Comprehensive analysis of the pre-ribosomal RNA maturation pathway in a methanoarchaeon exposes the conserved circularization and linearization mode in archaea.

The ribosomal RNA (rRNA) genes are generally organized as an operon and cotranscribed into a polycistronic precursor; therefore, processing and maturation of pre-rRNAs are essential for ribosome biogenesis. However, rRNA maturation pathways of archaea, particularly of methanoarchaea, are scarcely known. Here, we thoroughly elucidated the maturation pathway of the rRNA operon (16S-tRNAAla-23S-tRNACys-5S) in Methanolobus psychrophilus, one representative of methanoarchaea. Enzymatic assay demonstrated that EndA, a tRNA splicing endoribonuclease, cleaved bulge-helix-bulge (BHB) motifs buried in the processing stems of pre-16S and pre-23S rRNAs. Northern blot and quantitative PCR detected splicing-coupled circularization of pre-16S and pre-23S rRNAs, which accounted for 2% and 12% of the corresponding rRNAs, respectively. Importantly, endoribonuclease Nob1 was determined to linearize circular pre-16S rRNA at the mature 3' end so to expose the anti-Shine-Dalgarno sequence, while circular pre-23S rRNA was linearized at the mature 5' end by an unknown endoribonuclease. The resultant 5' and 3' extension in linearized pre-16S and pre-23S rRNAs were finally matured through 5'-3' and 3'-5' exoribonucleolytic trimming, respectively. Additionally, a novel processing pathway of endoribonucleolysis coupled with exoribonucleolysis was identified for the pre-5S rRNA maturation in this methanogen, which could be also conserved in most methanogenic euryarchaea. Based on evaluating the phylogenetic conservation of the key elements that are involved in circularization and linearization of pre-rRNA maturation, we predict that the rRNA maturation mode revealed here could be prevalent among archaea.