Genetic instability in the Saccharomyces cerevisiae rad9 mutant correlates with failure to arrest the cell cycle in response to DNA damage. We quantitated the DNA damage-associated stimulation of directed translocations in RAD9؉ and rad9 mutants. Directed translocations were generated by selecting for His ؉ prototrophs that result from homologous, mitotic recombination between two truncated his3 genes, GAL1::his3-⌬5 and trp1::his3-⌬3::HOcs. Compared to RAD9 ؉ strains, the rad9 mutant exhibits a 5-fold higher rate of spontaneous, mitotic recombination and a greater than 10-fold increase in the number of UV-and X-ray-stimulated His ؉ recombinants that contain translocations. The higher level of recombination in rad9 mutants correlated with the appearance of nonreciprocal translocations and additional karyotypic changes, indicating that genomic instability also occurred among non-his3 sequences. Both enhanced spontaneous recombination and DNA damage-associated recombination are dependent on RAD1, a gene involved in DNA excision repair. The hyperrecombinational phenotype of the rad9 mutant was correlated with a deficiency in cell cycle arrest at the G 2 -M checkpoint by demonstrating that if rad9 mutants were arrested in G 2 before irradiation, the numbers both of UV-and ␥-ray-stimulated recombinants were reduced. The importance of G 2 arrest in DNA damageinduced sister chromatid exchange (SCE) was evident by a 10-fold reduction in HO endonuclease-induced SCE and no detectable X-ray stimulation of SCE in a rad9 mutant. We suggest that one mechanism by which the RAD9-mediated G 2 -M checkpoint may reduce the frequency of DNA damage-induced translocations is by channeling the repair of double-strand breaks into SCE.It has been postulated that DNA damage-induced cell cycle arrest at cell cycle checkpoints maintains genomic stability by allowing time for DNA repair prior to the replication or division of damaged chromatids (67, 68). Consistent with this idea, mutations in genes controlling cell cycle arrest at the G 1 -S checkpoint and G 2 -M checkpoint confer enhanced genetic instability. For example, p53 mutations, which confer deficiencies in the G 1 -S checkpoint, are correlated with enhanced spontaneous and UV-stimulated amplification of CAD genes (35, 73). Cells cultured from patients with ataxia telangiectasia that are deficient in cell cycle arrest at both the G 1 -S and G 2 -M cell cycle checkpoints (7, 49) also exhibit higher frequencies of chromosomal rearrangements, including translocations (37) and deletions (38), and chromosome end-to-end joining (41).In Saccharomyces cerevisiae, DNA-damaging agents stimulate mitotic, homologous recombination and induce cell cycle arrest at cell cycle checkpoints (31, 59). For example, DNA damage-associated recombination between his3 fragments positioned at predetermined loci can result in chromosomal rearrangements, including translocations (18, 19), duplications (16), and deletions (54). HO endonuclease-generated doublestrand breaks (DSBs) stimulate ectopic gene conversi...

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Using a directional cloning strategy, DNA sequence information was obtained corresponding to the site of early radiation-induced apoptotic DNA fragmentation within the human lymphoblastoid cell line TK6. Data were obtained from 88 distinct clones comprising approximately 65 kbp of sequenced material. Analysis of all cloned material showed that sequences in the 10 bp immediately adjacent to the cleavage sites were enriched in short oligoT tracts. The proportion of repetitive DNA within the entire cloned material was found to be within the normal range. However the distribution of Alu and LINE repetitive DNA were biased to positions at or adjacent to the apoptotic cleavage site. In particular, a non-random distribution of five cleavage sites was found clustered within the second ORF of the LINE L1 that partially overlapped with two binding sites for the nuclear matrix-associated protein SATB1. Three other clones, containing alpha satellite elements, were also linked to a DNA matrix binding function. These data indicate that the site of chromatin loop formation at the nuclear matrix may be a specific target for early DNA fragmentation events during apoptosis.

No abstract

Using a directional cloning strategy, DNA sequence information was obtained corresponding to the site of early radiation-induced apoptotic DNA fragmentation within the human lymphoblastoid cell line TK6. Data were obtained from 88 distinct clones comprising approximately 65 kbp of sequenced material. Analysis of all cloned material showed that sequences in the 10 bp immediately adjacent to the cleavage sites were enriched in short oligoT tracts. The proportion of repetitive DNA within the entire cloned material was found to be within the normal range. However the distribution of Alu and LINE repetitive DNA were biased to positions at or adjacent to the apoptotic cleavage site. In particular, a non-random distribution of five cleavage sites was found clustered within the second ORF of the LINE L1 that partially overlapped with two binding sites for the nuclear matrix-associated protein SATB1. Three other clones, containing alpha satellite elements, were also linked to a DNA matrix binding function. These data indicate that the site of chromatin loop formation at the nuclear matrix may be a specific target for early DNA fragmentation events during apoptosis.