Printed in Great Britain PATTERN OF INCORPORATION OF [»H]URIDINE INTO RNA OF AMPHIBIAN OOCYTE NUCLEOLI

Amphibian oocytes were incubated in vitro in the presence of [sH]uridine, and autoradio-graphs were made of nucleoli isolated from these oocytes and of sections of oocytes. After incubations of a h or less the nucleoli of oocytes larger than o-6 mm diameter are asymmetrically labelled. With longer incubations nucleoli from oocytes of o-6 to r i mm dia-meter become more uniformly labelled. Those of oocytes larger than i-2 mm diameter remain asymmetrically labelled whatever the incubation time. Autoradiographs of i-/t sections through oocytes larger than o-6 mm diameter show, after short incubations, asymmetrically labelled nucleoli. In these autoradiographs silver grains are concentrated over a distinct component of each nucleolus which is eccentrically placed towards the nuclear envelope. Thin sections of oocytes show nucleoli consisting of core and cortex. The core material is always concentrated into the half of the nucleolus which, lies nearer the nuclear envelope. Autoradiographs of separated nucleolar cores and cortices from oocytes larger than o-6 mm diameter show, after short incubations, silver grains over cores but not over cortices. Similar autoradiographs prepared from oocytes of o-6 to r i mm diameter, after longer incubations, show grains over cores and cortices. These results appear to indicate that nucleolar RNA is synthesized in the nucleolar core, in association with the nucleolar DNA, and is thence transferred to the cortex where it is built into ribonucleoprotein particles. Initial asymmetrical labelling is a consequence of the eccentric location of the nucleolar core. The nucleoli of oocytes smaller than o-6 mm diameter always label symmetrically; such nucleoli consist entirely of core material. It is suggested that the nucleoli of oocytes larger than 1-2 mm diameter always label asymmetrically because transfer of RNA from core to cortex proceeds more slowly than in smaller oocytes