Structure of the Bombyx mori rDNA: initiation site for its transcription

The initiation site of rDNA transcription in Bombyx mori was determined to be located at 909bp upstream from the 5'- end of mature 18s rRNA by S1-nuclease mapping and primer extension experiment. An in vitro transcription system, which was constructed using posterior silk glands of Bombyx larvae, initiated the transcription of cloned rDNA at exactly the same site as determined for the in vivo transcription above. The primary transcript seemed to be processed at about 200b downstream of the initiation site both in vivo and in vitro. Sequence analyses of the flanking region of the initiation site revealed that short repetitive sequences are widely distributed throughout the NTS region, and that a highly AT-rich region resides immediately upstream of the initiation region.