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Subcellular transport and ribosomal incorporation of microinjected protein S6 in oocytes from Xenopus laevis.

Protein S6 was isolated from 40S ribosomal subunits of Xenopus laevis oocytes, labeled with sodium boro[3H]hydride, and microinjected back into oocytes. In the first 4 h of incubation, the uptake of S6 into the nucleus increased to a maximum, with, however, no detectable incorporation into 40S ribosomal subunits. After this lag period, S6 was progressively integrated into the small ribosomal subunit. When rRNA transcription was inhibited by actinomycin D, the uptake of S6 into the nucleus and its consequent incorporation into the 40S subunit were significantly reduced. Moreover, when enucleated oocytes were microinjected, little or no S6 was found in the 40S subunits, also suggesting that integration of S6 into ribosomes is linked to rRNA precursor synthesis. In contrast to S6, the acidic protein eL12 isolated from Artemia salina or X. laevis oocyte 60S subunits was integrated into the large subunit independently of the nucleus or active rRNA synthesis.