Transcribing Genes the Hard Way: In Vitro Reconstitution of Nanoarchaeal RNA Polymerase Reveals Unusual Active Site Properties

Nanoarchaea represent a highly diverged archaeal phylum that displays many unusual biological features. The Nanoarchaeum equitans genome encodes a complete set of RNA polymerase (RNAP) subunits and basal factors. Several of the standard motifs in the active center contain radical substitutions that are normally expected to render the polymerase catalytically inactive. Here we show that, despite these unusual features, a RNAP reconstituted from recombinant Nanoarchaeum subunits is transcriptionally active. Using a sparse-matrix high-throughput screening method we identified an atypical stringent requirement for fluoride ions to maximize its activity under in vitro transcription conditions.

The hyperthermophilic partners Nanoarchaeum and Ignicoccus stabilize their tRNA T-loops via different but structurally equivalent modifications

The universal L-shaped tertiary structure of tRNAs is maintained with the help of nucleotide modifications within the D- and T-loops, and these modifications are most extensive within hyperthermophilic species. The obligate-commensal Nanoarchaeum equitans and its phylogenetically-distinct host Ignicoccus hospitalis grow physically coupled under identical hyperthermic conditions. We report here two fundamentally different routes by which these archaea modify the key conserved nucleotide U54 within their tRNA T-loops. In N. equitans, this nucleotide is methylated by the S-adenosylmethionine-dependent enzyme NEQ053 to form m5U54, and a recombinant version of this enzyme maintains specificity for U54 in Escherichia coli. In N. equitans, m5U54 is subsequently thiolated to form m5s2U54. In contrast, I. hospitalis isomerizes U54 to pseudouridine prior to methylating its N1-position and thiolating the O4-position of the nucleobase to form the previously uncharacterized nucleotide m1s4Ψ. The methyl and thiol groups in m1s4Ψ and m5s2U are presented within the T-loop in a spatially identical manner that stabilizes the 3′-endo-anti conformation of nucleotide-54, facilitating stacking onto adjacent nucleotides and reverse-Hoogsteen pairing with nucleotide m1A58. Thus, two distinct structurally-equivalent solutions have evolved independently and convergently to maintain the tertiary fold of tRNAs under extreme hyperthermic conditions.

Are Antarctic Nanohaloarchaeota Emerging Viral Lineages?

A recent report in PNAS that Candidatus Nanohaloarchaeum antarcticus requires haloarchaeon Halorubrum lacusprofundi for growth expands the list of known symbiotic or parasitic associations between the members of DPANN archaea, which are relatively small cells with reduced genomes and limited metabolic capacity, and free-living archaea. In line with previous studies addressing the enigmatic mechanism(s) for the transfer of metabolites from Ignicoccus hospitalis to Nanoarchaeum equitans, this new study presents additional evidence supporting a direct cytoplasmic connection facilitated by the fusion of parasite’s membrane with that of its host. Here I show that this novel mechanism for accessing the host resources by a membrane fusing mechanism, which eliminates the need for sophisticated multivalent transport systems, is fundamentally similar to that employed by several viral lineages. These new findings support an evolutionary model on the origin of incipient viral lineages from parasitic cellular lineages that started their parasitic life cycle by fusing with their host cells.

An archaeal symbiont-host association from the deep terrestrial subsurface

DPANN archaea have reduced metabolic capacities and are diverse and abundant in deep aquifer ecosystems, yet little is known about their interactions with other microorganisms that reside there. Here, we provide evidence for an archaeal host-symbiont association from a deep aquifer system at the Colorado Plateau (Utah, USA). The symbiont, Candidatus Huberiarchaeum crystalense, and its host, Ca. Altiarchaeum hamiconexum, show a highly significant co-occurrence pattern over 65 metagenome samples collected over six years. The physical association of the two organisms was confirmed with genome-informed fluorescence in situ hybridization depicting small cocci of Ca. H. crystalense attached to Ca. A. hamiconexum cells. Based on genomic information, Ca. H. crystalense has a similar metabolism as Nanoarchaeum equitans and potentially scavenges vitamins, sugars, nucleotides, and reduced redox-equivalents from its host. These results provide insight into host-symbiont interactions among members of two uncultivated archaeal phyla that thrive in a deep subsurface aquifer.

The origin of life as a probabilistic event in the Universe

By means of a probabilistic mathematical model, we bring into discussion the origin of life as a stochastic process. We consider only the chance of information emergence in the proteome and genome under the ideal thermodynamic and chemical conditions. For a more realistic model, we used, as a parameter, the information amount in Nanoarchaeum equitans genome, the simplest known nowadays, as the equivalent to the first living cell that could have emerged in primitive Earth. We estimated the probability of information emergence by chance as about 10-500’000. Considering the necessary ideal conditions for information emergence, the probability of the origin of life would be even smaller.