Table 1: Ribosomal RNA (rRNA) and rRNA processing across the three domains.

ArchaeaEukaryaBacteria

rRNAs23S, 5S (LSU); 16S (SSU)28S, 5.8S, 5S (LSU); 18S (SSU)23S, 5S (LSU); 16S (SSU)
Ribosome70S80S70S
Large subunit (LSU)50S60S50S
Small subunit (SSU)30S40S30S
Presence of introns in rDNA genes?In some species [2024]In some speciesRare* [25]
Main rRNA modifying machinessRNPs [2628]snoRNPs [2933]Site-specific protein enzymes [34, 35]
Number of rRNA Nms and sS. solfataricus: ~76 [36]Yeast: 99
Human: 202 [34, 3740]
E. coli: 14 [34, 37, 38, 40, 41]
Number of C/D and H/ACA s(no)RNAsS. solfataricus: ~9–17 [42, 43]Yeast: 75
Human: 207 [40]
0 [34, 35]

Uncultured giant sulfur bacteria, such as Thiomargarita namibiensis, have been shown to contain self-splicing introns, varying in number and size, in the 16S rRNA gene. To date, this has been the only report of bacterial rDNA genes containing introns.