The 75 S RNA transcription unit in Balbiani ring 2 and its relation to chromosome structure.

A defined transcription unit in the Balbiani ring 2 (BR 2) region of chromosome IV in the salivary glands of Chironomus tentans has been characterized on the basis of analysis of the corresponding primary transcript, 75S RNA, and its functional significance. The available information on the transcription unit and its relations to chromosome structure can be summarized in the following way: 1. The size of the 75S RNA transcription unit in BR 2 is on the order of 30 000 base pairs. 2. The unit is likely to contain a long coding segment (at least 6000 base pairs), probably corresponding to information for salivary polypeptides. 3. The sequences are distributed in more than one chromomere (probably in 3--5 chromomeres). Further studies are needed before it can be stated whether or not there is a simple one-to-one relation between chromomeres and transcription units in the BR 2 region.