The emergence of eukaryotes from a merger between an archaeon and a bacterial cell ~two billion years ago involved a profound change in cellular organisation. While the order in which different featur...

The ongoing evolutionary arm race between archaea and their viruses has led to the development of diverse defense systems against viruses. While recent computational approaches have uncovered many bacterial antiviral defense systems, the viral infection strategies and antiviral responses of archaea remain poorly explored. In this study, we identified antiviral defense systems encoded in the genomes of 20 recently sequenced haloarchaeal strains. These systems are representative of the broader repertoire of defense systems present across all 253 complete sequenced Halobacteria (class) genomes in the RefSeq database. Detailed analysis showed that these haloarchaea usually harbor multiple different defense systems, with a particularly high abundance of uncharacterized predicted defense systems against viruses (Phage Defense Candidates (PDCs)). To further explore the impact of anti-viral defense mechanisms on host range, an extensive virus-host pair screening was performed using a panel of known virulent viruses. By correlating the genomic defense profiles with observed viral infectivity and adsorption profiles, a weak correlation between the number of encoded defense systems and viral susceptibility was detected. Specifically, hosts infected by fewer viruses tended to encode a broader repertoire of antiviral defense systems, whereas those with fewer defense systems were frequently infected. It was found that the host range of haloarchaeal viruses is majorly determined by the availability of viral receptors, whereas the presence of anti-viral defense systems plays a smaller but significant role. These findings offer valuable insights into the evolutionary pressure shaping archaeal antiviral strategies and lay the groundwork for future functional studies of archaeal defense systems.

Abstract. Glycans are complex carbohydrates that exhibit extraordinary structural complexity and stereochemical diversity while playing essential roles in

Pan-Mod-seq enables systematic, multi-modification mapping of rRNA across life. Applying it to diverse organisms reveals that hyperthermophiles dynamically install stabilizing modifications to support...

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Ribosome hibernation preserves translation machinery during stress, yet its mechanisms in Archaea remain poorly defined. Here we identify Hib, a previously unrecognized family of archaeal hibernation factors. Genetic, structural and biochemical analyses show that Hib binds ribosomal subunits, blocking the mRNA channel and tRNA sites to inactivate translation. Deletion of hib in Thermococcus barophilus delays recovery from stationary phase and reduces 70S ribosome pools, establishing its role in ribosome preservation. Hib displays a unique modular architecture, combining a bacterial-like HPF module with tandem CBS domains. Cryo-EM structures reveal conformational heterogeneity of Hib:ribosome complexes, consistent with dynamic engagement, and the CBS-containing N-terminal domain binds adenine nucleotides, suggesting a link between hibernation and energy status. These findings define Hib as a key archaeal hibernation factor and provide a framework for understanding ribosome dormancy and adaptation across all domains of life. ### Competing Interest Statement The authors have declared no competing interest. Agence Nationale de la Recherche, https://ror.org/00rbzpz17, ANR-25-CE12-4161

This structure of an archaeal tailed virus (arTV) provides detailed insights into arTV assembly and infection mechanisms.