DNA double-strand breaks are repaired by homologous recombination or DNA end-joining, but the latter process often causes legitimate recombination and chromosome rearrangements. One of the factors involved in the end-joining process is Hdf1, a yeast homologue of Ku protein. We used the yeast two-hybrid assay to show that Hdf1 interacts with Sir4, which is involved in transcriptional silencing at telomeres and HM loci. Analyses of sir4 mutants showed that Sir4 is required for deletion by illegitimate recombination and DNA end-joining in the pathway involving Hdf1. Sir2 and Sir3, but not Sir1, were also found to participate in these processes. Furthermore, mutations of the SIR2, SIR3 and SIR4 genes conferred increased sensitivity to gamma-radiation in a genetic background with a mutation of the RAD52 gene, which is essential for double-strand break repair mediated by homologous recombination. These results indicate that Sir proteins are involved in double-strand break repair mediated by end-joining. We propose that Sir proteins act with Hdf1 to alter broken DNA ends to create an inactivated chromatin structure that is essential for the rejoining of DNA ends.

Bloom's syndrome (BS) and Werner's syndrome (WS) are genetic disorders in which an increased rate of chromosomal aberration is detected. The genes responsible for these diseases, BLM and WRN, have been found to be homologs of Escherichia coli recQ and Saccharomyces cerevisiae SGS1 genes. Here we show that yeast Sgs1 helicase acts as a suppressor of illegitimate recombination through homologous recombination and that human BLM and WRN helicases can suppress the increased homologous and illegitimate recombinations in the S. cerevisiae sgs1 mutant. The results imply a role of BLM and WRN helicases to control genomic stability in human cells. Similar to Sgs1 helicase, BLM helicase suppressed the cell growth in the top3 sgs1 mutation background and restored the increased sensitivity of the sgs1 mutant to hydroxyurea, but the WRN helicase did not. We discussed differential roles of BLM and WRN helicases in human cells. BLM-and WRN-bearing yeasts provide new useful models to investigate human BS and WS diseases.Bloom's syndrome (BS) is an autosomal recessive disorder causing short stature, immunodeficiency, and an increasing risk of cancer. BS cells have a high level of genomic instability at the cellular level, as shown by an increased rate of sister chromatid exchange and chromosomal aberration (1). Werner's syndrome (WS) also is a rare genetic disorder and is characterized by the premature appearance of normal aging in young adults. Cultured cells derived from patients with WS show an increased rate of somatic mutations, chromosome losses, and deletions (2-4). These experiments may suggest an increased rate of illegitimate recombination in both BS and WS cells. Recently, the genes BLM and WRN, responsible for BS and WS, respectively, have been found to be homologs of Escherichia coli recQ and Saccharomyces cerevisiae SGS1 genes, which encode DNA helicases (5-9). In E. coli, RecQ helicase is involved in homologous and illegitimate recombination (7, 10). In yeast, Sgs1 protein physically interacts with topoisomerase II and III (11,12). The mutation of the SGS1 gene suppresses the growth defect generated by top3 mutation and also shows an increased recombination at the repeated ribosomal DNA locus and other loci (9, 12). The sgs1 mutation also reduces the average lifespan of yeast cells while it enhances relocalization of Sir proteins to nucleolus and accumulation of extrachromosomal rDNA circles (13,14). But whether the sgs1 mutation affects illegitimate recombination is unknown.Illegitimate recombination takes place between nonhomologous DNA sequences or very short regions of homology. Nonhomologous end-joining, which is known to be mediated by Rad50, Xrs2, Mre11, Hdf1, Ku80, and Dnl4 as well as silencing factors Sir2, Sir3,, has an essential role in illegitimate recombination. In this paper, we show that yeast Sgs1 helicase acts as a suppressor of illegitimate recombination through homologous recombination and that human BLM and WRN helicases can suppress the increased homologous and illegitimate recombinati...

Bloom syndrome and Werner syndrome are genetic disorders in which an increased rate of chromosomal abnormality is observed. The genes responsible for these diseases, BLM and WRN, have been cloned and identified as homologs of the Escherichia coli recQ genes. We studied the effect of recQ mutations on illegitimate recombination, which is an aberrant biological event related to the chromosomal abnormality in humans, and found that a variety of recQ mutations increased spontaneous illegitimate recombination by 20-to 300-fold and increased UV light-induced illegitimate recombination by 10-to 100-fold. Most bio or pro transducing phages are formed by the recombination events at several hot spots, which are enhanced by the recQ mutation. The analysis of nucleotide sequences at the recombination junction in the transducing phages indicates that recombination at the hot spot sites as well as the non-hot spot sites takes place between short homologous sequences. Enhancement of the recombination in the recQ mutants also occurs in the recA, recBC sbcBC, or recBC sbcA backgrounds, indicating that these recombination events are mediated by none of the known recombination pathways, RecBC, RecF, and RecE. We therefore concluded that the RecQ function suppresses illegitimate recombination that depends on short homologous regions. We discuss a model, based on the 3-to-5 helicase activity of RecQ, to explain the role of this protein as a suppressor of illegitimate recombination.