UC San Diego

SUMO-Targeted Ubiquitin Ligases (STUbLs) are evolutionarily conserved from yeast to eukaryotes and contribute broadly to the maintenance of genome integrity. The budding yeast, Saccharomyces cerevisiae, contains two distinct STUbLs, the Slx5-Slx8 heterodimeric complex (Slx complex), and Uls1. These enzymes use SUMO-Interacting Motifs (SIMs) to identify targets and a Really Interesting New Gene (RING) domain to ubiquitinate them. In this work, the gene encoding the lesser-characterized S. cerevisiae STUbL, ULS1, was characterized for chromatin-related functions. In addition, a mutational and complementation analysis of yeast STUbLs was carried out using RING and SIM mutants and the human STUbL, RNF4. Here we report that ULS1 contributes to DNA silencing at the HMR locus, within the cryptic mating-type loci, and at the 25S rDNA gene. ULS1 suppresses the formation of Gross Chromosomal Rearrangements (GCRs), but is not involved in the DNA damage response to ultra-violet (UV) radiation or HydroxyUrea (HU). However, ULS1 caused increased sensitivity to the drug camptothecin (CPT), and this sensitivity was dosage-dependent. The mutational analysis definitively shows that STUbLs require an intact RING domain for their in vivo chromatin functions, whereas the SIM domains of Slx5 are dispensable for its in vivo functions. Finally, RNF4 was able to complement both the DNA damage response and DNA silencing defects found in slx8[Delta] mutants