45S

45S refers to the primary transcript produced by RNA polymerase I from ribosomal DNA (rDNA) repeats in the genome. In most eukaryotes, the 45S pre-rRNA is processed to yield the mature ribosomal RNAs 18S, 5.8S, and 28S. The 5S rRNA is encoded separately by RNA polymerase III and is not part of the 45S transcript. The 45S precursor contains external transcribed spacers (ETS) at its ends and internal transcribed spacers (ITS1 and ITS2) between the mature rRNA sequences.

Structure and processing: The 45S pre-rRNA is transcribed as a single long RNA and subjected to a series of cleavages and chemical modifications, guided by small nucleolar RNAs and associated proteins. Through these processing steps, the 18S rRNA becomes part of the small ribosomal subunit, while the 5.8S and 28S rRNAs join the large subunit. Proper processing is essential for functional ribosome assembly.

Genomic organization: rDNA is present in tandem repeats arranged in nucleolar organizer regions (NORs) on multiple chromosomes. Copy number of rDNA repeats varies among species and individuals; only a subset of repeats is actively transcribed at any time, regulated by chromatin state and cellular growth cues.

Biological and clinical notes: Robust production of 45S rRNA reflects ribosome biogenesis and cellular growth. Dysregulation can contribute to nucleolar stress and affect p53 signaling. Alterations in rDNA copy number or transcription have been observed in cancer and aging studies, making 45S rRNA a focus of research on cellular proliferation.

Detection and measurement: 45S rRNA levels are assessed by methods such as RT-qPCR, Northern blotting, and in situ hybridization; assays may target the precursor or mature rRNA species, depending on the study.