Distribution and utilization of 5 S-RNA-binding proteins during the development of Xenopus oocytes

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Abstract
At early stages of oogenesis in Xenopus laevis most of the ribosomal 5S RNA is complexed with 3 proteins to form 2 types of cytoplasmic RNP [ribonucleoprotein] storage particle. A particle sedimenting at 42S contains 5S RNA and tRNA together with 2 proteins of MW 48,000 (P48) and MW-43,000 (P43) and a second particle sedimenting at 7S contains 5S RNA plus a protein of MW 40,000 (P40, also known as the transcription factor, TFIIIA). Antibodies monospecific for each protein were used to follow the movement of 5S RNA from nucleus to cytoplasm to nucleolus to cytoplasm and to determine the fate of each of the proteins that associate with 5S RNA during these transitions. Both P48 and P43 have roles additional to the formation of the 42S RNP storage particle; P48 is detected in the nucleus during early oogenesis and is cleaved to yield an MW-33,000 fragment that remains associated with 5S RNA that is excess to ribosome requirement during late oogenesis; P43 appears to be cleaved to yield fragments of MW-28,000 and 17,000, the latter being present in ribosomal fractions. Apparently, there is no function for P40 in addition to those already described in transcription of 5S RNA genes and in storage of 5S RNA as a 7S RNP particle.