Role of SGS1 and SLX4 in maintaining rDNA structure in Saccharomyces cerevisiae

To address the role of SGS1 in controlling genome stability we previously identified several slx mutants that require SGS1 for viability. Here, we report the isolation and characterization of temperature-sensitive (ts) SGS1 alleles in cells lacking SLX4. At the non-permissive temperature (37 degrees C) sgs1-ts slx4 cells progress through S-phase and arrest growth as large-budded cells with at least a 2C DNA content. Analysis of the integrity of the replicated DNA by pulsed-field gel electrophoresis revealed that chromosome XII (ChrXII) was uniquely altered, as it was unable to enter the gel. This defect was specific to the tandem rDNA repeats on ChrXII and occurred as cells progressed through S-phase at 37 degrees C. Reciprocal-shift experiments revealed that viability and ChrXII migration can be restored by allowing Sgs1 to act between G2/M and the subsequent G1 phase. These results suggest that Sgs1 and Slx4 are not required for bulk DNA synthesis but play redundant roles in maintaining rDNA structure during DNA replication.