In this report, we describe the identification and molecular characterization of a human RAD50 homolog, hRAD50. hRAD50 was included in a collection of cDNAs which were isolated by a direct cDNA selection strategy focused on the chromosomal interval spanning 5q23 to 5q31. Alterations of the 5q23-q31 interval are frequently observed in myelodysplasia and myeloid leukemia. This strategy was thus undertaken to create a detailed genetic map of that region. Saccharomyces cerevisiae RAD50 (ScRAD50) is one of three yeast RAD52 epistasis group members (ScRAD50, ScMRE11, and ScXRS2) in which mutations eliminate meiotic recombination but confer a hyperrecombinational phenotype in mitotic cells. The yeast Rad50, Mre11, and Xrs2 proteins appear to act in a multiprotein complex, consistent with the observation that the corresponding mutants confer essentially identical phenotypes. In this report, we demonstrate that the human Rad50 and Mre11 proteins are stably associated in a protein complex which may include three other proteins. hRAD50 is expressed in all tissues examined, but mRNA levels are significantly higher in the testis. Other human RAD52 epistasis group homologs exhibit this expression pattern, suggesting the involvement of human RAD52 epistasis group proteins in meiotic recombination. Human RAD52 epistasis group proteins are highly conserved and act in protein complexes that are analogous to those of their yeast counterparts. These findings indicate that the function of the RAD52 epistasis group is conserved in human cells.Acute myeloid leukemia (AML) and myelodysplastic disease are associated with alterations of the 5q23-q31 chromosomal interval (25,60,63). Extensive analysis of patient material has narrowed the commonly deleted region to 5q31, leading to the proposal that an AML tumor suppressor is contained within this region. This interval has thus been designated the critical region (45,46,60). We undertook the creation of a detailed transcriptional and physical map of the 5q23-q31 interval and used direct cDNA selection to identify potential tumor suppressors in this region (16a). Among the cDNAs isolated by this method was hRAD50, a human homolog of the Saccharomyces cerevisiae DNA repair gene, ScRAD50. We have mapped the hRAD50 locus to 5q31, placing it in the critical region proposed to contain the AML tumor suppressor (45). Its homology to ScRAD50 argues that hRAD50 encodes a protein that is involved in human recombinational DNA repair. Double-strand-break (DSB) repair proteins are involved in diverse DNA recombination processes in mammalian and yeast cells. In addition to conferring sensitivity to DNA-damaging agents, DSB repair deficiency affects meiotic and mitotic recombination, mating-type switching, and the assembly of antigen receptor genes (20,86). Although defects in DSB repair and DNA recombination are coincident in both mammals and S. cerevisiae, the prevalent mechanisms of recombinational DNA repair differ significantly in these systems (8,26). Nonhomologous recombination is much more frequen...

Transcription-replication conflicts (TRCs) occur when intensive transcriptional activity compromises replication fork stability, potentially leading to gene mutations. Transcriptiondeposited H3K4 methylation (H3K4me) is associated with regions that are susceptible to TRCs; however, the interplay between H3K4me and TRCs is unknown. Here we show that H3K4me aggravates TRC-induced replication failure in checkpoint-defective cells, and the presence of methylated H3K4 slows down ongoing replication. Both S-phase checkpoint activity and H3K4me are crucial for faithful DNA synthesis under replication stress, especially in highly transcribed regions where the presence of H3K4me is highest and TRCs most often occur. H3K4me mitigates TRCs by decelerating ongoing replication, analogous to how speed bumps slow down cars. These findings establish the concept that H3K4me defines the transcriptional status of a genomic region and defends the genome from TRC-mediated replication stress and instability.

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Green tea polyphenols may protect cells from UV damage through antioxidant activities and by stimulating the removal of damaged or cross-linked DNA. Recently, DNA repair pathways have been predicted as possible targets of epigallocatechin gallate (EGCG)-initiated signaling. However, whether and how green tea polyphenols can promote nucleotide excision repair and homologous recombination in diverse organisms requires further investigation. In this report, we used the budding yeast, Saccharomyces cerevisiae , as a model to investigate the effects of green tea extract on DNA repair pathways. We first showed that green tea extract increased the survival rate and decreased the frequency of mutations in yeast exposed to UVB-irradiation. Furthermore, green tea extract increased the expression of homologous recombination genes, RFA1 , RAD51 and RAD52 , and nucleotide excision repair genes, RAD4 and RAD14 . Importantly, we further used a specific strand invasion assay to show that green tea extract promotes homologous recombination at double-strand breaks. Thus, green tea extract acts to preserve genome stability by activating DNA repair pathways in yeast. Because homologous recombination repair is highly conserved in yeast and humans, this study demonstrates yeast may be a useful platform for future research to investigate the underlying mechanisms of the bioactive compounds in DNA repair.