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

Abstract

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 Beatty1) or might be, perhaps in parts, transcribed into RNA molecules which, however, are not covalently. linked with pre-rRNA (“spacers” sensu Reeder and Brown2 ; for demonstration of partial transcripts from spacers see Scheer et al.3). The relative arrangement of the “spacer” units and the stretches coding for pre-rRNA (corresponding to the matrix units of Miller and Beatty1,4) has been elucidated by different methods in chromosomal and extrachromosomal rDNA of diverse amphibian species, including urodeles and anurans1,3–13. In the clawed toads, Xenopus laevis and X. muelleri, according to differential DNA denaturation studies12,13, 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 oleracea14, 23–35% in Dytiscid beetles15 and 14–31% in Acheta domesticus 16–19 ). As has been shown by numerous authors18,20–24 a favourable material in this respect is the house cricket, A. domesticus.