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

At early stages of oogenesis in Xenopus laevis most of the ribosomal 5S RNA is complexed with three proteins to form two types of cytoplasmic RNP storage particle. A particle sedimenting at 42S contains 5S RNA and tRNA together with two proteins of Mr 48000 (P48) and Mr 43000 (P43) and a second particle sedimenting at 7S contains 5S RNA plus a protein of Mr 40000 (P40, also known as the transcription factor, TFIIIA). In this report we use antibodies monospecific for each protein 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 Mr-33000 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 Mr 28000 and 17000, 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.