Meiotic recombination is initiated by the formation of programmed DNA double-strand breaks (DSBs) catalyzed by the Spo11 protein. DSBs are not randomly distributed along chromosomes. To better understand factors that control the distribution of DSBs in budding yeast, we have examined the genome-wide binding and cleavage properties of the Gal4 DNA binding domain (Gal4BD)-Spo11 fusion protein. We found that Gal4BD-Spo11 cleaves only a subset of its binding sites, indicating that the association of Spo11 with chromatin is not sufficient for DSB formation. In centromere-associated regions, the centromere itself prevents DSB cleavage by tethered Gal4BD-Spo11 since its displacement restores targeted DSB formation. In addition, we observed that new DSBs introduced by Gal4BD-Spo11 inhibit surrounding DSB formation over long distances (up to 60 kb), keeping constant the number of DSBs per chromosomal region. Together, these results demonstrate that the targeting of Spo11 to new chromosomal locations leads to both local stimulation and genome-wide redistribution of recombination initiation and that some chromosomal regions are inherently cold regardless of the presence of Spo11.In most sexually reproducing organisms, homologous recombination is a prominent feature of meiosis, which creates the genetic diversity of the meiotic products by promoting a safe exchange of DNA information between the paternal and maternal chromosomes. As importantly, the crossovers maintain a physical connection between the homologs which ensures their proper disjunction at the first meiotic division (55). Therefore, understanding how the cells control the process of meiotic recombination is important because defects in the number or the localization of recombination events lead to failure in homolog disjunction or unviable gametes. For instance, errors in chromosome segregation are correlated with unusual crossover positions in many cases of human maternally derived trisomy 21 (27, 28). In particular, segregation defects are observed when a crossover occurs close to a telomere, probably because the structure is less efficient at providing a strong physical link between homologs (36, 43). Also, crossovers very close to the centromere lead to the premature loss of sister chromatid cohesion and nondisjunction at meiosis II (26, 42). Not surprisingly, then, the meiotic recombination process is tightly controlled from initiation to completion stages.Once recombination is initiated, a decision is made to channel the early intermediates along a pathway ending in crossover formation or in noncrossover events. This step is influenced both by "crossover interference," in which a crossover in one region makes it unlikely that another will occur nearby (18), and by "crossover homeostasis," which regulates the crossover/noncrossover ratio to ensure a minimal amount of crossover per chromosome (34).Before this stage, the frequency and the localization of the initiation events are the earliest determinants of how meiotic recombination events are distributed a...

Thus, while TNF-alpha is not required for the induction of these cytokines by LPS, it modulates the kinetics of their expression. The lethality studies simultaneously confirm a role for TNF as a mediator of early lethality and establish that, in the absence of these cytokines, other mediators take over, resulting in the absence of long-term protection from LPS toxicity.

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The ras proteins (Harvey, Kirsten and N-ras) are key regulators of signal transduction and a perturbation of their GDP/GTP cycle is frequently observed in tumors. In mammals, N-ras constitutes with unr (upstream of N-ras) a tightly linked tandem of ubiquitously expressed genes. Although unr and N-ras appear to be involved in distinct functions, this unusual genetic organization could be important for the regulation of N-ras expression. Specifically, transcription of unr could negatively regulate that of N-ras by transcriptional interference. To investigate this possibility, we have deleted the unr promoter by homologous recombination in murine embryonic stem cells. Analysis of tissues of heterozygous mice revealed an increase in N-ras mRNA accumulation ranging between 20 and 65%, in agreement with the suppression of a transcriptional interference.z 1997 Federation of European Biochemical Societies.