NUCLEOLAR TARGETING OF 5S RNA IN XENOPUS-LAEVIS OOCYTES - SOMATIC-TYPE NUCLEOTIDE SUBSTITUTIONS ENHANCE NUCLEOLAR LOCALIZATION

In Xenopus laevis oocytes, 5S RNA is stored in the cytoplasm until vit ellogenesis, at which time it is imported into the nucleus and targete d to nucleoli for ribosome assembly. This article shows that throughou t oogenesis there is a pool of nuclear 5S RNA which is not nucleolar-a ssociated. This distribution reflects that of oocyte-type 5S RNA, whic h is the major 5S RNA species in oocytes; only small amounts of somati c-type, which differs by six nucleotides, are synthesized. Indeed, P-3 2-labeled oocyte-type 5S RNA showed a degree of nucleolar localization similar to endogenous 5S RNA (33%) after microinjection. In contrast, P-32-labeled somatic-type 5S RNA showed significantly enhanced locali zation, whereby 70% of nuclear RNA was associated with nucleoli. A chi meric RNA molecule containing only one somatic-specific nucleotide sub stitution also showed enhanced localization, in addition to other soma tic-specific phenotypes, including enhanced nuclear import and ribosom e incorporation. The distribution of S-35-labeled ribosomal protein L5 was similar to that of oocyte-type 5S RNA, even when preassembled wit h somatic-type 5S RNA. The distribution of a series of 5S RNA mutants was also analyzed. These mutants showed various degrees of localizatio n, suggesting that the efficiency of nucleolar targeting can be influe nced by many discrete regions of the 5S RNA molecule. (C) 1998 Wiley-L iss, inc.