Early nucleolar preribosomal RNA - protein in mammalian cells.

Early steps of ribosomal maturation have been studied by analysis of nucleolar extracts using different extraction procedures. Early 45-S nucleolar RNA is found to be associated with slowly sedimenting elements, distinct from previously described 80-S and 55-S nucleolar preribosomes. This early 45-S RNA has been shown to be of preribosomal type according to the following criteria. (a) When hybridized with nucleolar DNA, competition with rRNA can be observed; (b) its biosynthesis is sensitive to low doses of actinomycin D; (c) it is methylated at an early stage; (d) it contains no linked poly(A) segments. This early 45-S RNA seems to be specifically linked with proteins. The protein content of these early RNA-protein complexes is significantly lower than that for 80-S preribosomes. 45-S RNA sensitivity to nucleolytic activities that can take place in the course of preribosome extraction has been found to be higher in these RNA-protein complexes than in 80-S preribosomes. Identical results were obtained when different mammalian cell species were studied (rat, hamster, or HeLa cells).