THE ACTIVITY OF TRANSCRIPTION FACTOR PBP, WHICH BINDS TO THE PROXIMALSEQUENCE ELEMENT OF MAMMALIAN U6 GENES, IS REGULATED DURING DIFFERENTIATION OF F9 CELLS

Mouse F9 embryonic carcinoma (EC) cells differentiate in culture to pa rietal endoderm (PE) cells upon induction with retinoic acid and cycli c AMP. In the course Of this process, the expression of polymerase III transcripts, e.g., 5S rRNA and U6 small nuclear RNA, is dramatically reduced. This reduction of endogenous RNA content is accompanied by a loss of transcriptional capacity in cell extracts from PE cells. Parti al purification of such extracts reveals that the DNA-binding activity of transcription factor PBP, binding specifically to the proximal seq uence element (PSE) sequence of vertebrate U6 genes, is significantly reduced. This finding is corroborated by a loss in the transcriptional activity of this factor in reconstitution assays with partially purif ied polymerase III transcription components. In contrast, the activity of TFIIIA and TFIIIB and the amount of free TATA-binding protein rema in unchanged during the differentiation process analyzed here. These d ata show for the first time that the PSE-binding protein PBP is essent ially involved in the differential regulation of polymerase III genes governed by external promoters.