rRNAsyntes

RRNA synthesis, or rRNA transcription, is the cellular process by which ribosomal RNA genes are transcribed to produce rRNA precursors that are processed and assembled into ribosomal subunits. In eukaryotes, this process is a central part of ribosome biogenesis and is closely linked to cell growth and metabolism.

In most eukaryotes, RNA polymerase I transcribes the 45S pre-rRNA from tandem rDNA repeats located in the nucleolus. The 45S precursor is processed and chemically modified to yield the mature 18S, 5.8S, and 28S rRNAs. RNA polymerase III transcribes the 5S rRNA from separate genes, typically in the nucleoplasm. The initial transcripts undergo chemical modifications guided by small nucleolar RNAs (snoRNAs) within small nucleolar ribonucleoprotein particles (snoRNPs), including 2’-O-methylation and pseudouridylation.

Ribosomal subunits assemble following rRNA maturation: the small 40S subunit contains the 18S rRNA, while the large 60S subunit contains the 5.8S, 28S, and 5S rRNAs, with ribosomal proteins added throughout. This assembly largely occurs in the nucleolus and nucleoplasm and is completed in the cytoplasm.

Regulation of rRNA synthesis is tightly controlled by growth conditions and nutrient signaling. In eukaryotes, RNA polymerase I activity is governed by transcription factors such as UBF and SL1, while RNA polymerase III is regulated by factors like BRF1/BRF2. Signaling pathways, including TOR, influence transcriptional activity; cellular stress can suppress rRNA synthesis via p53-dependent and independent mechanisms.

In bacteria and archaea, rRNA genes are organized in operons and transcribed by RNA polymerase, with synthesis driven by growth rate and environmental cues. rRNA synthesis is essential for protein production and cell viability in all domains of life.