Length heterogeneity of amplified circular rDNA molecules in oocytes of the house cricket Acheta domesticus (Orthoptera: Gryllidae).

Amplification of the genes coding for rRNA occurs in the oocytes of a wide variety of organisms. The amplification process appears to be mediated through a rolling-circle mechanism. The approximate molecular weight of the smallest rDNA circles is equivalent to the estimated combined molecular weight of DNA which codes for a single ribosomal RNA precursor molecule and an associated non-transcribed spacer DNA sequence. RNA-DNA hybridization studies carried out on oocytes of the house cricket, Acheta domesticus, suggest that DNA coding for rRNA accounts for only a small fraction of the rDNA satellite, all of which is amplified in the oocyte. In order to test the possibility that the remainder of the amplified rDNA represents spacer and to determine whether a rolling-circle mechanism might also be involved in amplification in A. domesticus oocytes, rDNA was isolated from ovaries of A. domesticus and spread for electron microscopy. A large proportion of the rDNA isolated from ovaries is circular, while main-band DNA and rDNA prepared from other tissues demonstrates few if any circles. The mean size of the smallest rDNA circles is approximately 8 times longer than the length estimated for DNA which codes for 18S and 28 S rRNA. Denaturation mapping shows the rDNA circles to contain two major readily denaturing regions located about equidistant from one another on the circle. Each readily denaturing region accounts for 4--6% of the total DNA in the circle. The fact that only 12% of the average molecule is required to code for A. domesticus 18S and 28S rRNA is consistent with the hybridization data. Considerable size heterogeneity exists in the length of the smallest class of rDNA molecules. In the rDNA of other species such heterogeneity has been shown to reside in the non-transcribed spacer.