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The role of Exo1p exonuclease in DNA end resection to generate gene conversion tracts in Saccharomyces cerevisiae.

Yin Y, Petes TD - Genetics (2014)

Bottom Line: In accordance with this expectation, gene conversion tract lengths associated with spontaneous crossovers in exo1 strains were reduced about twofold relative to wild type.For UV-induced events, conversion tract lengths associated with crossovers were also shorter for the exo1 strain than for the wild-type strain (3.2 and 7.6 kb, respectively).Unexpectedly, however, the lengths of conversion tracts that were unassociated with crossovers were longer in the exo1 strain than in the wild-type strain (6.2 and 4.8 kb, respectively).

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular Genetics and Microbiology and University Program in Genetics and Genomics, Duke University Medical Center, Durham, North Carolina 27710.

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Two models for the formation of long gene conversion tracts that are unassociated with crossovers. The data that require explanation are (1) crossover-associated conversion events are shorter in the exo1 strain than in wild type and (2) crossover-unassociated conversions are longer than crossover-associated conversions in the exo1 strain. The labels are the same as in Figure 6. The observations can be explained by an oversynthesis model (A and B) or by a BIR model (C and D). (A) UV-induced conversion in the wild-type strain (oversynthesis model). The synthesis from the invading strand is more extensive than the amount of DNA resected in the NCO pathway. The reinvasion during SDSA displaces the 5′ end of invaded homolog, and the resulting single-stranded branch is removed (shown by a triangle). In the CO pathway, the heteroduplex region is limited by resection. (B) UV-induced conversion in the exo1 strain (oversynthesis model). The conversion tract in the NCO pathway is similar to that observed in wild type, whereas the conversion tract for the CO pathway is shorter due to less resection. (C) UV-induced conversion in the wild-type strain (BIR model). In the NCO pathway, the conversion is a consequence of BIR. Following copying of the red chromosome, the invasion is reversed, and the end generated by BIR reassociates with the broken black chromosome. In this model, the conversion tract is not a consequence of repair of mismatches in a heteroduplex. The conversion events in the CO pathway occur by the same mechanism as in A and B. (D) UV-induced conversion in the exo1 strain (BIR model). As in C, the BIR event generating the conversion tract in the NCO pathway is not limited by resection, unlike the conversion tract in the CO pathway.
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fig7: Two models for the formation of long gene conversion tracts that are unassociated with crossovers. The data that require explanation are (1) crossover-associated conversion events are shorter in the exo1 strain than in wild type and (2) crossover-unassociated conversions are longer than crossover-associated conversions in the exo1 strain. The labels are the same as in Figure 6. The observations can be explained by an oversynthesis model (A and B) or by a BIR model (C and D). (A) UV-induced conversion in the wild-type strain (oversynthesis model). The synthesis from the invading strand is more extensive than the amount of DNA resected in the NCO pathway. The reinvasion during SDSA displaces the 5′ end of invaded homolog, and the resulting single-stranded branch is removed (shown by a triangle). In the CO pathway, the heteroduplex region is limited by resection. (B) UV-induced conversion in the exo1 strain (oversynthesis model). The conversion tract in the NCO pathway is similar to that observed in wild type, whereas the conversion tract for the CO pathway is shorter due to less resection. (C) UV-induced conversion in the wild-type strain (BIR model). In the NCO pathway, the conversion is a consequence of BIR. Following copying of the red chromosome, the invasion is reversed, and the end generated by BIR reassociates with the broken black chromosome. In this model, the conversion tract is not a consequence of repair of mismatches in a heteroduplex. The conversion events in the CO pathway occur by the same mechanism as in A and B. (D) UV-induced conversion in the exo1 strain (BIR model). As in C, the BIR event generating the conversion tract in the NCO pathway is not limited by resection, unlike the conversion tract in the CO pathway.

Mentions: We propose two new models to explain our data (Figure 7). Both require the assumption that the conversion events associated with crossovers are fundamentally different from those unassociated with crossovers. We suggest that the conversion events associated with crossovers are limited by the length of the resection tract, consistent with the observation that the conversion events associated with crossovers in wild type are significantly longer than those observed in the exo1 strain. In contrast, for the models shown in Figure 7, the lengths of the conversion tracts that are unassociated with crossovers are not limited by resection. In the first model (Figure 7A, wild type; and Figure 7B, exo1), the invading strand synthesizes a region of DNA longer than the gap resulting from end processing. When the invading strand is displaced by SDSA, the displaced end reinvades the processed black chromosome, producing a displaced 5′ end. This end could be removed by Rad27p or a related flap-processing enzyme. By this model, the conversion events unassociated with crossovers can be longer than conversions associated with crossovers (Figure 7B). The processing of a 5′ flap was previously proposed as a step in meiotic recombination (Osman et al. 2003), although in this model, the flap was generated during resolution of a recombination intermediate into a crossover rather than as a step in producing a conversion event unassociated with a crossover.


The role of Exo1p exonuclease in DNA end resection to generate gene conversion tracts in Saccharomyces cerevisiae.

Yin Y, Petes TD - Genetics (2014)

Two models for the formation of long gene conversion tracts that are unassociated with crossovers. The data that require explanation are (1) crossover-associated conversion events are shorter in the exo1 strain than in wild type and (2) crossover-unassociated conversions are longer than crossover-associated conversions in the exo1 strain. The labels are the same as in Figure 6. The observations can be explained by an oversynthesis model (A and B) or by a BIR model (C and D). (A) UV-induced conversion in the wild-type strain (oversynthesis model). The synthesis from the invading strand is more extensive than the amount of DNA resected in the NCO pathway. The reinvasion during SDSA displaces the 5′ end of invaded homolog, and the resulting single-stranded branch is removed (shown by a triangle). In the CO pathway, the heteroduplex region is limited by resection. (B) UV-induced conversion in the exo1 strain (oversynthesis model). The conversion tract in the NCO pathway is similar to that observed in wild type, whereas the conversion tract for the CO pathway is shorter due to less resection. (C) UV-induced conversion in the wild-type strain (BIR model). In the NCO pathway, the conversion is a consequence of BIR. Following copying of the red chromosome, the invasion is reversed, and the end generated by BIR reassociates with the broken black chromosome. In this model, the conversion tract is not a consequence of repair of mismatches in a heteroduplex. The conversion events in the CO pathway occur by the same mechanism as in A and B. (D) UV-induced conversion in the exo1 strain (BIR model). As in C, the BIR event generating the conversion tract in the NCO pathway is not limited by resection, unlike the conversion tract in the CO pathway.
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fig7: Two models for the formation of long gene conversion tracts that are unassociated with crossovers. The data that require explanation are (1) crossover-associated conversion events are shorter in the exo1 strain than in wild type and (2) crossover-unassociated conversions are longer than crossover-associated conversions in the exo1 strain. The labels are the same as in Figure 6. The observations can be explained by an oversynthesis model (A and B) or by a BIR model (C and D). (A) UV-induced conversion in the wild-type strain (oversynthesis model). The synthesis from the invading strand is more extensive than the amount of DNA resected in the NCO pathway. The reinvasion during SDSA displaces the 5′ end of invaded homolog, and the resulting single-stranded branch is removed (shown by a triangle). In the CO pathway, the heteroduplex region is limited by resection. (B) UV-induced conversion in the exo1 strain (oversynthesis model). The conversion tract in the NCO pathway is similar to that observed in wild type, whereas the conversion tract for the CO pathway is shorter due to less resection. (C) UV-induced conversion in the wild-type strain (BIR model). In the NCO pathway, the conversion is a consequence of BIR. Following copying of the red chromosome, the invasion is reversed, and the end generated by BIR reassociates with the broken black chromosome. In this model, the conversion tract is not a consequence of repair of mismatches in a heteroduplex. The conversion events in the CO pathway occur by the same mechanism as in A and B. (D) UV-induced conversion in the exo1 strain (BIR model). As in C, the BIR event generating the conversion tract in the NCO pathway is not limited by resection, unlike the conversion tract in the CO pathway.
Mentions: We propose two new models to explain our data (Figure 7). Both require the assumption that the conversion events associated with crossovers are fundamentally different from those unassociated with crossovers. We suggest that the conversion events associated with crossovers are limited by the length of the resection tract, consistent with the observation that the conversion events associated with crossovers in wild type are significantly longer than those observed in the exo1 strain. In contrast, for the models shown in Figure 7, the lengths of the conversion tracts that are unassociated with crossovers are not limited by resection. In the first model (Figure 7A, wild type; and Figure 7B, exo1), the invading strand synthesizes a region of DNA longer than the gap resulting from end processing. When the invading strand is displaced by SDSA, the displaced end reinvades the processed black chromosome, producing a displaced 5′ end. This end could be removed by Rad27p or a related flap-processing enzyme. By this model, the conversion events unassociated with crossovers can be longer than conversions associated with crossovers (Figure 7B). The processing of a 5′ flap was previously proposed as a step in meiotic recombination (Osman et al. 2003), although in this model, the flap was generated during resolution of a recombination intermediate into a crossover rather than as a step in producing a conversion event unassociated with a crossover.

Bottom Line: In accordance with this expectation, gene conversion tract lengths associated with spontaneous crossovers in exo1 strains were reduced about twofold relative to wild type.For UV-induced events, conversion tract lengths associated with crossovers were also shorter for the exo1 strain than for the wild-type strain (3.2 and 7.6 kb, respectively).Unexpectedly, however, the lengths of conversion tracts that were unassociated with crossovers were longer in the exo1 strain than in the wild-type strain (6.2 and 4.8 kb, respectively).

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular Genetics and Microbiology and University Program in Genetics and Genomics, Duke University Medical Center, Durham, North Carolina 27710.

Show MeSH
Related in: MedlinePlus