The intracellular location of 7S and 42S RNP particles in Xenopus oocytes has been determined by immunohistochemistry. Using antibodies directed against the 48-mol-wt protein component of the 42S particle and against transcription factor IIIA, the protein moiety of the 7S particle, we show that these ribonucleoprotein particles are detectable only in the oocyte cytoplasm, being excluded from the nucleus. The mechanism of this nuclear exclusion, and its possible significance in the regulation of 5S RNA gene expression, are discussed.

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