Enhancer 1 binding factor (E1BF), a Ku-related protein, is a growth-regulated RNA polymerase I transcription factor: association of a repressor activity with purified E1BF from serum-deprived cells.

Previous studies from this laboratory have demonstrated that the enhancer 1 binding factor (E1BF), a Ku-related protein, purified from the serum-enriched cells functions as a positive factor in an RNA polymerase (pol I) transcription system. We have now shown that E1BF purified from the serum-deprived cells (E1BFs) can inhibit rDNA transcription completely in a fractionated extract from the cells grown in serum-enriched medium. The suppression of transcription was overcome by the addition of control E1BF (E1BFc). Immunoprecipitation of purified E1BFs by the anti-Ku monoclonal antibody and addition of the supernatant to the transcription reaction mixture prevented the inhibition significantly, whereas immunoprecipitation with the control mouse IgG did not restore the transcription. The transcriptional repressor activity associated with the final DNA affinity column fractions copurified with E1BF. Neither the amount of E1BF nor its promoter binding activity was altered following serum depletion. E1BFs selectively inhibited the initiation of rDNA transcription. The inhibitory activity of E1BFs was not due to a nonspecific RNase activity. These data suggest that E1BF is post-translationally modified following serum starvation of cells, and that the repressor activity of E1BFs is largely responsible for the down-regulation of pol I transcription in serum-deprived cells.