Excess 5′-terminal sequences in the rat nucleolar 28S ribosomal RNA

Abstract

The 5′-termini of purified rat liver nucleolar and cytoplasmic 28S ribosomal RNA (rRNA) are precisely located within the homologous rDNA sequence by S 1 nuclease protection mapping using an appropriate rDNA restriction fragment. The 5′-termini of nucleolar 28S rRNA are heterogeneous in length. The bulk of the nucleolar 28S rRNA map within two CTC motifs in rDNA located in the internal transcribed spacer 2 at the 50–60 and 5–15 bp upstream from the site of the homogeneous 5′-terminus of the cytoplasmic 28S rRNA. These results provide direct proof that nucleolar 28S rRNA molecules contain excess sequences at their 5′-termini and require further processing to generate the mature cytoplasmic 28S rRNA.