2 Whole Chromosome Analysis

Resistance to Arsenite and Arsenate in Saccharomyces cerevisiae Arises through the Subtelomeric Expansion of a Cluster of Yeast Genes

Arsenic is one of the most prevalent toxic elements in the environment, and its toxicity affects every organism. Arsenic resistance has mainly been observed in microorganisms, and, in bacteria, it has been associated with the presence of the Ars operon. In Saccharomyces cerevisiae, three genes confer arsenic resistance: ARR1, ARR2, and ARR3. Unlike bacteria, in which the presence of the Ars genes confers per se resistance to arsenic, most of the S. cerevisiae isolates present the three ARR genes, regardless of whether the strain is resistant or sensitive to arsenic. To assess the genetic features that make natural S. cerevisiae strains resistant to arsenic, we used a combination of comparative genomic hybridization, whole-genome sequencing, and transcriptomics profiling with microarray analyses. We observed that both the presence and the genomic location of multiple copies of the whole cluster of ARR genes were central to the escape from subtelomeric silencing and the acquisition of resistance to arsenic. As a result of the repositioning, the ARR genes were expressed even in the absence of arsenic. In addition to their relevance in improving our understanding of the mechanism of arsenic resistance in yeast, these results provide evidence for a new cluster of functionally related genes that are independently duplicated and translocated.

Assessing the complex architecture of polygenic traits in diverged yeast populations

Phenotypic variation arising from populations adapting to different niches has a complex underlying genetic architecture. A major challenge in modern biology is to identify the causative variants driving phenotypic variation. Recently, the baker's yeast, Saccharomyces cerevisiae has emerged as a powerful model for dissecting complex traits. However, past studies using a laboratory strain were unable to reveal the complete architecture of polygenic traits. Here, we present a linkage study using 576 recombinant strains obtained from crosses of isolates representative of the major lineages. The meiotic recombinational landscape appears largely conserved between populations; however, strain-specific hotspots were also detected. Quantitative measurements of growth in 23 distinct ecologically relevant environments show that our recombinant population recapitulates most of the standing phenotypic variation described in the species. Linkage analysis detected an average of 6.3 distinct QTLs for each condition tested in all crosses, explaining on average 39% of the phenotypic variation. The QTLs detected are not constrained to a small number of loci, and the majority are specific to a single cross-combination and to a specific environment. Moreover, crosses between strains of similar phenotypes generate greater variation in the offspring, suggesting the presence of many antagonistic alleles and epistatic interactions. We found that subtelomeric regions play a key role in defining individual quantitative variation, emphasizing the importance of the adaptive nature of these regions in natural populations. This set of recombinant strains is a powerful tool for investigating the complex architecture of polygenic traits.

Saccharomyces cerevisiae: gene annotation and genome variability, state of the art through comparative genomics

In the early days of the yeast genome sequencing project, gene annotation was in its infancy and suffered the problem of many false positive annotations as well as missed genes. The lack of other sequences for comparison also prevented the annotation of conserved, functional sequences that were not coding. We are now in an era of comparative genomics where many closely related as well as more distantly related genomes are available for direct sequence and synteny comparisons allowing for more probable predictions of genes and other functional sequences due to conservation. We also have a plethora of functional genomics data which helps inform gene annotation for previously uncharacterised open reading frames (ORFs)/genes. For Saccharomyces cerevisiae this has resulted in a continuous updating of the gene and functional sequence annotations in the reference genome helping it retain its position as the best characterized eukaryotic organism's genome. A single reference genome for a species does not accurately describe the species and this is quite clear in the case of S. cerevisiae where the reference strain is not ideal for brewing or baking due to missing genes. Recent surveys of numerous isolates, from a variety of sources, using a variety of technologies have revealed a great deal of variation amongst isolates with genome sequence surveys providing information on novel genes, undetectable by other means. We now have a better understanding of the extant variation in S. cerevisiae as a species as well as some idea of how much we are missing from this understanding. As with gene annotation, comparative genomics enhances the discovery and description of genome variation and is providing us with the tools for understanding genome evolution, adaptation and selection, and underlying genetics of complex traits.

Localization of telomeres and telomere-associated proteins in telomerase-negative Saccharomyces cerevisiae

Cells lacking telomerase cannot maintain their telomeres and undergo a telomere erosion phase leading to senescence and crisis in which most cells become nonviable. On rare occasions survivors emerge from these cultures that maintain their telomeres in alternative ways. The movement of five marked telomeres in Saccharomyces cerevisiae was followed in wild-type cells and through erosion, senescence/crisis and eventual survival in telomerase-negative (est2::HYG) yeast cells. It was found that during erosion, movements of telomeres in est2::HYG cells were indistinguishable from wild-type telomere movements. At senescence/crisis, however, most cells were in G(2) arrest and the nucleus and telomeres traversed back and forth across the bud neck, presumably until cell death. Type I survivors, using subtelomeric Y' amplification for telomere maintenance, continued to show this aberrant telomere movement. However, Type II survivors, maintaining telomeres by a sudden elongation of the telomere repeats, became indistinguishable from wild-type cells, consistent with growth properties of the two types of survivors. When telomere-associated proteins Sir2p, Sir3p and Rap1p were tagged, the same general trend was seen-Type I survivors retained the senescence/crisis state of protein localization, while Type II survivors were restored to wild type.

Inferences of evolutionary relationships from a population survey of LTR-retrotransposons and telomeric-associated sequences in the Saccharomyces sensu stricto complex

The Saccharomyces sensu stricto complex consists of six closely related species and one natural hybrid. Intra- and inter- species variability in repetitive elements can help elucidate the population structure and evolution of these close relatives. The chromosome positions of several telomeric associated sequences (TASs) and LTR-retrotransposons have been determined, using PFGE, in 112 isolates. Most of the repetitive elements studied are found in multiple copies in each strain, although in some subpopulations these elements are present in low copy number or are absent. Hybridization patterns and copy numbers of the repetitive elements correlate with geographic distribution. These patterns may yield interesting clues as to the origins and evolution of some TASs and retrotransposons, e.g. we can infer that Y' originated on the left end of chromosome XIV. There is strong evidence for horizontal transfer of Ty2 between S. cerevisiae and S. mikatae. Ty1 and Ty5 are either lost easily or frequently horizontally transferred. We have also found some gross chromosomal rearrangements in isolates within species and a few new natural hybrids between species, indicating that these processes occur in the wild and are not limited to conditions of human influence. DNA sequences have been deposited with the EMBL/GenBank database under Accession Nos AJ632279-AJ632293.

A role for the MutL homologue MLH2 in controlling heteroduplex formation and in regulating between two different crossover pathways in budding yeast

BACKGROUND AND AIMS

Mismatch repair proteins play important roles during meiotic recombination in the budding yeast Saccharomyces cerevisiae and most eukaryotic organisms studied to date. To study the functions of the mismatch repair protein Mlh2p in meiosis, we constructed mlh2Delta strains and measured rates of crossing over, gene conversion, post-meiotic segregation and spore viability. We also analysed mlh1Delta, mlh3Delta, msh4Delta, msh5Delta, exo1Delta and mus81Delta mutant strains singularly and in various combinations.

RESULTS

Loss of MLH2 resulted in a small but significant decrease in spore viability and a significant increase in gene conversion frequencies but had no apparent effect on crossing over. Deletion of MLH2 in mlh3Delta, msh4Delta or msh5Delta strains resulted in significant proportion of the "lost" crossovers found in single deletion strains being regained in some genetic intervals. We and others propose that there are at least two pathways to generate crossovers in yeast (Ross-Macdonald and Roeder, 1994; Zalevsky et al., 1999; Khazanehdari and Borts, 2000; Novak et al., 2001; de los Santos et al., 2003). Most recombination intermediates are processed by the "major", Msh4-dependent pathway, which requires the activity of Mlh1p/Mlh3p/Msh4p/Msh5p as well as a number of other proteins. The minor pathway(s) utilizes Mms4p/Mus81p. We suggest that the absence of Mlh2p allows some crossovers from the MSH4 pathway to traverse the MUS81-dependent pathway.

Isolation of the repertoire of VSG expression site containing telomeres of Trypanosoma brucei 427 using transformation-associated recombination in yeast

Trypanosoma brucei switches between variant surface glycoproteins (VSGs) allowing immune escape. The active VSG is in one of many telomeric bloodstream form VSG expression sites (BESs), also containing expression site-associated genes (ESAGs) involved in host adaptation. The role of BES sequence diversity in parasite virulence can best be understood through analysis of the full repertoire of BESs from a given T. brucei strain. However, few BESs have been cloned, as telomeres are highly underrepresented in standard libraries. We devised a strategy for isolating the repertoire of T. brucei 427 BES-containing telomeres in Saccaromyces cerevisiae by using transformation-associated recombination (TAR). We isolated 182 T. brucei 427 BES TAR clones, 167 of which could be subdivided into minimally 17 BES groups. This set gives us the first view of the breadth and diversity of BESs from one T. brucei strain. Most BESs ranged between 40 and 70 kb (average, 57 +/- 17 kb) and contained most identified ESAGs. Phylogenetic comparison of the cohort of BES promoter and ESAG6 sequences did not show similar trees, indicating rapid evolution most likely mediated by sequence exchange between BESs. This cloning strategy could be used for any T. brucei strain, facilitating research on the biodiversity of telomeric gene families and host-pathogen interactions.

NEJ1 prevents NHEJ-dependent telomere fusions in yeast without telomerase

In a search for genes involved in cell-type-dependent chromosome instability, we have found a role for NEJ1, a regulator of nonhomologous end joining (NHEJ), in cells that survive in the absence of telomerase. In yeast, NHEJ is regulated by mating-type status through NEJ1, which is repressed in a/alpha cells. For efficient NHEJ, NEJ1 is required as part of a complex with LIF1 and DNL4, which catalyzes DNA ligation. In haploid cells without telomerase, we find that the absence of NEJ1 results in high frequencies of circular chromosomes in type II survivors (i.e., those typified by lengthened telomere repeat tracts). These telomere fusion events are DNL4 dependent. NEJ1 therefore has a role in protecting telomeres from end fusions by NHEJ in the absence of telomerase that contrasts with its role in promoting repair at sites of DNA double-strand breaks.

Evolution of gene order in the genomes of two related yeast species

Changes in gene order between the genomes of two related yeast species, Saccharomyces cerevisiae and Saccharomyces bayanus var. uvarum were studied. From the dataset of a previous low coverage sequencing of the S. bayanus var. uvarum genome, 35 different synteny breakpoints between neighboring genes and two cases of local gene inversion were characterized in detail. The number and the type of the chromosomal rearrangements that have led to these differences were identified. We show that evolution of gene order in the genomes of these two yeast species is driven mainly by gene duplication onto different chromosomes followed by differential loss of the repeated copies. In addition, local gene inversions also would result from a mechanism of gene duplication, but in an inverted orientation, followed by loss of the original copy. The identification of traces of anciently duplicated genes, called relics, show that the loss of duplicates is more frequently caused by the accumulation of numerous mutations in one of the two copies than by DNA deletion. Surprisingly, gross chromosomal rearrangements such as translocations have only a minor effect on gene order reshuffling as they account for <10% of the synteny breakpoints.

Chromosomal evolution in Saccharomyces

The chromosomal speciation model invokes chromosomal rearrangements as the primary cause of reproductive isolation. In a heterozygous carrier, chromosomes bearing reciprocal translocations mis-segregate at meiosis, resulting in reduced fertility or complete sterility. Thus, chromosomal rearrangements act as a post-zygotic isolating mechanism. Reproductive isolation in yeast is due to post-zygotic barriers, as many species mate successfully but the hybrids are sterile. Reciprocal translocations are thought to be the main form of large-scale rearrangement since the hypothesized duplication of the whole yeast genome 10(8) years ago. To test the chromosomal speciation model in yeast, we have characterized chromosomal translocations among the genomes of six closely related species in the Saccharomyces 'sensu stricto' complex. Here we show that rearrangements have occurred between closely related species, whereas more distant ones have colinear genomes. Thus, chromosomal rearrangements are not a prerequisite for speciation in yeast and the rate of formation of translocations is not constant. These rearrangements appear to result from ectopic recombination between Ty elements or other repeated sequences.

Use of a recombination reporter insert to define meiotic recombination domains on chromosome III of Saccharomyces cerevisiae

In Saccharomyces cerevisiae, meiotic recombination is initiated by DNA double-strand breaks (DSBs). DSBs usually occur in intergenic regions that display nuclease hypersensitivity in digests of chromatin. DSBs are distributed nonuniformly across chromosomes; on chromosome III, DSBs are concentrated in two "hot" regions, one in each chromosome arm. DSBs occur rarely in regions within about 40 kb of each telomere and in an 80-kb region in the center of the chromosome, just to the right of the centromere. We used recombination reporter inserts containing arg4 mutant alleles to show that the "cold" properties of the central DSB-deficient region are imposed on DNA inserted in the region. Cold region inserts display DSB and recombination frequencies that are substantially less than those seen with similar inserts in flanking hot regions. This occurs without apparent change in chromatin structure, as the same pattern and level of DNase I hypersensitivity is seen in chromatin of hot and cold region inserts. These data are consistent with the suggestion that features of higher-order chromosome structure or chromosome dynamics act in a target sequence-independent manner to control where recombination events initiate during meiosis.

The leptotene-zygotene transition of meiosis

The leptotene/zygotene transition of meiosis, as defined by classical cytological studies, is the period when homologous chromosomes, already being discernible individualized entities, begin to be close together or touching over portions of their lengths. This period also includes the bouquet stage: Chromosome ends, which have already become integral components of the inner nuclear membrane, move into a polarized configuration, along with other nuclear envelope components. Chromosome movements, active or passive, also occur. The detailed nature of interhomologue interactions during this period, with special emphasis on the involvement of chromosome ends, and the overall role for meiosis and recombination of chromosome movement and, especially, the bouquet stage are discussed.