Amplification of ribosomal DNA in Acheta. IV. The number of cistrons for 28S and 18S ribosomal RNA.

Abstract

In the cricket Acheta domesticus, the DNA of somatic tissues (heads and legs), centrifuged to equilibrium in CsCl gradients in the Model E ultracentrifuge, shows a DNA satellite with a buoyant densitiy of 1.716 g/ml. This satellite occurs in the same amount (0.8 % of the total DNA) and has the same buoyant density as the satellite found in testes. In ovaries (in which most oocytes are at pachytene) the satellite is 5 % of the total ovarian DNA and has the same buoyant density. The amount of DNA complementary to ribosomal RNA has been determined using the low temperature-formamide hybridization technique and by carrying out separate experiments involving 288 and 18s H3 RNA of Acheta. The data indicate that the numerical ratio between genes for 28s and 18S rRNA is one, in ovaries, testes and somatic tissues. The degree of amplification found by saturation experiments in the ovaries agrees with the size of the high buoyant density satellite observed in the analytical centrifuge. Under the conditions of our experiments there are 1,600 rRNA genes per ovarian cell (pachytene) whereas in somatic cells the value obtained is 342 sites. The amount of DNA non-homologous to rRNA present in the high buoyant density satellite is approximately 96.3 %. This may consists of “spacer” and other DNA sequences.