Multiple downstream promoter modules regulate the transcription of the Drosophila melanogaster I, Doc and F elements

The basal promoters of three Drosophila long interspersed nuclear elements (LINEs), the I factor and the F and Doc elements, have the same architecture. In each, transcription is directed by an initiator which is faithfully and efficiently recognized only when flanked 3' by a DNA segment approximately 20 bp in length called the B region. The B regions of the three promoters are interchangeable and have a complex structure, comprising three functionally distinct elements: de1, de2 and de3. While de2 is relatively conserved, fitting the consensus RGACGTGY, de1 and de3 vary among the three promoters. At different levels, each downstream element is able to ensure accurate recognition of the initiator. The de2 domain stimulates transcription of the F, I and Doc promoters to the same extent. In contrast, the I de1 domain stimulates transcription much more efficiently than the corresponding domains of the F and Doc elements. The finding that de2 is selectively required in order to detect full activity of enhancer sequences found in the F element suggests that de1 and de2 interact with different proteins. The B regions can be replaced by and synergize with a TATA element, can functionally substitute for downstream promoter sequences in the Drosophila hsp70 gene, and significantly activate the mouse terminal deoxynucleotidyl transferase initiator. Our data suggest that the B regions stimulate transcription by providing sites of interaction for the TFIID complex. Sequences homologous to the del to de3 array are found downstream from the transcription start site(s) both in TATA-less and TATA-containing promoters.