| Summary: | <p>The <em>Saccharomyces cerevisiae</em> Rmi1 protein is a component of the highly conserved Sgs1-Top3-Rmi1 complex, which is required for the maintenance of genome stability. The <em>rmi1Δ</em> deletion mutant has proven difficult to study because it exhibits very poor growth, and rapidly accumulates second site suppressor mutations. Furthermore, deletion of the putative HJ resolvase genes, <em>MUS81</em>-<em>MUS81</em> or <em>SLX1</em>-<em>SLX4</em> in <em>rmi1Δ</em> mutants causes synthetic lethality. In order to study phenotypes caused by loss of functional Rmi1, and to explore the genetic interactions between <em>RMI1</em> and the <em>MUS81</em>, <em>MUS81</em>, <em>SLX1</em> and <em>SLX4</em> genes, a temperature sensitive mutant of <em>RMI1</em> was isolated, named <em>rmi1-1</em>. Similar to <em>rmi1Δ</em> deletion mutants, <em>rmi1-1</em> cells are highly sensitive to the DNA damaging agent, MMS and the replication inhibitor, HU. In addition, <em>rmi1-1</em> mutants accumulate replication-associated branched DNA structures, and arrest in G<sub>2</sub>/M after a transient exposure to MMS. These cells are proficient in DNA damage checkpoint activation. Deletion of <em>SLX1</em>, <em>SLX4</em>, <em>MUS81</em> or <em>MUS81</em> in the <em>rmi1-1</em> strain causes synthetic lethality, which is associated with cell cycle defects. Following a transient exposure to MMS, <em>rmi1-1</em> mutants accumulate homologous recombination intermediates. These intermediates are slowly resolved at the restrictive temperature, revealing a redundant resolution activity in the absence of functional Rmi1. This resolution depends upon Mus81-Mms4, but not on Slx1-Slx4 or Yen1. I propose that while the Sgs1-Top3-Rmi1 complex constitutes the main pathway for removal of homologous recombination intermediates following a perturbed S-phase, Mus81-Mms4 can act as a back up for resolution of these intermediates, which most likely represent double Holliday junctions. In this study, I also present screens for high copy suppressors of <em>rmi1-1</em> phenotypes, and for novel Rmi1 interaction partners.</p>
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