasis of minerals in Saccharomyces cerevisiae. Using inductively coupled plasma-atomic emission spectroscopy (ICP-AES), we assayed 4,385 mutant strains for the accumulation of 13 elements (calcium, cobalt, copper, iron, potassium, magnesium, manganese, nickel, phosphorus, selenium, sodium, sulfur, and zinc). We refer to the resulting accumulation profile as the yeast 'ionome'.

RESULTS

We identified 212 strains that showed altered ionome profiles when grown on a rich growth medium. Surprisingly few of these mutants (four strains) were affected for only one element. Rather, levels of multiple elements were altered in most mutants. It was also remarkable that only six genes previously shown to be involved in the uptake and utilization of minerals were identified here, indicating that homeostasis is robust under these replete conditions. Many mutants identified affected either mitochondrial or vacuolar function and these groups showed similar effects on the accumulation of many different elements. In addition, intriguing positive and negative correlations among different elements were observed. Finally, ionome profile data allowed us to correctly predict a function for a previously uncharacterized gene, YDR065W. We show that this gene is required for vacuolar acidification.

CONCLUSION

Our results indicate the power of ionomics to identify new aspects of mineral homeostasis and how these data can be used to develop hypotheses regarding the functions of previously uncharacterized genes.

Comparative genomics and disorder prediction identify biologically relevant SH3 protein interactions

Protein interaction networks are an important part of the post-genomic effort to integrate a part-list view of the cell into system-level understanding. Using a set of 11 yeast genomes we show that combining comparative genomics and secondary structure information greatly increases consensus-based prediction of SH3 targets. Benchmarking of our method against positive and negative standards gave 83% accuracy with 26% coverage. The concept of an optimal divergence time for effective comparative genomics studies was analyzed, demonstrating that genomes of species that diverged very recently from Saccharomyces cerevisiae(S. mikatae, S. bayanus, and S. paradoxus), or a long time ago (Neurospora crassa and Schizosaccharomyces pombe), contain less information for accurate prediction of SH3 targets than species within the optimal divergence time proposed. We also show here that intrinsically disordered SH3 domain targets are more probable sites of interaction than equivalent sites within ordered regions. Our findings highlight several novel S. cerevisiae SH3 protein interactions, the value of selection of optimal divergence times in comparative genomics studies, and the importance of intrinsic disorder for protein interactions. Based on our results we propose novel roles for the S. cerevisiae proteins Abp1p in endocytosis and Hse1p in endosome protein sorting.

How can we tackle the complexity of a living cell? It is commonly said that living organisms are complex and display “emergent” properties. Emergence is perceived in this context as behaviors that appear at the system level but are not observable at the level of the system's components. In the cell this would be equivalent to saying that the cellular complexity could be explained if we could understand the interplay between the cellular components: that is, not just describe the “parts” that make up a cell but understand how they interact with each other to perform the necessary tasks.

A big step on the road to understanding cellular complexity will be a complete list of all relevant interactions between the cellular components. Although a lot of progress as been made in this direction, we are often dependent on experimental methods that are costly and time consuming. It's a big challenge for computational biology to process the current available knowledge and to propose new ways of predicting the interactions between cellular components.

Here the researchers studied protein interactions that are mediated by small linear peptide motifs,specifically interactions between a protein's SH3 domain and its targets, usually small peptide stretches containing a PXXP motif (where P is proline and X is any amino acid). The results showed that the putative target motifs that are conserved in ortholog proteins and are within regions that do not have a defined secondary structure are more likely to be relevant binding sites. Besides proposing a way to combine secondary structure information with comparative genomics to predict protein–protein interactions, the researchers highlight a possible role of intrinsically disordered proteins in SH3 protein interactions. The results also show that when looking for conservation of these motifs, it is important to carefully select the species used in the study: comparisons between species that have diverged to a certain extent—not too little and not too much—are the most informative.

Fungal biology reaps the benefit of genomics

A report on the 23rd Fungal Genetics Conference, Pacific Grove, USA, 15-20 March 2005.

A report on the 23rd Fungal Genetics Conference, Pacific Grove, USA, 15-20 March 2005.

Paracoccidioides brasiliensis translation and protein fate machineries revealed by functional genome analysis

The translational and post-translational modification machineries of Paracoccidioides brasiliensis were assessed by means of comparative analyses of PbAESTs (P. brasiliensis assembled expressed sequence tags) with sequences deposited on different databases. Of the 79 sequences corresponding to cytosolic ribosomal proteins, we were able to find 78 in the P. brasiliensis transcriptome. Nineteen of the 27 Saccharomyces cerevisiae genes related to translation initiation were also found. All eukaryotic elongation factors were detected in P. brasiliensis transcriptome, with eEF1A as one of the most expressed genes. Translation termination is performed, in eukaryotes, by factors 1 and 3 (eRF1, eRF3). In P. brasiliensis transcriptome it was possible to identify eRF3, but not eRF1. Sixteen PbAESTs showing aminoacyl-tRNA synthetase-predicted activities were found in our analyses, but no cysteinyl-, leucyl-, asparagyl- and arginyl-tRNA synthetases were detected. Among the mitochondrial ribosomal proteins, we have found 20 and 18 orthologs to S. cerevisiae large and small ribosomal subunit proteins, respectively. We have also found three PbAESTs similar to Neurospora crassa mitochondrial ribosomal genes, with no similarity with S. cerevisiae genes. Although orthologs to S. cerevisiae mitochondrial EF-Tu, EF-G and RF1 have been found in P. brasiliensis transcriptome, no sequences corresponding to functional EF-Ts were detected. In addition, 64 and 28 PbAESTs associated to protein modification and degradation, respectively, were found. These results suggest that these machineries are well conserved in P. brasiliensis, when compared to other organisms.

Bioinformatics of the Paracoccidioides brasiliensis EST Project

Paracoccidioides brasiliensis is the etiological agent of paracoccidioidomycosis, an endemic mycosis of Latin America. This fungus presents a dimorphic character; it grows as a mycelium at room temperature, but it is isolated as yeast from infected individuals. It is believed that the transition from mycelium to yeast is important for the infective process. The Functional and Differential Genome of Paracoccidioides brasiliensis Project--PbGenome Project was developed to study the infection process by analyzing expressed sequence tags--ESTs, isolated from both mycelial and yeast forms. The PbGenome Project was executed by a consortium that included 70 researchers (professors and students) from two sequencing laboratories of the midwest region of Brazil; this project produced 25,741 ESTs, 19,718 of which with sufficient quality to be analyzed. We describe the computational procedures used to receive process, analyze these ESTs, and help with their functional annotations; we also detail the services that were used for sequence data exploration. Various programs were compared for filtering and grouping the sequences, and they were adapted to a user-friendly interface. This system made the analysis of the differential transcriptome of P. brasiliensis possible.

Antifungal susceptibility and genotypical pattern ofMicrosporum canisstrains

Dermatophytes are a group of fungi that are capable of invading keratinized tissues of humans and other animals. Antifungal susceptibility analysis and genetic studies by random amplification of polymorphic DNA (RAPD), have been used to detect polymorphism as well as determining the possible resistance of dermatophytes to antifungals. The aim of this study was to evaluate the possible correlation between the antifungal susceptibility and genotypical pattern of Microsporum canis strains isolated in dogs and cats with dermatophytosis in Northeast Brazil. The antifungal susceptibility study was conducted using the broth microdilution test with griseofulvine, ketoconazole, itraconazole, and fluconazole. The genotypical analysis was performed using the RAPD method. The antifungal susceptibility analysis showed that all the strains of M. canis analyzed (n = 22) were sensitive to griseofulvine (0.25 µg/mL ≤ minimum inhibitory concentration (MIC) ≤ 1 µg/mL), ketoconazole (0.25 µg/mL ≤ MIC ≤ 2 µg/mL), itraconazole (0.25 µg/mL ≤ MIC ≤ 1 µg/mL), and fluconazole (1 µg/mL ≤ MIC ≤ 16 µg/mL). The RAPD results showed that all analyzed strains are genetically similar. Thus, based on antifungal susceptibility analysis and RAPD data, a possible correlation can be shown between the antifungal susceptibility and the genotypical pattern of the strains of M. canis from Northeast Brazil.Key words: Microsporum canis, antifungal susceptibility testing, RAPD.

Large-scale 13C-flux analysis reveals mechanistic principles of metabolic network robustness to null mutations in yeast

Genome-scale 13^C-flux analysis in Saccharomyces cerevisiae revealed that the apparent dispensability of knockout mutants with metabolic function can be explained by gene inactivity under a particular condition, by network redundancy through duplicated genes or by alternative pathways.

Background

Quantification of intracellular metabolite fluxes by ¹³C-tracer experiments is maturing into a routine higher-throughput analysis. The question now arises as to which mutants should be analyzed. Here we identify key experiments in a systems biology approach with a genome-scale model of Saccharomyces cerevisiae metabolism, thereby reducing the workload for experimental network analyses and functional genomics.

Results

Genome-scale ¹³C flux analysis revealed that about half of the 745 biochemical reactions were active during growth on glucose, but that alternative pathways exist for only 51 gene-encoded reactions with significant flux. These flexible reactions identified in silico are key targets for experimental flux analysis, and we present the first large-scale metabolic flux data for yeast, covering half of these mutants during growth on glucose. The metabolic lesions were often counteracted by flux rerouting, but knockout of cofactor-dependent reactions, as in the adh1, ald6, cox5A, fum1, mdh1, pda1, and zwf1 mutations, caused flux responses in more distant parts of the network. By integrating computational analyses, flux data, and physiological phenotypes of all mutants in active reactions, we quantified the relative importance of 'genetic buffering' through alternative pathways and network redundancy through duplicate genes for genetic robustness of the network.

Conclusions

The apparent dispensability of knockout mutants with metabolic function is explained by gene inactivity under a particular condition in about half of the cases. For the remaining 207 viable mutants of active reactions, network redundancy through duplicate genes was the major (75%) and alternative pathways the minor (25%) molecular mechanism of genetic network robustness in S. cerevisiae.

Molecular genetic study of introgression betweenSaccharomyces bayanus andS. cerevisiae

The genomic constitution of different S. bayanus strains and natural interspecific Saccharomyces hybrids has been studied by genetic and molecular methods. Unlike S. bayanus var. uvarum, some S. bayanus var. bayanus strains (the type culture CBS 380, CBS 378, CBS 425, CBS 1548) harbour a number of S. cerevisiae subtelomeric sequences: Y', pEL50, SUC, RTM and MAL. The two varieties, having 86-100% nDNA-nDNA reassociation, are partly genetically isolated from one another but completely isolated from S. cerevisiae. Genetic and molecular data support the maintaining of var. bayanus and var. uvarum strains in the species S. bayanus. Using Southern hybridization with species-specific molecular markers, RFLP of the MET2 gene and flow cytometry analysis, we showed that the non-S. cerevisiae parents are different in lager brewing yeasts and in wine hybrid strains. Our results suggest that S. pastorianus is a hybrid between S. cerevisiae and S. bayanus var. bayanus, while S. bayanus var. uvarum contributed to the formation of the wine hybrids S6U and CID1. According to the partial sequence of ACT1 gene and flow cytometry analysis, strain CID1 is a triple hybrid between S. cerevisiae, S. kudriavzevii and S. bayanus var. uvarum.

A comparative genomic analysis of ESTs from Ustilago maydis

A large-scale comparative genomic analysis of unisequence sets obtained from an Ustilago maydis EST collection was performed against publicly available EST and genomic sequence datasets from 21 species. We annotated 70% of the collection based on similarity to known sequences and recognized protein signatures. Distinct grouping of the ESTs, defined by the presence or absence of similar sequences in the species examined, allowed the identification of U. maydis sequences present only (1) in fungal species, (2) in plants but not animals, (3) in animals but not plants, or (4) in all three eukaryotic lineages assessed. We also identified 215 U. maydis genes that are found in the ascomycete but not in the basidiomycete genome sequences searched. Candidate genes were identified for further functional characterization. These include 167 basidiomycete-specific sequences, 58 fungal pathogen-specific sequences (including 37 basidiomycete pathogen-specific sequences), and 18 plant pathogen-specific sequences, as well as two sequences present only in other plant pathogen and plant species.

Predicting gene function in Saccharomyces cerevisiae

MOTIVATION

S.cerevisiae is one of the most important model organisms, and has has been the focus of over a century of study. In spite of these efforts, 40% of its open reading frames (ORFs) remain classified as having unknown function (MIPS: Munich Information Center for Protein Sequences). We wished to make predictions for the function of these ORFs using data mining, as we have previously successfully done for the genomes of M.tuberculosis and E.coli. Applying this approach to the larger and eukaryotic S.cerevisiae genome involves modifying the machine learning and data mining algorithms, as this is a larger organism with more data available, and a more challenging functional classification.

RESULTS

Novel extensions to the machine learning and data mining algorithms have been devised in order to deal with the challenges. Accurate rules have been learned and predictions have been made for many of the ORFs whose function is currently unknown. The rules are informative, agree with known biology and allow for scientific discovery.

AVAILABILITY

All predictions are freely available from http://www.genepredictions.org, all datasets used in this study are freely available from http://www.aber.ac.uk/compsci/Research/bio/dss/yeastdataand software for relational data mining is available from http://www.aber.ac.uk/compsci/Research/bio/dss/polyfarm.

[Expression of phosphoprotein P2 of Cysticercus cellulosae in Pichia pastoris and its application]

Cysticercosis is caused by the metacestode form of Taenia solium-Cysticercus cellulosae and it causes great economic losses and threatens the people's health. There are some problems on how to control cysticercosis, in order to resolve the key problem that the native antigen to diagnose and prevent cysticercosis is very limited and is not satisfied, Pichia pastoris Expression System was used to express recombinant P2 protein. The interested P2 gene was got by digesting the pGEM - P2 vector using restriction endonuclease, then it was inserted into the secretory pPIC9K Pichia pastoris expression vector and transformed into E. coli. Positive recombinant plasmids were selected sequenced and named pPIC9K-P2 and it was linearized by Sal I and Bgl II, then the linear DNA transfored into Pichia pastoris GS115 by electroporation. The recombinant expression vector pPIC9K - P2 integrated into GS115 via homologous recombination between the transforming DNA and regions of homology within the genome. The transformants were screened for multicopy recombinants using G418 and were distinguished for Mut phenotypes by MD and MM. Two different phenotypes were generated-HIS+ MUT+ (Methanol utilization plus) and HIS+ MUT(S) (Methanol utilization slow). PCR analysis of the multicopy recombinants indicated that the P2 gene was integrated within the genome of pichia Pastoris. The multicopy recombinants were named GS115/pPIC9K - P2HIS+ MUT+ and GS115/pPIC9K-P2HIS+ MUT(S), both HIS+ MUT+ and HIS+ MUT(S) were induced with methanol. The results of SDS-PAGE and Western blot demonstrated that the culture supernatant of the induced Pichia pastoris contained P2 protein which was accumulated up to 33 % of total proteins in the culture supernant and its molecular weight is 12.6kD. The results of the clinical study indicated that the expression P2 protein could be used to diagnose human cysticercosis and swine cysticercosis as diagnosis antigen.

Genome-wide transposon mutagenesis in yeast

This unit provides comprehensive protocols for the use of insertional libraries generated by shuttle mutagenesis. From the basic protocol, a small aliquot of insertional library DNA may be used to mutagenize yeast, producing strains containing a single transposon insertion within a transcribed and translated region of the genome. This transposon-mutagenized bank of yeast strains may be screened for any desired mutant phenotype. Alternatively, since the transposon contains a reporter gene lacking its start codon and promoter, transposon-tagged strains may also be screened for specific patterns of gene expression. Strains of interest may be characterized by vectorette PCR (protocol provided) in order to locate the precise genomic site of transposon insertion within each mutant. A method by which Cre/lox recombination may be used to reduce the transposon in yeast to a small insertion element encoding an epitope tag is described. This tag serves as a tool by which transposon-mutagenized gene products may be analyzed further (e.g., localized to a discrete subcellular site).