Localization of pre-messenger RNA at discrete nuclear sites.

We have studied the nuclear localization of rhodamine-labeled pre-mRNA after microinjection into nuclei of cultured rat kidney epithelial cells. Intranuclear localization of the injected RNA was followed in the living cells by fluorescence microscopy and digital image processing. Injected human beta-globin pre-mRNA became localized in 30-60 discrete nuclear sites that were coincident with loci defined by monoclonal antibodies against small nuclear ribonucleoproteins (Sm) or another spliceosome component (SC-35) in parallel immunocytochemical studies on the same nuclei. Similar patterns of nuclear localization were observed with a rat proenkephalin pre-mRNA. Nuclear microinjection of an intronlacking beta-globin RNA, a splicing-defective beta-globin mutant pre-mRNA, or an antisense beta-globin pre-mRNA did not result in localization at discrete sites. These results indicate that pre-mRNA binds preferentially to nuclear Sm and SC-35 antibody-reactive sites in vivo and that the binding requires intron sequences.