Developmental inactivity of 5S RNA genes persists when chromosomes are cut between genes

Abstract

Xenopus and other amphibians possess two major classes of 5S RNA genes¹. One class, which codes for somatic-type 5S RNA, is expressed in both oocytes and somatic cells². The other class, which is 50 times more abundant and encodes oocyte-type 5S RNA, is expressed in oocytes but not in somatic cells^2–4. The oocyte-type 5S genes are located in tandem arrays of hundreds of genes at the ends of most chromosomes⁵, and it has been postulated that the lack of expression of oocyte-type 5S RNA genes in somatic cells is due to some general characteristic of the chromosomal region which the genes occupy⁶, for example, a folding into heterochromatin or higher-order structures. An alternative possibility is that the inactivity of these genes is controlled individually, and is independent of adjacent genetic material. We have now distinguished between these two possibilities by testing the transcription of oocyte-type 5S RNA genes either in their normal arrangement on intact chromosomes or after each gene has been separated from its neighbours by micrococcal nuclease digestion. Transcription was assayed by injecting intact nuclei, containing the 5S RNA genes as chromatin, into Xenopus oocytes⁷. We find that the developmental inactivity of oocyte-type 5S RNA genes is retained after the chromatin is cleaved into fragments the size of individual gene repeats.