Misregulation of 2μm Circle Copy Number in a SUMO Pathway Mutant

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Abstract
Attachment of the ubiquitin-like protein SUMO to other proteins is an essential process in Saccharomyces cerevisiae. However, yeast mutants lacking the SUMO ligases Siz1 and Siz2/Nfi1 are viable, even though they show dramatically reduced levels of SUMO conjugation. This siz1Δ siz2Δ double mutant is cold sensitive and has an unusual phenotype in that it forms irregularly shaped colonies that contain sectors of wild-type-appearing cells as well as sectors of enlarged cells that are arrested in G2/M. We have found that these phenotypes result from misregulation of the copy number of the endogenous yeast plasmid, the 2μm circle. siz1Δ siz2Δ mutants have up to 40-fold-higher levels of 2μm than do wild-type strains. Furthermore, 2μm is responsible for the siz1Δ siz2Δ mutant9s obvious growth defects, as siz1Δ siz2Δ [cir0] strains, which lack 2μm, are no longer heterogeneous and show growth characteristics similar to those of the wild type. Possible mechanisms for SUMO9s effect on 2μm are suggested by the finding that both Flp1 recombinase and Rep2, two of the four proteins encoded by 2μm, are covalently modified by SUMO. Our data suggest that SUMO attachment negatively regulates Flp1 levels, which may partially account for the increased 2μm copy number in the siz1Δ siz2Δ strain.

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