| Summary: | The yeast proteins Sas3p (something about silencing 3) and Gcn5p (general control nonderepressible 5) are the catalytic subunits of the histone acetyltransferase (HAT) complexes NuA3 and SAGA, ADA, SLIWSALSA, and HAT-A2 respectively. When incorporated into their respective complexes they gain the ability to acetylate nucleosomal histone H3 at the overlapping K14 residue. Previous work reveals gcn5Δsas3Δ strains are inviable, suggesting they have an essential function in the redundant acetylation of histone H3. Furthermore, researchers have shown that viability is dependent on the HAT domains of Sas3p and Gcn5p. We show that a H3 tail with an arginine substitution mutation at K14 but not K9,18,23,27R is synthetically lethal in a gcn5Δ background. Thus, NuA3’s essential role in the cell is acetylating K14. To determine how Sas3p acetylation is targeted, we performed a yeast 2-hybrid screen against Sas3p’s essential N-terminus. Hsl7p and Rldp are tentatively shown to interact with Sas3p but did not interact or share genetic interactions with NuA3. Previous research shows the synthetic lethality between loss of Gcn5p and loss of components of NuA3 is independent of the adaptor proteins Ada2p and Ada3p; thus, Gcn5p’s essential function appears independent of known HAT complexes, suggesting it could be acetylating a non-histone protein. Consistent with this idea, we created a Gcn5p truncation that cannot interact with Ada2p or rescue a SAGA phenotype but rescues the synthetic lethality. Furthermore, we showed that histone H3 tail mutations in K9,14,18,23R or K9,18,23,27R are viable in a sas3Δ background. We attempted to find the essential subunit of Gcn5p and ruled out Sinlp and Htzlp as the sole substrate. In a synthetic lethality suppression screen, we identified several mutants that suppress the sas3Δgcn5Δ synthetic lethality. These mutants appear to have the characteristics of petites and/or prions. Thus, a possible role for these HATS in DNA damage repair is proposed. === Medicine, Faculty of === Biochemistry and Molecular Biology, Department of === Graduate
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