Nucleolar localization of murine nuclear DNA helicase II (RNA helicase A)

Nuclear DNA helicase II (NDH II) is a highly conserved member of the DEXH s uperfamily of eukaryotic helicases, whose physiological role is still uncle ar. To explore the function of NDH II, we studied the intracellular distrib ution of NDH II of different mammalian species by immunofluorescence and co mpared these findings with the known role of the Drosophila homologue MLE t hat is involved in sex-specific gene dosage compensation. NDH II displayed an apparent nucleolar localization in murine cells, whereas in cells from a ll other mammalian species examined so far the protein was confined to the nucleoplasm and apparently excluded from the nucleoli, The nucleolar locali zation of mouse NDH II strongly suggests a role in ribosomal RNA biosynthes is, Immunoelectron microscopic studies revealed that the mouse NDH II was f ound at the dense fibrillar components of the nucleoli, and a significant p ercentage of NDH II molecules colocalized with the RNA polymerase I(Pol I) transcription factor UBF (upstream binding factor). Additionally, the nucle olar localization of NDH II coincided with a preferential immunolabeling pa ttern of nascent transcripts with bromouridine (BrUMP). Furthermore, mouse NDH II redistributed in mitosis in a manner highly correlated with Pol I ac tivity. Conditions leading to the inhibition of Pol I activity in the inter phase decreased the amount of NDH II in the nucleoli that diffused into the nucleoplasm and the cytosol, Contrary to the effect of inhibiting rRNA syn thesis, treatment of mouse cells with the translation inhibitor cycloheximi de did not compromise the nucleolar localization of murine NDH II.