TRANSCRIPTION INITIATION FROM A POLY(DA) TRACT

The mammalian ME1 gene encodes a non-tissue-specific, helix-loop-helix transcription factor that is enriched in morphogenetically active reg ions during development, Regulation of mouse ME1 gene expression is co ntrolled by a novel initiator (ME1 Inr) that promotes transcription fr om the center of a 13 bp poly(dA) tract. We show here that the ME1 Inr autonomously directs initiation from the poly(dA) tract both in vitro and in vivo. This transcription was dependent upon two protein comple xes; MBP alpha, which associated directly with the poly(dA) tract, and MBP beta, which introduced an similar to 60 degrees bend immediately downstream of the poly(dA) tract. The MBP alpha and MBP beta binding s ites were strikingly conserved in homologous DNA from several mammalia n species and the frog Xenopus laevis. These results suggest that the ME1 Inr constitutes a robust nucleation site that promotes transcripti on initiation in the absence of conventional promoter elements.