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Nanoarchaeum equitansandIgnicoccus hospitalisrepresent a unique, intimate association of two archaea. Both form a stable coculture which is mandatory forN.equitansbut not for the hostI.hospitalis. Here, we investigated interactions and mutual influence between these microorganisms. Fermentation studies revealed that during exponential growth only about 25% ofI.hospitaliscells are occupied byN.equitanscells (one to three cells). The latter strongly proliferate in the stationary phase ofI.hospitalis, until 80 to 90% of theI.hospitaliscells carry around 10N.equitanscells. Furthermore, the expulsion of H2S, the major metabolic end product ofI.hospitalis, by strong gas stripping yields huge amounts of freeN.equitanscells.N.equitanshad no influence on the doubling times, final cell concentrations, and growth temperature, pH, or salt concentration ranges or optima ofI.hospitalis. However, isolation studies using optical tweezers revealed that infection withN.equitansinhibited the proliferation of individualI.hospitaliscells. This inhibition might be caused by deprivation of the host of cell components like amino acids, as demonstrated by13C-labeling studies. The strong dependence ofN.equitansonI.hospitaliswas affirmed by live-dead staining and electron microscopic analyses, which indicated a tight physiological and structural connection between the two microorganisms. No alternative hosts, including otherIgnicoccusspecies, were accepted byN.equitans. In summary, the data show a highly specialized association ofN.equitansandI.hospitaliswhich so far cannot be assigned to a classical symbiosis, commensalism, or parasitism.

Clustered regularly interspaced short palindromic repeat (CRISPR)-Cas systems are well-known acquired immunity systems that are widespread in archaea and bacteria. The RNA-guided nucleases from CRISPR-Cas systems are currently regarded as the most reliable tools for genome editing and engineering. The first hint of their existence came in 1987, when an unusual repetitive DNA sequence, which subsequently was defined as a CRISPR, was discovered in the Escherichia coli genome during an analysis of genes involved in phosphate metabolism. Similar sequence patterns were then reported in a range of other bacteria as well as in halophilic archaea, suggesting an important role for such evolutionarily conserved clusters of repeated sequences. A critical step toward functional characterization of the CRISPR-Cas systems was the recognition of a link between CRISPRs and the associated Cas proteins, which were initially hypothesized to be involved in DNA repair in hyperthermophilic archaea. Comparative genomics, structural biology, and advanced biochemistry could then work hand in hand, not only culminating in the explosion of genome editing tools based on CRISPR-Cas9 and other class II CRISPR-Cas systems but also providing insights into the origin and evolution of this system from mobile genetic elements denoted casposons. To celebrate the 30th anniversary of the discovery of CRISPR, this minireview briefly discusses the fascinating history of CRISPR-Cas systems, from the original observation of an enigmatic sequence in E. coli to genome editing in humans.

Erratum in : Correction for Adam et al., Evolutionary history of carbon monoxide dehydrogenase/acetyl-CoA synthase, one of the oldest enzymatic complexes. [Proc Natl Acad Sci U S A. 2018]; International audience; Carbon monoxide dehydrogenase/acetyl-CoA synthase (CODH/ ACS) is a five-subunit enzyme complex responsible for the carbonyl branch of the Wood-Ljungdahl (WL) pathway, considered one of the most ancient metabolisms for anaerobic carbon fixation, but its origin and evolutionary history have been unclear. While traditionally associated with methanogens and acetogens, the presence of CODH/ACS homologs has been reported in a large number of un-cultured anaerobic lineages. Here, we have carried out an exhaustive phylogenomic study of CODH/ACS in over 6,400 archaeal and bacterial genomes. The identification of complete and likely functional CODH/ACS complexes in these genomes significantly expands its distribution in microbial lineages. The CODH/ACS complex displays astounding conservation and vertical inheritance over geological times. Rare intradomain and interdomain transfer events might tie into important functional transitions, including the acquisition of CODH/ACS in some archaeal methanogens not known to fix carbon, the tinkering of the complex in a clade of model bacterial acetogens, or emergence of archaeal-bacterial hybrid complexes. Once these transfers were clearly identified, our results allowed us to infer the presence of a CODH/ACS complex with at least four subunits in the last universal common ancestor (LUCA). Different scenarios on the possible role of ancestral CODH/ACS are discussed. Despite common assumptions, all are equally compatible with an autotrophic, mixotrophic, or heterotrophic LUCA. Functional characterization of CODH/ACS from a larger spectrum of bacterial and archaeal lineages and detailed evolutionary analysis of the WL methyl branch will help resolve this issue.