Coincident start sites for divergent transcripts at a randomly selected CpG-rich island of mouse.

We determined the nucleotide sequence of two HTF islands that were selected at random from mouse chromosomal DNA. Both were non-methylated, G + C rich, and contained CpG at close to the expected frequency. When used as probes, the two islands detected multiple transcripts in RNA from several mouse tissues. Cloned cDNAs for the major transcripts of one island (HTF9) were isolated and used to construct a transcriptional map. We found that HTF9 contains the origin of a pair of divergent transcripts that are probably messenger RNAs. The bidirectional promoter is different from those previously observed as the major transcription start sites for each orientation are coincident on opposite strands of the DNA. The results support the view that HTF islands often mark genes, and they suggest that bidirectional transcription may be a common feature of island promoters.