Characterization of REC104, a gene required for early meiotic recombination in the yeast Saccharomyces cerevisiae

Divergence and conservation of the meiotic recombination machinery

Sexually reproducing eukaryotes use recombination between homologous chromosomes to promote chromosome segregation during meiosis. Meiotic recombination is almost universally conserved in its broad strokes, but specific molecular details often differ considerably between taxa, and the proteins that constitute the recombination machinery show substantial sequence variability. The extent of this variation is becoming increasingly clear because of recent increases in genomic resources and advances in protein structure prediction. We discuss the tension between functional conservation and rapid evolutionary change with a focus on the proteins that are required for the formation and repair of meiotic DNA double-strand breaks. We highlight phylogenetic relationships on different time scales and propose that this remarkable evolutionary plasticity is a fundamental property of meiotic recombination that shapes our understanding of molecular mechanisms in reproductive biology.

Mechanism and Control of Meiotic DNA Double-Strand Break Formation in S. cerevisiae

Developmentally programmed formation of DNA double-strand breaks (DSBs) by Spo11 initiates a recombination mechanism that promotes synapsis and the subsequent segregation of homologous chromosomes during meiosis. Although DSBs are induced to high levels in meiosis, their formation and repair are tightly regulated to minimize potentially dangerous consequences for genomic integrity. In S. cerevisiae, nine proteins participate with Spo11 in DSB formation, but their molecular functions have been challenging to define. Here, we describe our current view of the mechanism of meiotic DSB formation based on recent advances in the characterization of the structure and function of DSB proteins and discuss regulatory pathways in the light of recent models.

Kel1p Mediates Yeast Cell Fusion Through a Fus2p- and Cdc42p-Dependent Mechanism

Cell fusion is ubiquitous among eukaryotes. Although little is known about the molecular mechanism, several proteins required for cell fusion in the yeast Saccharomyces cerevisiae have been identified. Fus2p, a key regulator of cell fusion, localizes to the shmoo tip in a highly regulated manner. C-terminal truncations of Fus2p cause mislocalization and fusion defects, which are suppressed by overexpression of Kel1p, a kelch-domain protein of unknown function previously implicated in cell fusion. We hypothesize that Fus2p mislocalization is caused by auto-inhibition, which is alleviated by Kel1p overexpression. Previous work showed that Fus2p localization is mediated by both Fus1p- and actin-dependent pathways. We show that the C-terminal mutations mainly affect the actin-dependent pathway. Suppression of the Fus2p localization defect by Kel1p is dependent upon Fus1p, showing that suppression does not bypass the normal pathway. Kel1p and a homolog, Kel2p, are required for efficient Fus2p localization, acting through the actin-dependent pathway. Although Kel1p overexpression can weakly suppress the mating defect of a FUS2 deletion, the magnitude of suppression is allele specific. Therefore, Kel1p augments, but does not bypass, Fus2p function. Fus2p mediates cell fusion by binding activated Cdc42p Although Kel1p overexpression suppresses a Cdc42p mutant that is defective for Fus2p binding, cell fusion remains dependent upon Fus2p These data suggest that Fus2p, Cdc42p, and Kel1p form a ternary complex, which is stabilized by Kel1p Supporting this hypothesis, Kel1p interacts with two domains of Fus2p, partially dependent on Cdc42p We conclude that Kel1p enhances the activity of Fus2p/Cdc42p in cell fusion.

The signal from the initiation of meiotic recombination to the first division of meiosis

Two of the unique events that occur in meiosis are high levels of genetic recombination and the reductional division. Our previous work demonstrated that the REC102, REC104, REC114, and RAD50 genes, required to initiate meiotic recombination in Saccharomyces cerevisiae, are needed for the proper timing of the first meiotic (MI) division. If these genes are absent, the MI division actually begins at an earlier time. This paper demonstrates that the meiotic recombination genes MER2/REC107, SPO11, and MRE2 and the synaptonemal complex genes HOP1 and RED1 are also required for the normal delay of the MI division. A rec103/ski8 mutant starts the MI division at the same time as in wild-type cells. Our data indicate no obvious correlation between the timing of premeiotic S phase and the timing of the first division in Rec- mutants. Cells with rec102 or rec104 mutations form MI spindles before wild-type cells, suggesting that the initiation signal acts prior to spindle formation. Neither RAD9 nor RAD24 is needed to transduce the signal, which delays the first division. The timing of the MI division in RAD24 mutants indicates that the pachytene checkpoint is not active in Rec+ cells and suggests that the coordination between recombination and the MI division in wild-type cells may occur primarily due to the initiation signal. Finally, at least one of the targets of the recombination initiation signal is the NDT80 gene, a transcriptional regulator of middle meiotic gene expression required for the first division.

Support for a meiotic recombination initiation complex: interactions among Rec102p, Rec104p, and Spo11p

Initiation of meiotic recombination in the yeast Saccharomyces cerevisiae requires at least 10 gene products. The initiation event creates double-strand breaks, which are then processed by other recombination enzymes. A variety of classical observations, such as the existence of recombination nodules, have suggested that the proteins catalyzing recombination form a complex. A variety of lines of evidence indicate that Rad50p, Mre11p, and Xrs2p interact, and genetic data suggesting interactions between Rec102p and Rec104p have been reported. It has recently been shown that Spo11p coimmunoprecipitates with Rec102p in meiosis as well. In this paper, we provide genetic and biochemical evidence that the meiosis-specific proteins Rec102p, Rec104p, and Spo11p all interact with each other in meiosis. Furthermore, we demonstrate that the interaction between Rec102p and Spo11p does not require Rec104p. Likewise, the interaction between Rec104p and Rec102p does not require Spo11p, although Spo11p may stabilize that association. The interactions suggest that Spo11p, Rec102p, and Rec104p may form a trimeric complex during the initiation of recombination.

Mechanism and control of meiotic recombination initiation

Homologous recombination is essential during meiosis in most sexually reproducing organisms. In budding yeast, and most likely in other organisms as well, meiotic recombination proceeds via the formation and repair of DNA double-strand breaks (DSBs). These breaks appear to be formed by the Spo11 protein, with assistance from a large number of other gene products, by a topoisomerase-like transesterase mechanism. Recent studies in fission yeast, multicellular fungi, flies, worms, plants, and mammals indicate that the role of Spo11 in meiotic recombination initiation is highly conserved. This chapter reviews the properties of Spo11 and the other gene products required for meiotic DSB formation in a number of organisms and discusses ways in which recombination initiation is coordinated with other events occurring in the meiotic cell.

A role for MMS4 in the processing of recombination intermediates during meiosis in Saccharomyces cerevisiae

The MMS4 gene of Saccharomyces cerevisiae was originally identified due to its sensitivity to MMS in vegetative cells. Subsequent studies have confirmed a role for MMS4 in DNA metabolism of vegetative cells. In addition, mms4 diploids were observed to sporulate poorly. This work demonstrates that the mms4 sporulation defect is due to triggering of the meiotic recombination checkpoint. Genetic, physical, and cytological analyses suggest that MMS4 functions after the single end invasion step of meiotic recombination. In spo13 diploids, red1, but not mek1, is epistatic to mms4 for sporulation and spore viability, suggesting that MMS4 may be required only when homologs are capable of undergoing synapsis. MMS4 and MUS81 are in the same epistasis group for spore viability, consistent with biochemical data that show that the two proteins function in a complex. In contrast, MMS4 functions independently of MSH5 in the production of viable spores. We propose that MMS4 is required for the processing of specific recombination intermediates during meiosis.

Coordination of the initiation of recombination and the reductional division in meiosis in Saccharomyces cerevisiae

Early exchange (EE) genes are required for the initiation of meiotic recombination in Saccharomyces cerevisiae. Cells with mutations in several EE genes undergo an earlier reductional division (MI), which suggests that the initiation of meiotic recombination is involved in determining proper timing of the division. The different effects of null mutations on the timing of reductional division allow EE genes to be assorted into three classes: mutations in RAD50 or REC102 that confer a very early reductional division; mutations in REC104 or REC114 that confer a division earlier than that of wild-type (WT) cells, but later than that of mutants of the first class; and mutations in MEI4 that do not significantly alter the timing of MI. The very early mutations are epistatic to mutations in the other two classes. We propose a model that accounts for the epistatic relationships and the communication between recombination initiation and the first division. Data in this article indicate that double-strand breaks (DSBs) are not the signal for the normal delay of reductional division; these experiments also confirm that MEI4 is required for the formation of meiotic DSBs. Finally, if a DSB is provided by the HO endonuclease, recombination can occur in the absence of MEI4 and REC104.

Suppressor analysis of the Saccharomyces cerevisiae gene REC104 reveals a genetic interaction with REC102

REC104 is a gene required for the initiation of meiotic recombination in Saccharomyces cerevisiae. To better understand the role of REC104 in meiosis, we used an in vitro mutagenesis technique to create a set of temperature-conditional mutations in REC104 and used one ts allele (rec104-8) in a screen for high-copy suppressors. An increased dosage of the early exchange gene REC102 was found to suppress the conditional recombinational reduction in rec104-8 as well as in several other conditional rec104 alleles. However, no suppression was observed for a null allele of REC104, indicating that the suppression by REC102 is not "bypass" suppression. Overexpression of the early meiotic genes REC114, RAD50, HOP1, and RED1 fails to suppress any of the rec104 conditional alleles, indicating that the suppression might be specific to REC102.

Nps1/Sth1p, a component of an essential chromatin-remodeling complex of Saccharomyces cerevisiae, is required for the maximal expression of early meiotic genes

BACKGROUND

The NPS1/STH1 gene of Saccharomyces cerevisiae is essential for mitotic growth, especially for the progression through the G2/M phase. It encodes a major component of the chromatin-remodelling complex, RSC, of unknown function. We attempted to address the function of NPS1 in meiosis.

RESULTS

The homozygote of the temperature sensitive nps1 mutant, nps1-105, showed reduced and delayed levels of sporulation, accompanied with a notable decrease and delay of the expression of several early meiotic genes (IME2, SPO11 and SPO13). Deletion analysis of the IME2 promoter revealed that the defect in the gene expression occurred through the URS1 site. The sporulation defect of nps1-105 was alleviated by the over-expression of either IME1 or IME2. However, over-expression of IME1 did not permit the full expression of IME2, SPO11 and SPO13 in nps1-105. In addition, the expression of NPS1 itself increased transiently upon initiation of meiosis, before the appearance of the IME2 message but after that of IME1. The impaired increase in NPS1 transcription led to inefficient sporulation.

CONCLUSION

The results suggest that Nps1p/RSC is required for the activation of gene expression at the initiation of meiosis.

Double strand breaks at the HIS2 recombination hot spot in Saccharomyces cerevisiae

Double strand breaks (DSBs) have been found at several meiotic recombination hot spots in Saccharomyces cerevisiae; more global studies have found that they occur at many places along several yeast chromosomes during meiosis. Indeed, the number of breaks found is consistent with the number of recombination events predicted from the genetic map. We have previously demonstrated that the HIS2 gene is a recombination hot spot, exhibiting a high frequency of gene conversion and associated crossing over. This paper shows that DSBs occur in meiosis at a site in the coding region and at a site downstream of the HIS2 gene and that the DSBs are dependent upon genes required for recombination. The frequency of DSBs at HIS2 increases when the gene conversion frequency is increased by alterations in the DNA around HIS2, and vice versa. A deletion that increases both DSBs and conversion can stimulate both when heterozygous; that is, it is semidominant and acts to stimulate DSBs in trans. These data are consistent with the view that homologous chromosomes associate with each other before the formation of the DSBs.

Control of meiotic gene expression in Saccharomyces cerevisiae

Sporulation of the yeast Saccharomyces cerevisiae is restricted to one type of cell, the a/alpha cell, and is initiated after starvation for nitrogen in the absence of a fermentable carbon source. More than 25 characterized genes are expressed only during sporulation and are referred to as meiotic genes or sporulation-specific genes. These genes are in the early, middle, and late expression classes. Most early genes have a 5' regulatory site, URS1, and one of two additional sequences, UASH or a T4C site. URS1 is required both to repress meiotic genes during vegetative growth and to activate these genes during meiosis. UASH and the T4C site also contribute to meiotic expression. A different type of site, the NRE, is found in at least two late genes. The NRE behaves as a repression site in vegetative cells and is neutral in meiotic cells. Many regulatory genes that either repress or activate meiotic genes have been identified. One group of regulators affects the expression of IME1, which specifies a positive regulator of meiotic genes and is expressed at the highest levels in meiotic cells. A second group of regulators acts in parallel with or downstream of IME1 to influence meiotic gene expression. This group includes UME6, which is required both for repression through the URS1 site in vegetative cells and for IME1-dependent activation of an upstream region containing URS1 and T4C sites. IME1 may activate meiotic genes by modifying a UME6-dependent repression complex at a URS1 site. Several additional mechanisms restrict functional expression of some genes to meiotic cells. Translation of IME1 has been proposed to occur only in meiotic cells; several meiotic transcripts are more stable in acetate medium than in glucose medium; and splicing of MER2 RNA depends on a meiosis-specific gene, MER1.

Meiotic recombination in yeast

Over the past several years, the yeast Saccharomyces cerevisiae has proven to be an extremely useful model system for understanding how cells acquire high recombinational ability during meiosis. Due to recent advances in the physical monitoring of DNA intermediates during meiosis, new cytological methods for visualization of chromosomes during pairing and exchange, and progress in the identification and analysis of recombination-defective mutants, a general picture of the order and dependencies of specific recombination events is now emerging.