Inhibitor of ribosomal RNA synthesis in Xenopus laevis embryos : IV. The release of inhibitor from cultured cells

Abstract

Previous reports from our laboratory have shown that a culture medium, conditioned by the growth of isolated cells of Xenopus laevis blastulae, contains a low-molecular-weight substance which selectively inhibits 18 and 28S ribosomal RNA (rRNA) synthesis. Although the occurrence of an inhibitor in an acid-soluble fraction of blastulae has recently been demonstrated, our observation of an inhibitor in a conditioned medium has not been confirmed by other laboratories. To resolve this discrepancy, we have reexamined the effects of conditioned media and acid-soluble extracts on rRNA synthesis by neurula cells. (1) The inhibitory activity for rRNA synthesis can consistently be observed in blastula-conditioned media, provided some of the cells have been broken down during conditioning. If cell rupture is avoided, an inactive conditioned medium is obtained. (2) A homogenate of blastulae inhibits total RNA synthesis and shows no selective inhibition of rRNA synthesis. (3) Charcoal treatment of the conditioned medium and homogenate enhances their specificity for rRNA synthesis. It is then likely that cell breakdown may be involved in the release of the inhibitor into the medium and that some differences in the methods of preparation of conditioned medium may account for the above discrepancy.