Abstract
Genome stability requires a set of RecQ-Top3 DNA helicase-topoisomerase complexes whose sole budding yeast homolog is encoded by
SGS1-TOP3. RMI1/NCE4
was identified as a potential intermediate in the
SGS1-TOP3
pathway, based on the observation that strains lacking any one of these genes require
MUS81
and
MMS4
for viability. This idea was tested by confirming that
sgs1
and
rmi1
mutants display the same spectrum of synthetic lethal interactions, including the requirements for
SLX1
,
SLX4
,
SLX5
, and
SLX8
, and by demonstrating that
rmi1 mus81
synthetic lethality is dependent on homologous recombination. On their own, mutations in
RMI1
result in phenotypes that mimic those of
sgs1
or
top3
strains including slow growth, hyperrecombination, DNA damage sensitivity, and reduced sporulation. And like
top3
strains, most
rmi1
phenotypes are suppressed by mutations in
SGS1
. We show that Rmi1 forms a heteromeric complex with Sgs1-Top3 in yeast and that these proteins interact directly in a recombinant system. The Rmi1-Top3 complex is stable in the absence of the Sgs1 helicase, but the loss of either Rmi1 or Top3 in yeast compromises its partner's interaction with Sgs1. Biochemical studies demonstrate that recombinant Rmi1 is a structure-specific DNA binding protein with a preference for cruciform structures. We propose that the DNA binding specificity of Rmi1 plays a role in targeting Sgs1-Top3 to appropriate substrates.