WERNER HELICASE IS LOCALIZED TO TRANSCRIPTIONALLY ACTIVE NUCLEOLI OF CYCLING CELLS

Mutations at the Werner helicase locus (WRN) are responsible for the W erner syndrome (WS), a ''caricature of aging.'' We have localized the Werner protein (WRNp) to the nucleoli of replicating mammalian cells, where its appearance is associated with transcriptional activity. A dr amatic reduction of the nucleolar signal and of [H-3]uridine incorpora tion occurred when cultures were made quiescent or were exposed to 4-n itroquinoline-1-oxide (4NQO), to which WS cells are particularly susce ptible. Total cellular levels of WRNp, however, did not change, and vi rtually all WRNp was in the nuclear fractions, consistent with translo cation to the nucleoplasm and/or masking of the epitopes. The 4NQO-ind uced altered state of WRNp was prevented by Na3VO4, but not by okadaic acid, suggesting that WRNp localization/function is partially regulat ed by kinases/phosphatases for Tyr substrates on WRNp or interacting p roteins. The repression of rDNA transcription by 4NQO was not reversed by Na3VO4, We suggest that physiological states and genotoxic agents modulate the interaction of WRNp with rDNA, consistent with a role of WRNp in rDNA transcription. (C) 1998 Academic Press.