Ribosomal RNA Synthesis

Eukaryotic ribosomes contain four types of ribosomal RNA: 5S, 5.8S, 18S, and 28S rRNA. Three of these rRNAs—5.8S, 18S, and 28S—are encoded together in a single DNA element and transcribed as one large precursor called 45S precursor rRNA. The 5S rRNA is encoded separately and transcribed independently by RNA Polymerase III as a 120-nucleotide transcript.

Small nucleolar RNA-protein complexes, or snoRNPs, catalyze two main modifications to precursor rRNA: nucleoside methylation and pseudouridylation. Each snoRNP consists of a snoRNA and four or five proteins, including the modifying enzyme. The snoRNA base-pairs with complementary sequences on the precursor rRNA, directing the enzyme to the exact site requiring modification.

After transcription by RNA Polymerase I, the 45S precursor rRNA undergoes extensive processing and modification. It is chemically modified through nucleoside methylation and pseudouridylation, then cleaved by nucleolar proteins into mature 5.8S, 18S, and 28S rRNAs. These mature rRNAs are subsequently incorporated into ribosomal subunits.

Modified bases in ribosomal RNA stabilize particular conformations of the rRNA and regulate ribosomal activity during translation. Although individual modifications may seem minor, collectively they concentrate in functional regions of the rRNA and enhance ribosome performance. These modifications are a distinct feature of eukaryotic ribosomes.

The 5S rRNA is encoded separately from the other three rRNAs and is transcribed by RNA Polymerase III outside the nucleolus as a 120-nucleotide transcript. Unlike 5.8S, 18S, and 28S rRNAs, the 5S rRNA remains unmodified after transcription. It is then imported into the nucleolus to be assembled with other ribosomal components.

Magnesium ions (Mg2+) play an important role in maintaining ribosome structure. When Mg2+ is removed during experiments, the ribosome dissociates into subunits. The cations likely interact with ionized phosphates of the RNA to bridge the two ribosomal subunits, though the precise mechanism remains uncertain.

Ribosomal components assemble in the nucleolus, nucleoplasm, and cytoplasm through a process involving more than 200 assembly factors. Once assembly is complete, some ribosomes associate with intracellular membranes, primarily the endoplasmic reticulum, forming bound ribosomes. Free ribosomes are distributed throughout the cytoplasm for protein synthesis.