Structural organization of the transcription of ribosomal DNA in oocytes of the house cricket.

THE DNA molecules which contain the cistrons for ribosomal RNA (rDNA) consist of repeats of alternating segments of (i) regions which are transcribed into the primary rRNA precursors (pre-rRNA) and (ii) regions which are either not transcribed (corresponding to "spacers" sensu Miller and Beattyl) or might be, perhaps in parts, transcribed into RNA molecules which, however, are not covalently linked with pre-rRNA ("spacers" sensu Reeder and Brown 2 ; for demonstration of partial transcripts from spacers see Scheer et aP). The relative arrangement of the "spacer" units and the stretches coding for pre-rRNA (corresponding to the matrix units of Miller and Beattyl,4) has been elucidated by different methods in chromosomal and extrachromosomal rDNA of diverse amphibian species, including urodeles and anurans 1 ,3-13. In the clawed toads, Xenopus laevis and X. muelleri, according to differential DNA denaturation studies 12 ,13, this pattern of arrangement is identical in both chromosomal ("nucleolus organizer") and extrachromosomal ("amplified") rDNA. Extrachromosomal nucleolar material consisting of the actively transcribing amplified rDNA is especially suitable for biochemical and electron microscopic studies because it provides a natural enrichment of this kind of DNA in a state which is topologically isolated from all the other chromatin, We have therefore chosen, in order to examine the generality of the organization of transcribing rDNA, another amplified rDNA system, the extrachromosomal DNA masses which occur in the oocytes of diverse insects and constitute a considerable amount of the total nuclear DNA (for example, 59 % in Tipula oleracea l 4, 23-35 % in Dytiscid beetles IS, and 14-31 % in Acheta domesticus l 6-19). As has been shown by numerous authors I 8 ,21)-24 a favourable material in this respect is the house cricket, A. domesticus. Using a modified "Miller technique" we have spread the isolated nuclei of A. domesticus diplotene oocytes, that is, oocytes at a stage in which the amplified rDNA is actively transcribing and is highly dispersed over most of the nuclear interior 2 11-26. Figure 1 shows an electron micrograph of a part of such transcriptionally active deoxyribonucleoproteins (DNP) containing the rRNA cistrons. When partially spread, the DNP and their associated RNP are clumped together in aggregates (Fig. la, top), the structure of which is revealed on further spreading (Fig. la, bottom). Regions on the long DNP axes covered in lateral fibrils of gradually increasing lengths, presumed to represent the protein-complexed growing pre-rRNAs (matrix units sensu refs 1 …