THE RNA-POLYMERASE I-SPECIFIC TRANSCRIPTION INITIATION-FACTOR UBF IS ASSOCIATED WITH TRANSCRIPTIONALLY ACTIVE AND INACTIVE RIBOSOMAL GENES

We have characterized an anti-NOR (nucleolar organizer region) serum ( P419) from a patient with rheumatoid arthritis and show that it contai ns antibodies directed against the RNA polymerase I-specific transcrip tion initiation factor UBF. This serum reacts with UBF from a variety of vertebrate cells as revealed both by immunoblotting and by indirect immunofluorescence. We have used the P419 serum to study the intracel lular localization of this transcription factor at the light and elect ron microscopic level. In interphase cells, UBF exhibits a pronounced punctate pattern and is found to be asssociated with necklace-like str uctures, which appear to reflect the transcriptionally active state of the nucleolus. Inhibition of rRNA synthetic activity caused either by nutritional starvation or by actinomycin D treatment resulted in a ma rked decrease in the number and in a significant increase in the size of UBF-positive granules. Under all experimental conditions applied, U BF was exclusively found within the nucleolus and was not released int o the nucleoplasm or cytoplasm. During mitosis, UBF was found to be co ncentrated at the chromosomal NOR indicating that a significant quanti ty, if not all, of this factor remains bound to the ribosomal transcri ption units. From this we conclude that UBF is associated both with tr anscriptionally active and inactive rRNA genes and, therefore, changes in the intracellular localization of UBF are very likely not involved in rDNA transcription regulation.