Early gene expression of both RNA polymerase I transcription factors UBF1 and UBF2 precedes ribosomal RNA synthesis during lymphocyte mitogenic stimulation.

Phytohaemagglutinin stimulates lymphoid cells to initiate active cell division which is tightly coordinated with transcription of ribosomal RNA genes. Nuclear Run-On assays demonstrated that treatment of peripheral blood lymphocytes with PHA (10 microg/ml) resulted in maximal rRNA synthesis after 64 hrs. In contrast, mRNA levels for upstream binding factor (UBF)1 and UBF2, as measured by reverse transcriptase-polymerase chain reaction (RT-PCR) and Southern blotting, increased relatively rapidly within 3 to 6 hrs. and remained elevated for at least the next 60 hrs. We further showed that exponentially growing cells of promyelocytic leukemia line HL-60 contained the same amounts of UBF1 and UBF2 mRNAs as phytohaemagglutinin (PHA)-stimulated lymphocytes for 6 hrs. Growth arrest of HL-60 cells, caused by 10 nM phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced monocytic/macrophage-like differentiation for 72 hrs., has been accompanied by a 50% decrease in UBF1/2 mRNAs expression. The lowest concentrations of UBF1/2 mRNAs were revealed in non-dividing terminally differentiated granulocytes. Regardless the activity of RNA polymerase I transcription and cell division rate, UBF1 mRNA levels prevailed over UBF2 mRNA levels in all human blood cell populations tested. Our results suggest that UBF gene expression is an important regulatory mechanism involved in the acceleration and possibly deceleration of rDNA transcription observed during mitogenic stimulation and inhibition of blood cells.