Oocyte extracts reactivate developmentally inert Xenopus 5S genes in somatic nuclei

Abstract

In Xenopus and other amphibians, there are two major classes of 5S RNA gene¹. One class, which codes for somatic-type 5S RNA, is expressed in both oocytes and somatic cells²; the other, which is 50 times more abundant³ and which codes for oocyte-type 5S RNA, is expressed in oocytes but not in somatic cells^2,4. The developmentally inactivated oocyte-type genes are reactivated when erythrocyte nuclei are injected into most oocytes (activating oocytes)⁵. As the injected nuclei are induced to take on the natural activity of the host cell, the activation of oocyte-type 5S genes may represent a normal control mechanism. A special opportunity to investigate this mechanism is provided by the existence of certain individual animals whose oocytes consistently fail to activate 5S genes (non-activating oocytes). We have injected extracts of activating or non-activating oocytes, together with erythrocyte nuclei, into oocytes of each kind. We show here that the transcription of 5S genes is induced by extracts of activating oocytes (positive regulation) and is not inhibited by similar extracts of non-activating oocytes. The activating factor is sensitive to protease and heat treatment, suggesting that a protein is involved.