STRUCTURAL REQUIREMENTS OF 5S RIBOSOMAL-RNA FOR NUCLEAR TRANSPORT, 7SRIBONUCLEOPROTEIN PARTICLE ASSEMBLY, AND 60S RIBOSOMAL-SUBUNIT ASSEMBLY IN XENOPUS-OOCYTES

Structural requirements of 5S rRNA for nuclear transport and RNA-prote in interactions have been studied by analyzing the behavior of oocyte- type 5S rRNA and of 31 different in vitro-generated mutant transcripts after microinjection into the cytoplasm of Xenopus oocytes. Experimen ts reveal that the sequence and secondary and/or tertiary structure re quirements of 5S rRNA for nuclear transport, storage in the cytoplasm as 7S ribonucleoprotein particles, and assembly into 60S ribosomal sub units are complex and nonidentical. Elements of loops A, C, and E, hel ices II and V, and bulged and hinge nucleotides in the central domain of 5S rRNA carry the essential information for these functional activi ties. Assembly of microinjected SS rRNA into 60S ribosomal subunits wa s shown to occur in the nucleus; thus, the first requirement for subun it assembly is nuclear targeting. The inhibitory effects of ATP deplet ion, wheat germ agglutinin, and chilling on the nuclear import of 5S r RNA indicate that it crosses the nuclear envelope through the nuclear pore complex by a pathway similar to that used by karyophilic proteins .