Immunostaining of nucleolus organizers in mammalian cells by a human autoantibody against the polymerase I transcription factor UBF.

A mammalian autoantigen of the nucleolus organizing regions (NORs) was identified by a human autoantibody from the serum of a rheumatoid arthritis patient. The distribution and changes of NORs during the cell cycle of mammals were followed by using this autoantiserum in indirect-immunofluorescence microscopy. In interphase cells the staining pattern indicated that the autoantigen is restricted exclusively within the nucleolus. This fluorescence appeared punctuated rather than uniform, and it was reorganized during inhibition of transcription in cells treated with actinomycin D. During mitosis, the autoantigen was detected by light microscopy at the chromosomal nucleolus organizer regions, indicating that presumably the protein remains bound to the rRNA genes. Biochemical analysis by immunoblotting showed that the NOR autoantigen consists of two polypeptides with molecular masses apparent of 90-92 kDa in all of the mammalian cell lines tested. The identity of some epitopes, recognized by this autoantibody as the ribosomal transcription factor UBF, is discussed.