LOCALIZATION OF NUCLEOLIN BINDING-SITES ON HUMAN AND MOUSE PRE-RIBOSOMAL-RNA

Nucleolin, a major RNA binding protein of the nucleolus is found assoc iated mainly to the pre-ribosomal particles and is absent from the cyt oplasmic mature ribosomes. The role of this protein in ribosome biogen esis remains largely unknown, and is likely to be reflected by its RNA binding properties. Nucleolin contains in its central domain four RNA recognition motifs (RRM, also called RED for RNA binding domain) whic h cue conserved among different species. RNA binding studies have reve aled that nucleolin interacts specifically with a short stem loop stru cture called NRE (nucleolin recognition element). We show that nucleol in extracted from human, hamster and mouse cells interacts with the sa me specificity and affinity to a mouse 5'ETS (external transcribed spa cer) RNA fragment which contains a NRE motif. A similar structure with in the human 5'ETS is also efficiently recognized by mouse nucleolin. We identified putative NRE not only in the 5'ETS but also in the 3'ETS , ITS (internal transcribed spacer) and in the 18S and 28S RNA sequenc es. This is in agreement with in vivo cross-linking data and a previou s immunocytological analysis of ribosomal transcription units. Interes tingly, we found that all the NRE localized in the 28S region are with in the variable domains. Despite considerable sequence divergence of t hese domains, several of the NRE have sequences perfectly conserved be tween these two species. This suggests that these nucleolin binding si tes might be functionally important, in particular for ribosome biogen esis.