Elongator Complex Influences Telomeric Gene Silencing and DNA Damage Response by Its Role in Wobble Uridine tRNA Modification

摘要

Elongator complex is required for formation of the side chains at position 5 of modified nucleosides 5-carbamoylmethyluridine (ncm~(5)U_(34)), 5-methoxycarbonylmethyluridine (mcm~(5)U_(34)), and 5-methoxycarbonylmethyl-2-thiouridine (mcm~(5)s~(2)U_(34)) at wobble position in tRNA. These modified nucleosides are important for efficient decoding during translation. In a recent publication, Elongator complex was implicated to participate in telomeric gene silencing and DNA damage response by interacting with proliferating cell nuclear antigen (PCNA). Here we show that elevated levels of tRNA~(Lys)_(s~(2))_(UUU), tRNA~(Gln)_(s~(2))_(UUG), and tRNA~(Glu)_(s~(2))_(UUC), which in a wild-type background contain the mcm~(5)s~(2)U nucleoside at position 34, suppress the defects in telomeric gene silencing and DNA damage response observed in the Elongator mutants. We also found that the reported differences in telomeric gene silencing and DNA damage response of various elp3 alleles correlated with the levels of modified nucleosides at U_(34). Defects in telomeric gene silencing and DNA damage response are also observed in strains with the tuc2 Δ mutation, which abolish the formation of the 2-thio group of the mcm~(5)s~(2)U nucleoside in tRNA~(Lys)_(mcm~(5)_(s~(2))_(UUU)), tRNA~(Gln)_(mcm~(5)_(s~(2))_(UUG)), and tRNA~(Glu)_(mcm~(5)_(s~(2))_(UUC)). These observations show that Elongator complex does not directly participate in telomeric gene silencing and DNA damage response, but rather that modified nucleosides at U_(34)are important for efficient expression of gene products involved in these processes. Consistent with this notion, we found that expression of Sir4, a silent information regulator required for assembly of silent chromatin at telomeres, was decreased in the elp3 Δ mutants. Author Summary Elongator is a conserved protein complex in eukaryotes. Studies in yeast, worms, and plants have revealed that Elongator complex is required for formation of mcm~(5)and ncm~(5)side chains at wobble uridines in a subset of tRNA species. The primary function of Elongator complex in yeast is to modify U_(34)in tRNAs. Lack of these tRNA modifications causes pleiotropic phenotypes in yeast Elongator mutants due to inefficient translation. In this report, we demonstrate that the defects in telomeric silencing and DNA damage response observed in yeast Elongator mutants are a consequence of a tRNA modification defect. We suggest that the requirement of Elongator complex in tRNA modification is conserved in all eukaryotes, and diseases linked to human Elongator mutations may involve impaired translation due to lack of tRNA modifications.