In vitro processing of the 16S rRNA of the thermophilic archaeon Sulfolobus solfataricus

Endonucleolytic processing plays a critical role in the maturation of ribosomal RNA in Methanococcus maripaludis

Ribosomal RNA (rRNA) processing and maturation are fundamentally important for ribosome biogenesis, but the mechanisms in archaea, the third form of life, remains largely elusive. This study aimed to investigate the rRNA maturation process in Methanococcus maripaludis, a representative archaeon lacking known 3'-5' exonucleases. Through cleavage site identification and enzymatic assays, the splicing endonuclease EndA was determined to process the bulge-helix-bulge (BHB) motifs in 16S and 23S rRNA precursors. After splicing, the circular processing intermediates were formed and this was confirmed by quantitative RT-PCR and Northern blot. Ribonuclease assay revealed a specific cleavage at a 10-nt A/U-rich motif at the mature 5' end of pre-16S rRNA, which linearized circular pre-16S rRNA intermediate. Further 3'-RACE and ribonuclease assays determined that the endonuclease Nob1 cleaved the 3' extension of pre-16S rRNA, and so generated the mature 3' end. Circularized RT-PCR (cRT-PCR) and 5'-RACE identified two cleavage sites near helix 1 at the 5' end of 23S rRNA, indicating that an RNA structure-based endonucleolytic processing linearized the circular pre-23S rRNA intermediate. In the maturation of pre-5S rRNA, multiple endonucleolytic processing sites were determined at the 10-nt A/U-rich motif in the leader and trailer sequence. This study demonstrates that endonucleolytic processing, particularly at the 10-nt A/U-rich motifs play an essential role in the pre-rRNA maturation of M. maripaludis, indicating diverse pathways of rRNA maturation in archaeal species.

The dark side of the ribosome life cycle

Thanks to genetics, biochemistry, and structural biology many features of the ribosome´s life cycles in models of bacteria, eukaryotes, and some organelles have been revealed to near-atomic details. Collectively, these studies have provided a very detailed understanding of what are now well-established prototypes for ribosome biogenesis and function as viewed from a 'classical' model organisms perspective. However, very important challenges remain ahead to explore the functional and structural diversity of both ribosome biogenesis and function across the biological diversity on earth. Particularly, the 'third domain of life', the archaea, and also many non-model bacterial and eukaryotic organisms have been comparatively neglected. Importantly, characterizing these additional biological systems will not only offer a yet untapped window to enlighten the evolution of ribosome biogenesis and function but will also help to unravel fundamental principles of molecular adaptation of these central cellular processes.

Ribosome Biogenesis in Archaea

Making ribosomes is a major cellular process essential for the maintenance of functional ribosome homeostasis and to ensure appropriate gene expression. Strikingly, although ribosomes are universally conserved ribonucleoprotein complexes decoding the genetic information contained in messenger RNAs into proteins, their biogenesis shows an intriguing degree of variability across the tree of life. In this review, we summarize our knowledge on the least understood ribosome biogenesis pathway: the archaeal one. Furthermore, we highlight some evolutionary conserved and divergent molecular features of making ribosomes across the tree of life.

Splicing Endonuclease Is an Important Player in rRNA and tRNA Maturation in Archaea

In all three domains of life, tRNA genes contain introns that must be removed to yield functional tRNA. In archaea and eukarya, the first step of this process is catalyzed by a splicing endonuclease. The consensus structure recognized by the splicing endonuclease is a bulge-helix-bulge (BHB) motif which is also found in rRNA precursors. So far, a systematic analysis to identify all biological substrates of the splicing endonuclease has not been carried out. In this study, we employed CRISPRi to repress expression of the splicing endonuclease in the archaeon Haloferax volcanii to identify all substrates of this enzyme. Expression of the splicing endonuclease was reduced to 1% of its normal level, resulting in a significant extension of lag phase in H. volcanii growth. In the repression strain, 41 genes were down-regulated and 102 were up-regulated. As an additional approach in identifying new substrates of the splicing endonuclease, we isolated and sequenced circular RNAs, which identified excised introns removed from tRNA and rRNA precursors as well as from the 5' UTR of the gene HVO_1309. In vitro processing assays showed that the BHB sites in the 5' UTR of HVO_1309 and in a 16S rRNA-like precursor are processed by the recombinant splicing endonuclease. The splicing endonuclease is therefore an important player in RNA maturation in archaea.

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-tRNA^Ala-23S-tRNA^Cys-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.

A versatile cis-acting element reporter system to study the function, maturation and stability of ribosomal RNA mutants in archaea

General molecular principles of ribosome biogenesis have been well explored in bacteria and eukaryotes. Collectively, these studies have revealed important functional differences and few similarities between these processes. Phylogenetic studies suggest that the information processing machineries from archaea and eukaryotes are evolutionary more closely related than their bacterial counterparts. These observations raise the question of how ribosome synthesis in archaea may proceed in vivo. In this study, we describe a versatile plasmid-based cis-acting reporter system allowing to analyze in vivo the consequences of ribosomal RNA mutations in the model archaeon Haloferax volcanii. Applying this system, we provide evidence that the bulge-helix-bulge motif enclosed within the ribosomal RNA processing stems is required for the formation of archaeal-specific circular-pre-rRNA intermediates and mature rRNAs. In addition, we have collected evidences suggesting functional coordination of the early steps of ribosome synthesis in H. volcanii. Together our investigation describes a versatile platform allowing to generate and functionally analyze the fate of diverse rRNA variants, thereby paving the way to better understand the cis-acting molecular determinants necessary for archaeal ribosome synthesis, maturation, stability and function.

Insights into RNA-processing pathways and associated RNA-degrading enzymes in Archaea

RNA-processing pathways are at the centre of regulation of gene expression. All RNA transcripts undergo multiple maturation steps in addition to covalent chemical modifications to become functional in the cell. This includes destroying unnecessary or defective cellular RNAs. In Archaea, information on mechanisms by which RNA species reach their mature forms and associated RNA-modifying enzymes are still fragmentary. To date, most archaeal actors and pathways have been proposed in light of information gathered from Bacteria and Eukarya. In this context, this review provides a state of the art overview of archaeal endoribonucleases and exoribonucleases that cleave and trim RNA species and also of the key small archaeal proteins that bind RNAs. Furthermore, synthetic up-to-date views of processing and biogenesis pathways of archaeal transfer and ribosomal RNAs as well as of maturation of stable small non-coding RNAs such as CRISPR RNAs, small C/D and H/ACA box guide RNAs, and other emerging classes of small RNAs are described. Finally, prospective post-transcriptional mechanisms to control archaeal messenger RNA quality and quantity are discussed.

Lack of strand-specific repair of UV-induced DNA lesions in three genes of the archaeon Sulfolobus solfataricus

In all organisms, specialized systems are devoted to repair of DNA lesions induced by exposure to UV light. In both Eucarya and Bacteria, UV-induced pyrimidine dimers in the transcribed strand of active genes are repaired at a faster rate compared to the non-transcribed strand and the rest of the genome. Preferential repair of transcribed strands requires the Transcription-Repair Coupling Factor in Escherichia coli and the CSA and CSB proteins in humans. These factors are needed for coupling of transcription to nucleotide excision repair (NER), a major pathway for repair of UV-induced lesions. Whereas transcription-coupled NER (TC-NER) is an evolutionary conserved process, not all active genes show preferential repair of transcribed strands. The existence of a NER pathway in the Archaea has not been demonstrated directly, yet it is suggested by the presence and properties of homologues of NER nucleases and helicases. However, none of the proteins responsible for the lesion recognition steps or for TC-NER has been found in archaeal genomes. Moreover, the kinetics of gene or strand-specific repair has never been investigated in any organism of this domain. We have analysed the kinetics of repair of UV-induced DNA damage in the transcribed and non-transcribed strands of three genes of the hyperthermophilic archaeon Sulfolobus solfataricus. We found that in all three genes the two strands are repaired with the same efficiency with each other and with the genome in general, thus providing no evidence of strand bias or transcription coupling of the repair process in the genes analysed. Further studies will be required to test the existence of a transcription-coupled repair pathway in other archaeal genes and to elucidate the mechanism of UV lesion recognition and repair in Archaea.

Identification of BHB splicing motifs in intron-containing tRNAs from 18 archaea: evolutionary implications

Most introns of archaeal tRNA genes (tDNAs) are located in the anticodon loop, between nucleotides 37 and 38, the unique location of their eukaryotic counterparts. However, in several Archaea, mostly in Crenarchaeota, introns have been found at many other positions of the tDNAs. In the present work, we revisit and extend all previous findings concerning the identification, exact location, size, and possible fit to the proposed bulge-helix-bulge structural motif (BHB, now renamed hBHBh') of the sequences spanning intron-exon junctions in intron-containing tRNAs of 18 archaea. A total of 103 introns were found located at the usual position 37/38 and 33 introns at 14 other different positions, that is, in the anticodon stem and loop, in the D-and T-loops, in the V-arm, or in the amino acid arm. For introns located at 37/38 and elsewhere in the pre-tRNA, canonical hBHBh' motifs were not always found. Instead, a relaxed hBH or HBh' motif including the constant central 4-bp helix H flanked by one helix (h or h') on either side generating only one bulge could be disclosed. Also, for introns located elsewhere than at position 37/38, the hBHBh' (or HBh') structure competes with the three-dimensional structure of the mature tRNA, attesting to important structural rearrangements during the complex multistep maturation-splicing processes. A homotetramer-type of splicing endonuclease (like in all Crenarchaeota) instead of a homodimeric-type of enzyme (as in most Euryarchaeota) appears to best fit the requirement for splicing introns at relaxed hBH or HBh' motifs, and may represent the most primitive form of this enzyme.

Two reactions of Haloferax volcanii RNA splicing enzymes: joining of exons and circularization of introns

Archaeal RNA splicing involves at least two protein enzymes, a specific endonuclease and a specific ligase. The endonuclease recognizes and cleaves within a characteristic bulge-helix-bulge (BHB) structure formed by pairing of the regions near the two exon-intron junctions, producing 2',3'-cyclic phosphate and 5'-hydroxyl termini. The ligase joins the exons and converts the cyclic phosphate into junction phosphate. The ligated product contains a seven-base hairpin loop, in which the splice junction is in between the two 3' terminal residues of the loop. Archaeal splicing endonucleases are also involved in rRNA processing, cutting within the BHB structures formed by pairing of the 5' and 3' flanking regions of the rRNAs. Large free introns derived from pre-rRNAs have been observed as stable and abundant circular RNAs in certain Crenarchaeota, a kingdom in the domain Archaea. In the present study, we show that the cells of Haloferax volcanii, a Euryarchaeote, contain circular RNAs formed by 3',5'-phosphodiester linkage between the two termini of the introns derived from their pre-tRNAs. H. volcanii ligase, in vitro, can also circularize both endonuclease-cleaved introns, and non-endonuclease-produced substrates. Exon joining and intron circularization are mechanistically similar ligation reactions that can occur independently. The size of the ligated hairpin loop and position of the splice junction within this loop can be changed in in vitro ligation reactions. Overall, archaeal RNA splicing seems to involve two sets of two symmetric transesterification reactions each.

The Sulfolobus solfataricus Lrp-like protein LysM regulates lysine biosynthesis in response to lysine availability

Although the archaeal transcription apparatus resembles the eukaryal RNA polymerase II system, many bacterial-like regulators can be found in archaea. Particularly, all archaeal genomes sequenced to date contain genes encoding homologues of Lrp (leucine-responsive regulatory protein). Whereas Lrp-like proteins in bacteria are involved in regulation of amino acid metabolism, their physiological role in archaea is unknown. Although several archaeal Lrp-like proteins have been characterized recently, no target genes apart from their own coding genes have been discovered yet, and no ligands for these regulators have been identified so far. In this study, we show that the Lrp-like protein LysM from Sulfolobus solfataricus is involved in the regulation of lysine and possibly also arginine biosynthesis, encoded by the lys gene cluster. Exogenous lysine is the regulatory signal for lys gene expression and specifically serves as a ligand for LysM by altering its DNA binding affinity. LysM binds directly upstream of the TFB-responsive element of the intrinsically weak lysW promoter, and DNA binding is favored in the absence of lysine, when lysWXJK transcription is maximal. The combined in vivo and in vitro data are most compatible with a model in which the bacterial-like LysM activates the eukarya-like transcriptional machinery. As with transcriptional activation by Escherichia coli Lrp, activation by LysM is apparently dependent on a co-activator, which remains to be identified.

RNomics in Archaea reveals a further link between splicing of archaeal introns and rRNA processing

The bulge-helix-bulge (BHB) motif recognised by the archaeal splicing endonuclease is also found in the long processing stems of archaeal rRNA precursors in which it is cleaved to generate pre-16S and pre-23S rRNAs. We show that in two species, Archaeoglobus fulgidus and Sulfolobus solfataricus, representatives from the two major archaeal kingdoms Euryarchaeota and Crenarchaeota, respectively, the pre-rRNA spacers cleaved at the BHB motifs surrounding pre-16S and pre-23S rRNAs subsequently become ligated. In addition, we present evidence that this is accompanied by circularization of ribosomal pre-16S and pre-23S rRNAs in both species. These data reveal a further link between intron splicing and pre-rRNA processing in Archaea, which might reflect a common evolutionary origin of the two processes. One spliced RNA species designated 16S-D RNA, resulting from religation at the BHB motif of 16S pre-rRNA, is a highly abundant and stable RNA which folds into a three-stem structure interrupted by two single-stranded regions as assessed by chemical probing. It spans a region of the pre-rRNA 5' external transcribed spacer exhibiting a highly conserved folding pattern in Archaea. Surprisingly, 16S-D RNA contains structural motifs found in archaeal C/D box small RNAs and binds to the L7Ae protein, a core component of archaeal C/D box RNPs. This supports the notion that it might have an important but still unknown role in pre-rRNA biogenesis or might even target RNA molecules other than rRNA.