Nuclear transport of 5S ribosomal RNA in Xenopus laevis oocytes.

This thesis presents investigations into aspects of oocyte-type 5S ribosomal RNA (5S rRNA) nuclear transport in Xenopus laevis oocytes. Oocyte-type 5S rRNA synthesis begins in the nucleus of early-stage oocytes before other ribosomal RNAs and proteins are available, and the RNA is subsequently stored in the cytoplasm in specific complexes with proteins as 42S and 7S ribonucleoprotein particles (RNPs). 5S rRNA remains sequestered in the cytoplasm until it receives the signal to return to the nucleus to be incorporated into 60S ribosomal subunits; a process occurring in the nucleolus. Movement of 5S rRNA into the nucleus is concurrent with the maximal synthesis of the other ribosomal RNAs and ribosomal proteins at the later stages of oogenesis. At this time, 5S rRNA interacts with ribosomal protein L5 to form pre-ribosomal 5S RNPs. By microinjecting in vitro-synthesised, labelled 5S rRNA and 5S RNPs into late-stage Xenopus oocytes, I have analysed the mechanism of 5S rRNA localisation to the nucleus. The results presented in this thesis provide evidence for a model for 5S rRNA nuclear import where the exchange of TFIIIA for L5 binding triggers the switch from cytoplasmic storage to its nuclear accumulation. It is likely that L5 provides a nuclear localisation signal which interacts with components of the nuclear transport machinery used by nuclear proteins, enabling the 5S RNP to be translocated into the nucleus through the channel provided by the nuclear pore complex. The process of 5S rRNA nuclear import requires physiological temperatures and metabolic energy, and involves interactions with nuclear pore complex glycoproteins