No abstract

We describe the identification of a new meiosis-specific gene of Saccharomyces cerevisiae, NDT80. The ndt80 null and point mutants arrest at the pachytene stage of meiosis, with homologs connected by full-length synaptonemal complexes and spindle pole bodies duplicated but unseparated. Meiotic recombination in an ndt80⌬ mutant is relatively normal, although commitment to heteroallelic recombination is elevated two-to threefold and crossing over is decreased twofold compared with those of the wild type. ndt80 arrest is not alleviated by mutations in early recombination genes, e.g., SPO11 or RAD50, and thus cannot be attributed to an intermediate block in prophase chromosome metabolism like that observed in several other mutants. The ndt80 mutant phenotype during meiosis most closely resembles that of a cdc28 mutant, which contains a thermolabile p34, the catalytic subunit of maturation-promoting factor. Cloning and molecular analysis reveal that the NDT80 gene maps on the right arm of chromosome VIII between EPT1 and a Phe-tRNA gene, encodes a 627-amino-acid protein which exhibits no significant homology to other known proteins, and is transcribed specifically during middle meiotic prophase. The NDT80 gene product could be a component of the cell cycle regulatory machinery involved in the transition out of pachytene, a participant in an unknown aspect of meiosis sensed by a pachytene checkpoint, or a SPO11-and RAD50-independent component of meiotic chromosomes that is the target of cell cycle signaling.During meiosis, two rounds of chromosome segregation follow a single round of DNA replication. In the first meiotic division, pairs of sister chromatids (homologs) disjoin; at the second meiotic division, sister chromatids segregate to opposite poles as in mitosis.Meiotic prophase, the period between DNA replication and the first nuclear division, involves a complex series of changes in the chromosomes, which include association of sister chro-

During meiosis, mutations that cause defects at intermediate stages in the recombination process confer arrest at the end of prophase (e.g., pachytene). In yeast, mutations of this type include rad50S, dmcl, rad51, and zipl. Rad50 is likely part of a recombination initiation complex. DMC1, RADS1, and ZIP1 encode two RecA homologs and a synaptonemal complex protein, respectively. We report here the effects of mutations in two other (meiosis-specific) genes, RED1 and MEK1/MRE4, that encode a chromosome structure component and a protein kinase, respectively. A redl or mekl/mre4 mutation alleviates completely rad50S, dmcl, rad51, and zipl arrest. Furthermore, the redl and mekl/mre4 mutations define a unique, previously unrecognized aspect of recombination imposed very early in the process, during DSB formation. Finally, the redl and mekl ~rare4 mutations appear to alleviate prophase arrest directly rather than by eliminating, or permitting bypass of, the rad50S, dmcl, rad51, or zipl defects. These and other observations suggest that a meiosis-specific regulatory surveillance process monitors the status of the protein/DNA interhomolog recombination machinery as an integral entity, in its proper chromosomal context, and dependent upon its appropriate Redl and Mekl/Mre4-promoted development. We speculate that a properly developed recombination complex emits an inhibitory signal to delay progression of meiotic cells out of prophase until or unless the recombination process has progressed, at least past certain critical steps, and perhaps to completion.[Key Words: Checkpoint; double-stranded breaks; meiosis; recombination; pachytene arrest; synaptonemal complex] Received May 2, 1996; revised version accepted November 22, 1996.Meiosis involves a single round of DNA replication followed by two rounds of chromosome segregation. At the unique meiosis I division, homologous chromosomes segregate to opposite poles. This process requires a physical connection between homologs that is usually provided by one or a few crossovers in conjunction with intersister cohesions (Carpenter 1994;Roeder 1995;Kleckner 1996).In Saccharomyces cerevisiae, most or all meiotic recombination is initiated by double-stranded breaks (DSBs), which are resected rapidly to give 3' singlestranded tails. DSBs are then converted to double Holliday junctions, at about the time that synaptonemal complex (SC) forms between the two homolog structural axes. Mature recombinants, both crossovers and noncrossovers, arise at about the end of pachytene, the stage when SC is full-length. Immediately after SC dissolution, mother and daughter spindle pole bodies (SPBs) separate to form a short spindle (e.g., Shuster and Byers 1989;Padmore et al. 1991 nodular structure, which undergoes progressive morphogenesis as the recombination process proceeds. In addition, the recombination complex is juxtaposed physically to the structural axes of the interacting homologs throughout the process and important functional interplay between the two features seems likely (Kleckner 199...

Zipl is a yeast synaptonemal complex (SC) central region component and is required for normal meiotic recombination and crossover interference. Physical analysis of meiotic recombination in a zipi mutant reveals the following: Crossovers appear later than normal and at a reduced level. Noncrossover recombinants, in contrast, seem to appear in two phases: (i) a normal number appear with normal timing and (ii) then additional products appear late, at the same time as crossovers. Also, Holliday junctions are present at unusually late times, presumably as precursors to late-appearing products. Redl is an axial structure component required for formation of cytologically discernible axial elements and SC and maximal levels of recombination. In a redi mutant, crossovers and noncrossovers occur at coordinately reduced levels but with normal timing. If Zipl affected recombination exclusively via SC polymerization, a zip] mutation should confer no recombination defect in a redi strain background. But a red] zip] double mutant exhibits the sum of the two single mutant phenotypes, including the specific deficit of crossovers seen in a zipi strain. We infer that Zipl plays at least one role in recombination that does not involve SC polymerization along the chromosomes. Perhaps some Zipl molecules act first in or around the sites of recombinational interactions to influence the recombination process and thence nucleate SC formation. We propose that a Zipldependent, pre-SC transition early in the recombination reaction is an essential component of meiotic crossover control. A molecular basis for crossover/noncrossover differentiation is also suggested.In meiosis, crossovers ensure the disjunction of homologs at the first division. The number and distribution of crossovers are tightly controlled (1-6). One manifestation of control is crossover interference: the presence of a crossover at one position along a chromosome reduces the probability that a crossover will also be found nearby. Crossover interference may act upon an array of undifferentiated recombinational interactions causing certain ones to mature into crossovers and others to mature into noncrossovers (e.g., refs. 3 and 4).In yeast, meiotic recombination initiates via meiosis-specific double strand breaks (DSBs) (7,8), which occur prior to bulk polymerization of the synaptonemal complex between the structural axes of paired homologs (9). Resected DSBs then invade an intact duplex to form double Holliday junctions; invasion and ensuing steps are approximately concomitant with initiation and progression of SC polymerization, respectively (10). Double Holliday junctions persist throughout much of the period when SC is full-length ("pachytene"). Mature crossover and noncrossover products form an hour or so after Holliday junctions appear, at about the time that SC disappears (9), but not dependent upon SC disassembly (11,12 In this model, all undifferentiated recombinational interactions are placed under "stress"; in addition, each interaction has an intrinsic "...

The HIS4LEU2 meiotic recombination hot spot specifies two double‐strand break (DSB) sites, I and II. Results presented demonstrate that DSBs at site I occur at many positions throughout a region of approximately 150 bp; we infer that breaks occur in a sequence non‐specific fashion. Single‐strand nicks at sites I and II are not detectable. Analysis of the effects of a 36 bp linker insertion at site I reveals the existence of communication along and between homologs prior to DSB formation. In cis, the insertion allele causes an increase in DSBs at site I but a decrease in DSBs at site II. In trans, two effects are observed. One effect likely reflects very early pre‐DSB interhomolog interactions; the second is suggestive of a later, more intimate interaction in which sites I and II on the two homologs all compete for DSBs. The existence of interhomolog interactions in early meiotic prophase can explain how the sites of crossovers come to lie between the homolog axes at pachytene.