Genome instability: McClintock revisited.

Recent studies in yeast have shed light on the molecular mechanisms by which telomere dysfunction leads to chromosome fusions. Furthermore, examination of the consequences of telomerase loss in mice suggests that only a few critically short telomeres may be sufficient to promote genomic instability.

Defects in mismatch repair promote telomerase-independent proliferation.

Mismatch repair has a central role in maintaining genomic stability by repairing DNA replication errors and inhibiting recombination between non-identical (homeologous) sequences. Defects in mismatch repair have been linked to certain human cancers, including hereditary non-polyposis colorectal cancer (HNPCC) and sporadic tumours. A crucial requirement for tumour cell proliferation is the maintenance of telomere length, and most tumours achieve this by reactivating telomerase. In both yeast and human cells, however, telomerase-independent telomere maintenance can occur as a result of recombination-dependent exchanges between often imperfectly matched telomeric sequences. Here we show that loss of mismatch-repair function promotes cellular proliferation in the absence of telomerase. Defects in mismatch repair, including mutations that correspond to the same amino-acid changes recovered from HNPCC tumours, enhance telomerase-independent survival in both Saccharomyces cerevisiae and a related budding yeast with a degree of telomere sequence homology that is similar to human telomeres. These results indicate that enhanced telomeric recombination in human cells with mismatch-repair defects may contribute to cell immortalization and hence tumorigenesis.

Saccharomyces cerevisiae CTF18 and CTF4 are required for sister chromatid cohesion.

CTF4 and CTF18 are required for high-fidelity chromosome segregation. Both exhibit genetic and physical ties to replication fork constituents. We find that absence of either CTF4 or CTF18 causes sister chromatid cohesion failure and leads to a preanaphase accumulation of cells that depends on the spindle assembly checkpoint. The physical and genetic interactions between CTF4, CTF18, and core components of replication fork complexes observed in this study and others suggest that both gene products act in association with the replication fork to facilitate sister chromatid cohesion. We find that Ctf18p, an RFC1-like protein, directly interacts with Rfc2p, Rfc3p, Rfc4p, and Rfc5p. However, Ctf18p is not a component of biochemically purified proliferating cell nuclear antigen loading RF-C, suggesting the presence of a discrete complex containing Ctf18p, Rfc2p, Rfc3p, Rfc4p, and Rfc5p. Recent identification and characterization of the budding yeast polymerase kappa, encoded by TRF4, strongly supports a hypothesis that the DNA replication machinery is required for proper sister chromatid cohesion. Analogous to the polymerase switching role of the bacterial and human RF-C complexes, we propose that budding yeast RF-C(CTF18) may be involved in a polymerase switch event that facilities sister chromatid cohesion. The requirement for CTF4 and CTF18 in robust cohesion identifies novel roles for replication accessory proteins in this process.

Cdc13 both positively and negatively regulates telomere replication.

Cdc13 is a single-strand telomeric DNA-binding protein that positively regulates yeast telomere replication by recruiting telomerase to chromosome termini through a site on Cdc13 that is eliminated by the cdc13-2 mutation. Here we show that Cdc13 has a separate role in negative regulation of telomere replication, based on analysis of a new mutation, cdc13-5. Loss of this second regulatory activity results in extensive elongation of the G strand of the telomere by telomerase, accompanied by a reduced ability to coordinate synthesis of the C strand. Both the cdc13-5 mutation and DNA polymerase alpha mutations (which also exhibit elongated telomeres) are suppressed by increased expression of the Cdc13-interacting protein Stn1, indicating that Stn1 coordinates action of the lagging strand replication complex with the regulatory activity of CDC13. However, the association between Cdc13 and Stn1 is abolished by cdc13-2, the same mutation that eliminates the interaction between Cdc13 and telomerase. We propose that Cdc13 participates in two regulatory steps-first positive, then negative-as a result of successive binding of telomerase and the negative regulator Stn1 to overlapping sites on Cdc13. Thus, Cdc13 coordinates synthesis of both strands of the telomere by first recruiting telomerase and subsequently limiting G-strand synthesis by telomerase in response to C-strand replication.

Cdc13 delivers separate complexes to the telomere for end protection and replication.

In Saccharomyces cerevisiae, the telomere binding protein Cdc13 mediates telomere replication by recruiting telomerase, and also performs an essential function in chromosome end protection. We show here that delivery of the Stn1 protein to the telomere, by fusing the DNA binding domain of Cdc13 (DBD(CDC13)) to Stn1, is sufficient to rescue the lethality of a cdc13 null strain and, hence, provide end protection. Telomere replication is still defective in this strain, but can be restored by delivering telomerase to the telomere as a DBD(CDC13)-telomerase fusion. These results establish Stn1 as the primary effector of chromosome end protection, whereas the principal function of Cdc13 is to provide a loading platform to recruit complexes that provide end protection and telomere replication.

Growth and manipulation of yeast.

This unit describes preparation of selected media for growing yeast and also discusses strain storage and revival. Protocols are provided for the assay of beta-galactosidase in liquid culture and for transformation using lithium acetate.

Preparation of yeast media.

Preparation of sterile media of consistently high quality is essential for the genetic manipulation of yeast. Recipes for media needed in the protocols in this chapter are provided in this unit. Specific suppliers are recommended for specific ingredients.

Manipulation of cloned yeast DNA.

A major advantage of working with yeast is the ability to replace the wild-type chromosomal copy of a gene with a mutant derivative that is constructed in vitro using a cloned copy of the gene. This technique unavailable in most other eukaryotes allows the phenotype of the mutation to be studied under accurate in vivo conditions, with the mutation present in single copy at its normal chromosomal location. In the first protocol, a plasmid harboring both a selectable marker and a cloned gene of interest is integrated at the chromosomal location of the cloned gene via homologous recombination (integrative transformation). Four methods are described for constructing a mutation in vitro in a cloned gene and reintroducing this mutation at the correct chromosomal site. This allows assessment of the genetic consequences of a mutation, and is often used to determine whether or not a gene is essential (by determining if a complete gene deletion is viable). Two of these techniques integrative disruption and one-step gene disruption generate either insertion or deletion mutations. The third technique transplacement is more generally applicable: it can be used to introduce insertion or deletion mutations containing a selectable marker, but it can also be used to introduce nonselectable mutations, such as conditional lethal mutations in an essential gene. Protocols are also provided to allow creation of modified genes by one-step integrative replacement, and also conditional alleles by a copper-inducible double-shutoff procedure.

Yeast cloning vectors and genes.

This unit describes some of the most commonly used yeast vectors, as well as the cloned yeast genes that form the basis for these plasmids. Yeast vectors can be grouped into five general classes, based on their mode of replication in yeast: YIp, YRp, YCp, YEp, and YLp plasmids. With the exception of the YLp plasmids (yeast linear plasmids), all of these plasmids can be maintained in E. coli as well as in S. cerevisiae and thus are referred to as shuttle vectors. The nomenclature of different classes of yeast vectors, as well as details about their mode of replication in yeast are discussed.

Yeast vectors and assays for expression of cloned genes.

This unit describes some of the most commonly used yeast vectors, as well as the cloned yeast genes that form the basis for these plasmids. Yeast vectors can be grouped into five general classes, based on their mode of replication in yeast: YIp, YRp, YCp, YEp, and YLp plasmids. With the exception of the YLp plasmids (yeast linear plasmids), all of these plasmids can be maintained in E. coli as well as in S. cerevisiae and thus are referred to as shuttle vectors. The nomenclature of different classes of yeast vectors, as well as details about their mode of replication in yeast are discussed.

Introduction of DNA into yeast cells.

The most commonly used yeast transformation protocol is the lithium acetate procedure (described here). It is reasonably fast and provides a transformation efficiency of 10(5) to 10(6) transformants/microg. This efficiency rivals that achieved for most, but not all, strains with the more difficult and time-consuming spheroplast procedure presented here. However, the fastest and easiest of the transformation methods is electroporation, as described in this unit. For a number of strains, electroporation offers the highest transformation efficiency, and may prove especially useful with limiting quantities of transforming DNA. Unlike the lithium acetate procedure, however, electroporation saturates at low DNA levels, restricting its general utility.

Cloning yeast genes by complementation.

This unit presents a generalized protocol and describes the principles involved in cloning yeast genes by complementation in yeast. The protocol is presented using a hypothetical mutation of yeast, the cdc101-1 mutation. This mutation was isolated as a cell cycle mutant and is both recessive and temperature-sensitive for growth: it can grow relatively normally at 30 degrees C but is unable to make a colony at 37 degrees C. A genomic DNA clone that complements this mutation will be isolated by transforming the cdc101-1 strain with a yeast genomic library and subsequently screening for temperature-resistant colonies. Once isolated, two steps are necessary to prove that the insert present on the plasmid contains the wild-type CDC101 gene. First, segregation of the complementing plasmid must result in co-loss of both the plasmid-borne selectable marker and the complementing phenotype, demonstrating that the observed complementation is plasmid-specific and is not due to reversion of the cdc101-1 mutation. Second, it must be ruled out whether the cloned gene encodes a phenotypic suppressor of the mutation, rather than the wild-type gene. This is done via a complementation test, which demonstrates whether or not a disruption of the cloned gene that is integrated into the genome can complement the original mutation.

Manipulation of plasmids from yeast cells.

This unit describes several procedures for manipulating plasmids in yeast cells. The first is a general method to segregate autonomously replicating plasmids from cells: plasmid-containing yeast cells are grown in nonselective medium, and colonies lacking the plasmid are identified by replica plating. The second, plasmid shuffling, represents a specialized version of plasmid segregation that is useful for analyzing the function of essential genes and for identifying conditional lethal mutations in essential genes. The third approach, plasmid gap repair, is based on the efficient homologous recombination characteristics of yeast cells. Plasmid gap repair can be be used as a method to incorporate mutagenized DNA fragments into a yeast plasmid, rescue genomic mutations onto plasmids, or map alleles of a given gene.

Positive and negative regulation of telomerase access to the telomere.

The protective caps on chromosome ends - known as telomeres - consist of DNA and associated proteins that are essential for chromosome integrity. A fundamental part of ensuring proper telomere function is maintaining adequate length of the telomeric DNA tract. Telomeric repeat sequences are synthesized by the telomerase reverse transcriptase, and, as such, telomerase is a central player in the maintenance of steady-state telomere length. Evidence from both yeast and mammals suggests that telomere-associated proteins positively or negatively control access of telomerase to the chromosome terminus. In yeast, positive regulation of telomerase access appears to be achieved through recruitment of the enzyme by the end-binding protein Cdc13p. In contrast, duplex-DNA-binding proteins assembled along the telomeric tract exert a feedback system that negatively modulates telomere length by limiting the action of telomerase. In mammalian cells, and perhaps also in yeast, binding of these proteins probably promotes a higher-order structure that renders the telomere inaccessible to the telomerase enzyme.

DNA ends: maintenance of chromosome termini versus repair of double strand breaks.

This review focuses on the factors that define the differences between the two types of DNA ends encountered by eukaryotic cells: telomeres and double strand breaks (DSBs). Although these two types of DNA termini are functionally distinct, recent studies have shown that a number of proteins is shared at telomeres and sites of DSB repair. The significance of these common components is discussed, as well as the types of DNA repair events that can compensate for a defective telomere.

Molecular biology. Telomeres keep on rappin'.

Many molecules help maintain the ends of chromosomes, which get chewed off as cells age. Lundblad in her provocative Perspective now tells us about another protein, hRap1, that regulates the length of telomeres in human cells with the help of the TRF proteins. The homology between hRap1 and its counterpart in yeast suggests how the complex molecular machinery needed to maintain chromosome ends may have evolved.

The Est3 protein is a subunit of yeast telomerase.

EST1, EST2, EST3 and TLC1 function in a single pathway for telomere replication in the yeast Saccharomyces cerevisiae [1] [2], as would be expected if these genes all encode components of the same complex. Est2p, the reverse transcriptase protein subunit, and TLC1, the templating RNA, are subunits of the catalytic core of yeast telomerase [3] [4] [5]. In contrast, mutations in EST1, EST3 or CDC13 eliminate telomere replication in vivo [1] [6] [7] [8] but are dispensable for in vitro telomerase catalytic activity [2] [9]. Est1p and Cdc13p, as components of telomerase and telomeric chromatin, respectively, cooperate to recruit telomerase to the end of the chromosome [7] [10]. However, Est3p has not yet been biochemically characterized and thus its specific role in telomere replication is unclear. We show here that Est3p is a stable component of the telomerase holoenzyme and furthermore, association of Est3p with the enzyme requires an intact catalytic core. As predicted for a telomerase subunit, fusion of Est3p to the high affinity Cdc13p telomeric DNA binding domain greatly increases access of telomerase to the telomere. Est1p is also tightly associated with telomerase; however, Est1p is capable of forming a stable TLC1-containing complex even in the absence of Est2p or Est3p. Yeast telomerase therefore contains a minimum of three Est proteins for which there is both in vivo and in vitro evidence for their role in telomere replication as subunits of the telomerase complex.

Identification of the single-strand telomeric DNA binding domain of the Saccharomyces cerevisiae Cdc13 protein.

The CDC13 gene of Saccharomyces cerevisiae is required both to protect telomeric DNA and to ensure proper function of yeast telomerase in vivo. We have previously demonstrated that Cdc13p has a high affinity single-strand telomeric DNA binding activity, although the primary amino acid sequence of Cdc13p has no previously characterized DNA binding motifs. We report here mapping of the Cdc13 DNA binding domain by a combination of proteolysis mapping and deletion cloning. The DNA binding domain maps to residues 557-694 of the 924-amino acid Cdc13 polypeptide, within the most basic region of Cdc13p. A slightly larger version of this domain can be efficiently expressed in Escherichia coli as a soluble small protein, with DNA binding properties comparable to those of the full-length protein. A single amino acid missense mutation within this domain results in thermolabile DNA binding and conditional lethality in yeast, consistent with the prediction that DNA binding should be essential for CDC13 function. These results show that Cdc13p contains a discrete substructure responsible for DNA binding and should facilitate structural characterization of this telomere binding protein.

Est1 and Cdc13 as comediators of telomerase access.

Cdc13 and Est1 are single-strand telomeric DNA binding proteins that contribute to telomere replication in the yeast Saccharomyces cerevisiae. Here it is shown that fusion of Cdc13 to the telomerase-associated Est1 protein results in greatly elongated telomeres. Fusion proteins consisting of mutant versions of Cdc13 or Est1 confer similar telomere elongation, indicating that close physical proximity can bypass telomerase-defective mutations in either protein. Fusing Cdc13 directly to the catalytic core of telomerase allows stable telomere maintenance in the absence of Est1, consistent with a role for Est1 in mediating telomerase access. Telomere length homeostasis therefore is maintained in part by restricting access of telomerase to chromosome termini, but this limiting situation can be overcome by directly tethering telomerase to the telomere.