Figure 38.1 The processing of eukaryotic pre-rRNA. The mammalian pre-rRNA transcript contains the RNA sequences destined to become the 18S, 5.8S, and 28S rRNAs of the small and large ribosomal subunits. First, nucleotides are modified (indicated by red lines). Next, the pre-rRNA is cleaved and packaged to form mature ribosomes in a highly regulated process in which more than 200 proteins take part.