Although the erythroid-specific promoter of human porphobilinogen deaminase [PBGD] gene has no TATA box, transcription is initiated at a single nucleotide. Using 5' and 3' deletions and point mutations, we have identified an element, located around the initiation site, which is necessary and sufficient for 'in vitro' accurate initiation of transcription. This 15 bp element extends 1 bp 5' and 14 bp 3' from the initiation site. It is composed of two regions, a proximal region centred on the cap site and a distal region which bears homology with the TdT initiator element. We show that a nuclear factor, present both in erythroid and non erythroid cells, binds the distal PBGD initiator element. Lack of heat inactivation suggests that initiation of transcription mediated by this element is not TFIID dependent. By transfection into erythroid cells, we also show that the proximal PBGD initiator element is essential for the selection of the initiation site but not for the regulation of transcription of the PBGD erythroid promoter during erythroid differentiation.