SMALL NUCLEOLAR RNAS AND NUCLEOLAR PROTEINS IN XENOPUS ANUCLEOLATE EMBRYOS

We investigated the presence and localization, in the cells of anucleo late mutant embryos of Xenopus laevis, of three representative small n ucleolar RNAs (snoRNAs), U3, U15 and U17, and of two nucleolar protein s, nucleolin and fibrillarin. The levels of the three snoRNAs in the a nucleolate mutant are the same as in normal embryos, in contrast to 5S RNA and ribosomal proteins. In situ hybridization showed that, in the absence of fully organized nucleoli, the three RNAs are diffusely dis tributed in the nucleus and partly associated with a number of small s tructures. Nucleolin and fibrillarin are also present in the anucleola te embryos as in normal embryos, although there is less nucleolin mRNA in the former. The two nucleolar proteins were localized by immunoflu orescence microscopy. Fibrillarin, similar to its associated U3 and U1 5 snoRNAs, is diffusely distributed in the anucleolate nucleus and is partly associated with small structures, probably prenucleolar bodies and pseudonucleoli. Nucleolin also appears diffusely distributed in th e nucleus with some spots of higher concentration, but with a differen t pattern with respect to fibrillarin.