The G ^ T transversion mutation, ade6-M26, creates the heptanucleotide sequence ATGACTG, which lies close to the 5' end of the open reading frame of the ade6 gene in Schizosaccharomyces pombe. The mutation generates a meiosis-specific recombination hot spot and a binding site for the Mtsl/Mts2 protein. We examined the chromatin structure at the ade6 locus in the M26 strain and compared it to that of the wild-type and hot spot-negative control M375. Micrococcal nuclease (MNase) digestion and indirect end-labeling methods were applied. In the M26 strain, we detected a new MNase-hypersensitive site at the position of the M26 mutation and no longer observed the phasing of nucleosomes seen in the wild-type and the M375 strains. Quantitative comparison of MNase sensitivity of the chromatin in premeiotic and meiotic cultures revealed a small meiotic induction of MNase hypersensitivity in the ade6 promoter region of the wild-type and M375 strains. The meiotic induction of MNase hypersensitivity was enhanced significantly in the ade6 promoter region of the M26 strain and also occurred at the M26 mutation site. The formation of the MNase-sensitive region around the heptamer sequence was abolished by the introduction of single-nucleotide substitutions in the heptamer sequence, which also abolish hot spot activity and binding of Mtsl/Mts2. These data suggest that Mtsl/Mts2 binding to the heptamer sequence results in a chromatin structure suitable for the recruitment of a meiosis-specific recombination function or functions.

The meiotic recombination hot spot ura4A (formerly ura4-aim) of Schizosaccharomyces pombe was observed at the insertion of the ura4 ϩ gene 15 kb centromere-proximal to ade6 on chromosome III. Crosses heterozygous for the insertion showed frequent conversion at the heterology with preferential loss of the insertion. This report concerns the characterization of 12 spontaneous ura4A mutants. A gradient of conversion ranging from 18% at the 5Ј end to 6% at the 3Ј end was detected. A novel phenomenon also was discovered: a matingtype-related bias of conversion. The allele entering with the h ϩ parent acts preferentially as the acceptor for conversion (ratio of 3:2). Tetrad analysis of two-factor crosses showed that heteroduplex DNA is predominantly asymmetrical, enters from the 5Ј end, and more often than not covers the entire gene. Restoration repair of markers at the 5Ј end was inferred. Random spore analyses of two-factor crosses and normalization of prototroph-recombinant frequencies to physical distance led to the demonstration of map expansion: Crosses involving distant markers yielded recombinant frequencies higher than the sum of the frequencies measured in the subintervals. Finally, marker effects on recombination were defined for two of the ura4A mutations.H OMOLOGOUS recombination contributes to the transfer changes only one strand of the recipient chromageneration of genetic diversity and is required for tid. The resulting spore bears both wild-type and mutant proper chromosome segregation in meiosis and for the information. Since the genetic marker segregates only repair of DNA damage. Since homologous recombinain the first mitotic division after meiosis, the observed tion occurs most frequently during meiosis, the underly-5ϩ:3Ϫ or 3ϩ:5Ϫ tetrads are called postmeiotic segregaing mechanisms are best studied in ascomycetous fungi tions (PMS). in which all four products of single meioses can beThe frequency of homologous recombination varies individually recovered and analyzed (Paques and Haber widely from interval to interval along the genome. DNA 1999). The fate of the four chromatids in a diploid cell regions with lower or higher than average frequency of undergoing meiosis can be followed by tetrad analysis. aberrant segregation and crossing over are called cold Examination of the segregation of genetic markers in spots or hot spots, respectively. They have been identitetrads has demonstrated two classes of recombination fied and described in many different organisms (Lichten events: the classical reciprocal exchange of DNA seand Goldman 1995; Paques and Haber 1999; Petes quences, called crossing over, and the unidirectional trans-2001). Specific nucleotide sequences have been identifer of genetic information between chromatids, desigfied in some cases, which are recognized by proteins nated non-Mendelian segregation (NMS). Two types of promoting, directly or indirectly, a rate-limiting step of NMS events are distinguished. One is the nonreciprocal the recombination process (Smith 2001). transfe...

Homologues of the bacterial mutS and mutL genes involved in DNA mismatch repair have been found in organisms from bacteria to humans. Here, we describe the structure and function of a newly identified Schizosaccharomyces pombe gene that encodes a predicted amino acid sequence of 794 residues with a high degree of homology to MutL related proteins. On the basis of its closer relationship to the eukaryotic “PMS” genes than to the “MLH” genes, we have designated the S. pombe homologue pms1. Disruption of the pms1 gene causes a significant increase of spontaneous mutagenesis as documented by reversion rate measurements. Tetrad analyses of crosses homozygous for the pms1 mutation reveal a reduction of spore viability from >92% to 80% associated with a low proportion (∼50%) of meioses producing four viable spores and a significant, allele-dependent increase of the level of post-meiotic segregation of genetic marker allele pairs. The mutant phenotypes are consistent with a general function of pms1 in correction of mismatched base pairs arising as a consequence of DNA polymerase errors during DNA synthesis, or of hybrid DNA formation between homologous but not perfectly complementary DNA strands during meiotic recombination.