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Open Access Research article

Effects of mutations in SGS1 and in genes functionally related to SGS1 on inverted repeat-stimulated spontaneous unequal sister-chromatid exchange in yeast

Dilip K Nag12* and Steffany J Cavallo1

Author Affiliations

1 Division of Molecular Medicine, Wadsworth Center, Center for Medical Sciences, 150 New Scotland Avenue, Albany, NY 12208, USA

2 Department of Biomedical Sciences, School of Public Health, State University of New York, Albany, NY 12201, USA

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BMC Molecular Biology 2007, 8:120  doi:10.1186/1471-2199-8-120

Published: 31 December 2007

Abstract

Background

The presence of inverted repeats (IRs) in DNA poses an obstacle to the normal progression of the DNA replication machinery, because these sequences can form secondary structures ahead of the replication fork. A failure to process and to restart the stalled replication machinery can lead to the loss of genome integrity. Consistently, IRs have been found to be associated with a high level of genome rearrangements, including deletions, translocations, inversions, and a high rate of sister-chromatid exchange (SCE). The RecQ helicase Sgs1, in Saccharomyces cerevisiae, is believed to act on stalled replication forks. To determine the role of Sgs1 when the replication machinery stalls at the secondary structure, we measured the rates of IR-associated and non-IR-associated spontaneous unequal SCE events in the sgs1 mutant, and in strains bearing mutations in genes that are functionally related to SGS1.

Results

The rate of SCE in sgs1 cells for both IR and non-IR-containing substrates was higher than the rate in the wild-type background. The srs2 and mus81 mutations had modest effects, compared to sgs1. The exo1 mutation increased SCE rates for both substrates. The sgs1 exo1 double mutant exhibited synergistic effects on spontaneous SCE. The IR-associated SCE events in sgs1 cells were partially MSH2-dependent.

Conclusions

These results suggest that Sgs1 suppresses spontaneous unequal SCE, and SGS1 and EXO1 regulate spontaneous SCE by independent mechanisms. The mismatch repair proteins, in contradistinction to their roles in mutation avoidance, promote secondary structure-associated genetic instability.