The Sgs1 DNA helicase and its mammalian homolog BLM control crossover formation in mitotic cells. Zakharyevich et al. and De Muyt et al. now uncover a key role for Sgs1 in meiotic crossover ...regulation, which in turn reveals a joint molecule resolution pathway that produces the majority of crossovers in budding yeast.
O 6 -alkylG adducts are highly mutagenic due to their capacity to efficiently form O 6 -alkylG:T mispairs during replication, thus triggering G→A transitions. Mutagenesis is largely prevented by ...repair strategies such as reversal by alkyltransferases or excision by nucleotide excision repair (NER). Moreover, methyl-directed mismatch repair (MMR) is known to trigger sensitivity to methylating agents via a mechanism that involves recognition by MutS of the O 6 -mG:T replication intermediates. We wanted to investigate the mechanism by which MMR controls the genotoxicity of environmentally relevant O 6 -alkylG adducts formed by ethylene oxide and propylene oxide. Recently, the alkyltransferase-like gene ybaZ (eATL) was shown to enhance repair of these slightly larger O 6 -alkylG adducts by NER. We analyzed the toxicity and mutagenesis induced by these O 6 -alkylG adducts using single-adducted plasmid probes. We show that the eATL gene product prevents MMR-mediated attack of the O 6 -alkylG:T replication intermediate for the larger alkyl groups but not for methyl. In vivo data are compatible with the occurrence of repeated cycles of MMR attack of the O 6 -alkylG:T intermediate. In addition, in vitro, the eATL protein efficiently prevents binding of MutS to the O 6 -alkylG:T mispairs formed by the larger alkyl groups but not by methyl. In conclusion, eATL not only enhances the efficiency of repair of these larger adducts by NER, it also shields these adducts from MMR-mediated toxicity.
Recombination between inverted repeats is
RAD52 dependent, but reduced only modestly in the
rad51Δ mutant.
RAD59 is required for
RAD51-independent inverted-repeat recombination, but no clear ...mechanism for how recombination occurs in the absence of
RAD51 has emerged. Because Rad59 is thought to function as an accessory factor for the single-strand annealing activity of Rad52 one possible mechanism for spontaneous recombination could be by strand annealing between repeats at a stalled replication fork. Here we demonstrate the importance of the Rad52 single-strand annealing activity for generating recombinants by showing suppression of the
rad52Δ,
rad51Δ rad52Δ and
rad52Δ rad59Δ inverted-repeat recombination defects by the
rfa1-D228Y mutation. In addition, formation of recombinants in the
rad51Δ mutant was sensitive to the distance between the inverted repeats, consistent with a replication-based mechanism. Deletion of
RAD5 or
RAD18, which are required for error-free post-replication repair, reduced the recombination rate in the
rad59Δ mutant, but not in wild type. These data are consistent with
RAD51-independent recombinants arising by a faulty template switch mechanism that is distinct from nascent strand template switching.
Rad51 requires a number of other proteins, including the Rad51 paralogs, for efficient recombination in vivo. Current evidence suggests that the yeast Rad51 paralogs, Rad55 and Rad57, are important ...in formation or stabilization of the Rad51 nucleoprotein filament. To gain further insights into the function of the Rad51 paralogs, reporters were designed to measure spontaneous or double-strand break (DSB)-induced sister or nonsister recombination. Spontaneous sister chromatid recombination (SCR) was reduced 6000-fold in the rad57 mutant, significantly more than in the rad51 mutant. Although the DSB-induced recombination defect of rad57 was suppressed by overexpression of Rad51, elevated temperature, or expression of both mating-type alleles, the rad57 defect in spontaneous SCR was not strongly suppressed by these same factors. In addition, the UV sensitivity of the rad57 mutant was not strongly suppressed by MAT heterozygosity, even though Rad51 foci were restored under these conditions. This lack of suppression suggests that Rad55 and Rad57 have different roles in the recombinational repair of stalled replication forks compared with DSB repair. Furthermore, these data suggest that most spontaneous SCR initiates from single-stranded gaps formed at stalled replication forks rather than DSBs.
MRE11 functions in several aspects of DNA metabolism, including meiotic recombination, double-strand break repair, and telomere maintenance. Although the purified protein exhibits 3' to 5' ...exonuclease and endonuclease activities in vitro, Mre11 is implicated in the 5' to 3' resection of duplex ends in vivo. The mre11-H125N mutation, which eliminates the nuclease activities of Mre11, causes an accumulation of unprocessed double-strand breaks (DSBs) in meiosis, but no defect in processing HO-induced DSBs in mitotic cells, suggesting the existence of redundant activities. Mutation of EXO1, which encodes a 5' to 3' exonuclease, was found to increase the ionizing radiation sensitivity of both mre11Delta and mre11-H125N strains, but the exo1 mre11-H125N strain showed normal kinetics of mating-type switching and was more radiation resistant than the mre11Delta strain. This suggests that other nucleases can compensate for loss of the Exo1 and Mre11 nucleases, but not of the Mre11-Rad50-Xrs2 complex. Deletion of RAD27, which encodes a flap endonuclease, causes inviability in mre11 strains. When mre11-H125N was combined with the leaky rad27-6, the double mutants were viable and no more gamma-ray sensitive than the mre11-H125N strain. This suggests that the double mutant defect is unlikely to be due to defective DSB processing.
The Rad51 paralogs Rad55 and Rad57 form a heterodimer required to mediate the formation and/or stabilization of the Rad51 filament. To further characterize the function of Rad55-Rad57, we used a ...combination of rad57 partial suppressors to determine whether the DNA repair and recombination defects of the rad57 mutant could be completely suppressed. The combination of all suppressors, elevated temperature, srs2, rad51-I345T, and mating-type (MAT) heterozygosity resulted in almost complete suppression of the rad57 mutant defect in the recruitment of Rad51 to DNA-damaged sites, as well as survival in response to ionizing radiation and camptothecin. In a physical assay to monitor the kinetics of double-strand-break (DSB)-induced gene conversion, the rad57 mutant defect was effectively suppressed by srs2 and MAT heterozygosity, but these same suppressors failed to suppress the spontaneous recombination defect. Thus the Rad55-Rad57 heterodimer appears to have a unique function in spontaneous recombination that is not essential for DSB repair. Furthermore, we investigated the currently unknown mechanism of rad57 suppression by MAT heterozygosity and found that it is independent of DNL4.
Crystal structure of a Rad51 filament Rice, Phoebe A; Conway, Adam B; Lynch, Thomas W ...
Nature structural & molecular biology,
08/2004, Letnik:
11, Številka:
8
Journal Article
Recenzirano
Rad51, the major eukaryotic homologous recombinase, is important for the repair of DNA damage and the maintenance of genomic diversity and stability. The active form of this DNA-dependent ATPase is a ...helical filament within which the search for homology and strand exchange occurs. Here we present the crystal structure of a Saccharomyces cerevisiae Rad51 filament formed by a gain-of-function mutant. This filament has a longer pitch than that seen in crystals of Rad51's prokaryotic homolog RecA, and places the ATPase site directly at a new interface between protomers. Although the filament exhibits approximate six-fold symmetry, alternate protein-protein interfaces are slightly different, implying that the functional unit of Rad51 within the filament may be a dimer. Additionally, we show that mutation of His352, which lies at this new interface, markedly disrupts DNA binding.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
69.
EXO1-A multi-tasking eukaryotic nuclease Tran, Phuoc T.; Erdeniz, Naz; Symington, Lorraine S. ...
DNA Repair,
12/2004, Letnik:
3, Številka:
12
Book Review, Journal Article
Recenzirano
Exo1 was first isolated as a 5′ → 3′ exonuclease activity induced during meiosis in fission yeast and since that time has been implicated in a multitude of eukaryotic DNA metabolic pathways that ...include DNA repair, recombination, replication, and telomere integrity. Involvement in multiple pathways affecting genomic stability makes
EXO1 a logical target for mutation during oncogenesis. Here, we review studies in several experimental systems that shed light on the role of Exo1 in these DNA transaction pathways, particularly those that may relate to oncogenesis.
PDF
