Properties of promoter regions of mdg1 Drosophila retrotransposon indicate that it belongs to a specific class of promoters.

A sequence 30 bp downstream from the start site of the Drosophila melanogaster retrotransposon mdg1 is shown to be responsible for correct and precise initiation of mdg1 RNA synthesis in combination with the RNA start-site sequence TCAGTT. A sequence-specific DNA binding protein is demonstrated to interact with the +30 sequence, and the efficient binding of this factor is necessary for in vivo transcriptional activity of the plasmid constructs containing mdg1 promoter fragments. The nucleotides -8/+34 of mdg1 represent a minimal promoter which is able to provide correct initiation of transcription by RNA polymerase II at basal levels. A comparison with properties of some other retrotransposable elements and several developmentally regulated cellular genes allows us to conclude that together they form a specific class of RNA polymerase II promoter. This promoter class characteristically lacks upstream sequences necessary for transcription initiation, such as TATA boxes, but requires a specific downstream promoter element within 40 bp downstream of the RNA start site. The level of transcription can, however, be modulated by upstream regulatory elements. The identified sequence-specific downstream initiation factor may be responsible for transcription initiation on promoters of some genes which belong to this class.