Initiation of meiotic recombination in the yeast Saccharomyces cerevisiae occurs by localized DNA double‐strand breaks (DSBs) at several locations in the genome, corresponding to hot spots for meiotic gene conversion and crossing over. The meiotic DSBs occur in regions of chromatin that are hypersensitive to nucleases. To gain insight into the molecular mechanism involved in the formation of these DSBs, we have determined their positions at the nucleotide level at the CYS3 hot spot of gene conversion on chromosome I. We found four major new features of these DSBs: (i) sites of DSBs are multiple with varying intensities and spacing within the promoter region of the CYS3 gene; (ii) no consensus sequence can be found at these sites, indicating that the activity involved in DSB formation has little or no sequence specificity; (iii) the breaks are generated by blunt cleavages; and (iv) the 5′ ends are modified in rad50S mutant strains, where the processing of these ends is known to be prevented. We present a model for the initiation of meiotic recombination taking into account the implications of these results.

The accurate, reproducible, and timely reporting of nucleated red blood cells (NRBC) is an important function of the clinical hematology laboratory. We used 960 samples from 5 worldwide sites to evaluate a new NRBC enumeration method for the ADVIA 2120 Hematology System. The method showed excellent correlation with microscopy (r = 0.93). Sensitivity and specificity for the presence of NRBC for all samples analyzed was 77.3% and 74.6%, respectively. Almost all false negative samples were at NRBC counts 10/100 WBC. The NRBC method automatically corrects WBC counts and differential results for the presence of NRBC; the uncorrected counts are available to the user on the run screen. This method allows the enumeration of NRBC with the ADVIA 2120 Hematology System in CBC/DIFF sample mode without need for additional hardware, sample preparation, or reagents.

In Two types of meiotic recombination events have been found to occur in all eukaryotes examined, crossovers (reciprocal exchanges) and gene conversions (unidirectional transfers of information). The frequencies of both events vary within a genome (1-4). The nature of the underlying molecular processes that control the distribution of recombination events remains to be elucidated. One approach is the study of gene conversion hotspots, defined as sites or regions enhancing gene conversion in their vicinity.In fungi, where all four products of individual meiosis remain grouped in a tetrad, gene conversion events are genetically detected when a diploid strain is heterozygous at a locus (alleles A and a) leading to a tetrad containing 3A:la or 1A:3a products rather than the normal Mendelian segregation (2A:2a). In Saccharomyces cerevisiae, the frequency of meiotic gene conversion is high in the vicinity of the ARG4 gene (5, 6). The highest frequency of gene conversion (17% of total meiosis) is found for a marker located in the ARG4 promoter region around position -119 (Fig. 1). The frequencies decrease in a gradient, termed polarity, for markers located on either side of the ARG4 promoter (10). Within the ARG4 coding region, the conversion frequencies decrease from 10% (position +3) to 0.4% (position +1274). Two previous deletion analyses mapped an initiation site for meiotic gene conversion in the promoter region of ARG4 between positions -316 and -37 (6, 7). These genetics data and the physical analysis of the DNA of this chromosomal region during meiosis, which demonstrated the occurrence of a transient double-strand break (DSB) around position -200 (8, 9, 11), strongly suggest that the implicated region includes a cis-acting initiation site which stimulates recombination in its vicinity.To know whether all the cis-acting elements had been identified by deletion analysis, we tested whether the -319/ -37 sequence was sufficient to promote gene conversion in a novel chromosomal context-i.e., when displaced so that it was adjacent to new flanking sequences. We chose to invert the ARG4 gene on the chromosome. This "minimal displacement" was preferred to the random insertion of DNA fragments on different chromosomes, which is known to be strongly subject to position effects for recombination between LEU2 (12) or ARG4 (13) heteroalleles. However, a 12-kilobase (kb) ARG4-containing fragment inserted on a yeast artificial chromosome has the same recombinational properties as at its normal location on chromosome VIII (14).We report here that the necessary -316/-37 cis-acting sequence previously defined by deletion analysis at the normal location is not sufficient to promote gene conversion at ARG4 in any chromosomal context. We show that the presence of a transcriptional terminator upstream from the initiation site in these inversions is absolutely required in order to maintain both the DSB and the recombination properties at ARG4. We also show that high and low levels of gene conversion at ARG4 obtained in variou...