The size of the transcription unit in balbiani ring 2 of Chironomus tentans as derived from analysis of the primary transcript and 75 S RNA

Abstract

The size of the transcription unit for 75 S RNA in Balbiani ring 2, a giant puff in the salivary glands of Chironomus tentans, has been derived from determinations of the size of salivary gland 75 S RNA as well as of the size of the primary transcript in BR2. § § Abbreviations used: BR2, Balbiani ring 2. Salivary gland 75 S RNA, predominantly located in the cytoplasm, was analyzed by electrophoresis in formaldehyde-containing agarose gels. The molecular weight was shown to be at least 12 × 10 6 but probably not exceeding 13 to 14 × 10 6. The size was also determined by contour length measurements in the electron microscope, which gave a molecular weight of 12.3 × 10 6. Since this value fell within the size range established by the electrophoretic method, it was accepted as an estimate of the size of 75 S RNA. The biochemical and electron microscopic evidence for considering 75 S RNA as a single, covalently linked polynucleotide is discussed. Balbiani ring 2 RNA, containing the finished primary transcript as well as growing RNA molecules, was analyzed in agarose/formaldehyde gels with salivary gland 75 S RNA as an internal size marker. From the distribution of the nascent RNA it was concluded that the finished primary transcript essentially migrated coincident with 75 S RNA. Thus, the molecular weight value of 12.3 × 10 6 was also adopted for the primary transcript. This would imply that the corresponding transcription unit in BR2 comprises about 37 × 10 3 base-pairs of DNA. The predicted size of the transcription unit in BR2 was related to the information available on the BR2 chromomere and the adjacent interchromomeres. It was concluded that only a minor part of the unit (less than 5× 10 3 base-pairs) can be accommodated in an interchromomere adjacent to the BR2 chromomere. Most, if not all, of the unit has to be present in the BR2 chromomere itself. On the other hand, there is more DNA in a BR2 chromomere (> 100 × 10 3 base-pairs) than can be accounted for by one single 75 S RNA transcription unit.