title: Effects of mutations in SGS1 and in genes functionally related to SGS1 on inverted repeat-stimulated spontaneous unequal sister-chromatid exchange in yeast.
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.
authors.affiliation: Division of Molecular Medicine, Wadsworth Center, Center for Medical Sciences, 150 New Scotland Avenue, Albany, NY 12208, USA. dilip.nag@wadsworth.org
authors.forename: Dilip K
authors.initials: DK
authors.lastname: Nag
authors.forename: Steffany J
authors.initials: SJ
authors.lastname: Cavallo
mesh_terms: D001483:Base Sequence; D004265:DNA Helicases / Q000235:genetics / Q000378:metabolism; D004271:DNA, Fungal / Q000235:genetics; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D004720:Endonucleases / Q000235:genetics / Q000378:metabolism; D005090:Exodeoxyribonucleases / Q000235:genetics / Q000378:metabolism; D005800:Genes, Fungal; D051718:MutS Homolog 2 Protein / Q000235:genetics / Q000378:metabolism; D009154:Mutation; D010957:Plasmids / Q000235:genetics; D051135:Rad51 Recombinase / Q000235:genetics / Q000378:metabolism; D053484:RecQ Helicases / Q000235:genetics* / Q000378:metabolism; D011995:Recombination, Genetic; D012091:Repetitive Sequences, Nucleic Acid; D012441:Saccharomyces cerevisiae / Q000235:genetics* / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics* / Q000378:metabolism; D012854:Sister Chromatid Exchange / Q000235:genetics*
chemical_list: D004271:DNA, Fungal; D004268:DNA-Binding Proteins; D029701:Saccharomyces cerevisiae Proteins; C061376:SRS2 protein, S cerevisiae; C495151:RAD51 protein, S cerevisiae; D051135:Rad51 Recombinase; D004720:Endonucleases; D005090:Exodeoxyribonucleases; C411624:MUS81 protein, S cerevisiae; C028450:exodeoxyribonuclease I; C090472:SGS1 protein, S cerevisiae; C489751:MSH2 protein, S cerevisiae; D051718:MutS Homolog 2 Protein; D004265:DNA Helicases; D053484:RecQ Helicases