Circular 23S rRNA within archaeal ribosomes

The ribosomal RNAs (rRNAs) that form the core of ribosomes are believed to occur as linear molecules. Here, we investigated rRNAs from diverse and mostly uncultivated archaea and found evidence that, in at least eight phylum-level archaeal lineages, the 23S rRNAs within mature ribosomes are circular. Sequencing of total cellular RNA indicates that the transcriptional abundances of archaeal circular 23S rRNAs vastly exceed those of linear counterparts, and linear versions are often undetectable. As the majority of rRNAs derive from mature ribosomes, the data suggested that the circular molecules are within the ribosomes. Thus, we directly sequenced RNA extracted from isolated ribosomes of a model archaeon, Methanosarcina acetivorans, and confirmed that the assembled 23S rRNAs are circular. Structural modeling places the 5 and 3 ends of the linear precursors of archaeal 23S rRNAs in close proximity to form a GNRA tetraloop (in which N is A, C, G, or U and R is A or G), consistent with their further circularization within ribosomes. We confirm the existence of circular 16S rRNA intermediates in transcriptomes of most archaea, yet a circular form is not evident in some distinct archaeal groups. Overall, the results uncover unexpected variations in the processing required to generate mature rRNAs and the conformation of functional molecules in archaeal ribosomes.