PRH represses transcription in hematopoietic cells by at least two independent mechanisms.

PRH (proline-rich homeodomain protein) is strongly expressed in the hematopoietic compartment. Here we show that PRH is a repressor of transcription in hematopoietic cells. A fragment of PRH that includes the homeodomain can bind to TATA box sequences in vitro and can also bind to the TATA box-binding protein. PRH represses transcription from TATA box-containing promoters in intact cells but does not repress transcription from a promoter lacking a TATA box. A mutation in the PRH homeodomain that blocks binding to DNA but that has little or no effect on binding to the TATA box-binding protein significantly reduces the ability of the protein to repress transcription and provides the first clear demonstration that a homeodomain can bring about transcriptional repression in vivo by binding to a TATA box. However, we also show that mutation of the PRH homeodomain does not block the ability of PRH to repress transcription when this protein is tethered upstream of the TATA box via a heterologous DNA-binding domain. PRH also contains an N-terminal proline-rich repression domain that is separate from the homeodomain. Deletion mapping suggests that this repression domain contains at least two regions that both independently contribute to transcriptional repression.