Start site selection at the TATA-less carbamoyl-phosphate synthase (glutamine-hydrolyzing)/aspartate carbamoyltransferase/dihydroorotase promoter.

Transcription of the carbamoyl-phosphate synthase (glutamine-hydrolyzing)/aspartate carbamoyltransferase/dihydroorotase (CAD) gene from the Syrian hamster, Mesocricetus auratus, starts at a single major site. We characterized the cis-acting elements that position RNA polymerase II at the correct start site in the CAD promoter. Sequence alignment showed that the CAD promoter lacks a TATA box, but contains a consensus initiator. Mutational analysis of the CAD promoter demonstrated that the sequences between -81 and +26 were sufficient for accurate and efficient transcription in vitro and in vivo; binding sites for the transcription factor Sp1 around -70 and -49 were necessary for transcriptional activity. The binding site at -49 directed initiation about 50 base pairs downstream. A ubiquitous activator protein, Honk, bound to the CAD promoter between -30 and -12, but did not participate in start site selection. The sequences around +1, which contain the consensus initiator, contributed to promoter activity; however, the presence of a consensus initiator in this region was neither necessary nor sufficient for transcription. We concluded from these results that the Sp1 binding site at -49 substituted for the missing TATA box and played a major role in start site selection at the CAD promoter.