Genome-wide identification of genes required for alternative peptidoglycan cross-linking in Escherichia coli revealed unexpected impacts of ß-lactams.

The D,D-transpeptidase activity of penicillin-binding proteins (PBPs) is the well-known primary target of ß-lactam antibiotics that block peptidoglycan polymerization. ß-lactam-induced bacterial killing involves complex downstream responses whose causes and consequences are difficult to resolve. Here, we use the functional replacement of PBPs by a ß-lactam-insensitive L,D-transpeptidase to identify genes essential to mitigate the effects of PBP inactivation by ß-lactams in actively dividing bacteria. The functions of the 179 conditionally essential genes identified by this approach extend far beyond L,D-transpeptidase partners for peptidoglycan polymerization to include proteins involved in stress response and in the assembly of outer membrane polymers. The unsuspected effects of ß-lactams include loss of the lipoprotein-mediated covalent bond that links the outer membrane to the peptidoglycan, destabilization of the cell envelope in spite of effective peptidoglycan cross-linking, and increased permeability of the outer membrane. The latter effect indicates that the mode of action of ß-lactams involves self-promoted penetration through the outer membrane.

Sequence motifs recognized by the casposon integrase of Aciduliprofundum boonei.

Casposons are a group of bacterial and archaeal DNA transposons encoding a specific integrase, termed casposase, which is homologous to the Cas1 enzyme responsible for the integration of new spacers into CRISPR loci. Here, we characterized the sequence motifs recognized by the casposase from a thermophilic archaeon Aciduliprofundum boonei. We identified a stretch of residues, located in the leader region upstream of the actual integration site, whose deletion or mutagenesis impaired the concerted integration reaction. However, deletions of two-thirds of the target site were fully functional. Various single-stranded 6-FAM-labelled oligonucleotides derived from casposon terminal inverted repeats were as efficiently incorporated as duplexes into the target site. This result suggests that, as in the case of spacer insertion by the CRISPR Cas1-Cas2 integrase, casposon integration involves splaying of the casposon termini, with single-stranded ends being the actual substrates. The sequence critical for incorporation was limited to the five terminal residues derived from the 3' end of the casposon. Furthermore, we characterize the casposase from Nitrosopumilus koreensis, a marine member of the phylum Thaumarchaeota, and show that it shares similar properties with the A. boonei enzyme, despite belonging to a different family. These findings further reinforce the mechanistic similarities and evolutionary connection between the casposons and the adaptation module of the CRISPR-Cas systems.

Novel Families of Archaeo-Eukaryotic Primases Associated with Mobile Genetic Elements of Bacteria and Archaea.

Cellular organisms in different domains of life employ structurally unrelated, non-homologous DNA primases for synthesis of a primer for DNA replication. Archaea and eukaryotes encode enzymes of the archaeo-eukaryotic primase (AEP) superfamily, whereas bacteria uniformly use primases of the DnaG family. However, AEP genes are widespread in bacterial genomes raising questions regarding their provenance and function. Here, using an archaeal primase-polymerase PolpTN2 encoded by pTN2 plasmid as a seed for sequence similarity searches, we recovered over 800 AEP homologs from bacteria belonging to 12 highly diverse phyla. These sequences formed a supergroup, PrimPol-PV1, and could be classified into five novel AEP families which are characterized by a conserved motif containing an arginine residue likely to be involved in nucleotide binding. Functional assays confirm the essentiality of this motif for catalytic activity of the PolpTN2 primase-polymerase. Further analyses showed that bacterial AEPs display a range of domain organizations and uncovered several candidates for novel families of helicases. Furthermore, sequence and structure comparisons suggest that PriCT-1 and PriCT-2 domains frequently fused to the AEP domains are related to each other as well as to the non-catalytic, large subunit of archaeal and eukaryotic primases, and to the recently discovered PriX subunit of archaeal primases. Finally, genomic neighborhood analysis indicates that the identified AEPs encoded in bacterial genomes are nearly exclusively associated with highly diverse integrated mobile genetic elements, including integrative conjugative plasmids and prophages.

topIb, a phylogenetic hallmark gene of Thaumarchaeota encodes a functional eukaryote-like topoisomerase IB.

Type IB DNA topoisomerases can eliminate torsional stresses produced during replication and transcription. These enzymes are found in all eukaryotes and a short version is present in some bacteria and viruses. Among prokaryotes, the long eukaryotic version is only observed in archaea of the phylum Thaumarchaeota. However, the activities and the roles of these topoisomerases have remained an open question. Here, we demonstrate that all available thaumarchaeal genomes contain a topoisomerase IB gene that defines a monophyletic group closely related to the eukaryotic enzymes. We show that the topIB gene is expressed in the model thaumarchaeon Nitrososphaera viennensis and we purified the recombinant enzyme from the uncultivated thaumarchaeon Candidatus Caldiarchaeum subterraneum. This enzyme is active in vitro at high temperature, making it the first thermophilic topoisomerase IB characterized so far. We have compared this archaeal type IB enzyme to its human mitochondrial and nuclear counterparts. The archaeal enzyme relaxes both negatively and positively supercoiled DNA like the eukaryotic enzymes. However, its pattern of DNA cleavage specificity is different and it is resistant to camptothecins (CPTs) and non-CPT Top1 inhibitors, LMP744 and lamellarin D. This newly described thermostable topoisomerases IB should be a promising new model for evolutionary, mechanistic and structural studies.

A highly divergent archaeo-eukaryotic primase from the Thermococcus nautilus plasmid, pTN2.

We report the characterization of a DNA primase/polymerase protein (PolpTN2) encoded by the pTN2 plasmid from Thermococcus nautilus. Sequence analysis revealed that this protein corresponds to a fusion between an N-terminal domain homologous to the small catalytic subunit PriS of heterodimeric archaeal and eukaryotic primases (AEP) and a C-terminal domain related to their large regulatory subunit PriL. This unique domain configuration is not found in other virus- and plasmid-encoded primases in which PriS-like domains are typically fused to different types of helicases. PolpTN2 exhibited primase, polymerase and nucleotidyl transferase activities and specifically incorporates dNTPs, to the exclusion of rNTPs. PolpTN2 could efficiently prime DNA synthesis by the T. nautilus PolB DNA polymerase, suggesting that it is used in vivo as a primase for pTN2 plasmid replication. The N-terminal PriS-like domain of PolpTN2 exhibited all activities of the full-length enzyme but was much less efficient in priming cellular DNA polymerases. Surprisingly, the N-terminal domain possesses reverse transcriptase activity. We speculate that this activity could reflect an ancestral function of AEP proteins in the transition from the RNA to the DNA world.

[Enigmatic archaeal viruses]. / Énigmatiques virus d'archées.

Viruses infecting microorganisms of the third domain of life, Archaea, are still poorly characterized: to date, only about fifty of these viruses have been isolated. Their hosts are hyperthermophilic, acidothermophilic, and extreme halophilic or methanogenic archaea. Their morphotypes are highly diverse and their gene content is very specific. Some of these viruses have developed extraordinary mechanisms to open the cell wall thanks to the formation of exceptional pyramidal nanostructures. The still limited knowledge about the biology of archaeoviruses should develop rapidly in the coming years.

Massive activation of archaeal defense genes during viral infection.

Archaeal viruses display unusually high genetic and morphological diversity. Studies of these viruses proved to be instrumental for the expansion of knowledge on viral diversity and evolution. The Sulfolobus islandicus rod-shaped virus 2 (SIRV2) is a model to study virus-host interactions in Archaea. It is a lytic virus that exploits a unique egress mechanism based on the formation of remarkable pyramidal structures on the host cell envelope. Using whole-transcriptome sequencing, we present here a global map defining host and viral gene expression during the infection cycle of SIRV2 in its hyperthermophilic host S. islandicus LAL14/1. This information was used, in combination with a yeast two-hybrid analysis of SIRV2 protein interactions, to advance current understanding of viral gene functions. As a consequence of SIRV2 infection, transcription of more than one-third of S. islandicus genes was differentially regulated. While expression of genes involved in cell division decreased, those genes playing a role in antiviral defense were activated on a large scale. Expression of genes belonging to toxin-antitoxin and clustered regularly interspaced short palindromic repeat (CRISPR)-Cas systems was specifically pronounced. The observed different degree of activation of various CRISPR-Cas systems highlights the specialized functions they perform. The information on individual gene expression and activation of antiviral defense systems is expected to aid future studies aimed at detailed understanding of the functions and interplay of these systems in vivo.

Genomics and genetics of Sulfolobus islandicus LAL14/1, a model hyperthermophilic archaeon.

The 2 465 177 bp genome of Sulfolobus islandicus LAL14/1, host of the model rudivirus SIRV2, was sequenced. Exhaustive comparative genomic analysis of S. islandicus LAL14/1 and the nine other completely sequenced S. islandicus strains isolated from Iceland, Russia and USA revealed a highly syntenic common core genome of approximately 2 Mb and a long hyperplastic region containing most of the strain-specific genes. In LAL14/1, the latter region is enriched in insertion sequences, CRISPR (clustered regularly interspaced short palindromic repeats), glycosyl transferase genes, toxin-antitoxin genes and MITE (miniature inverted-repeat transposable elements). The tRNA genes of LAL14/1 are preferential targets for the integration of mobile elements but clusters of atypical genes (CAG) are also integrated elsewhere in the genome. LAL14/1 carries five CRISPR loci with 10 per cent of spacers matching perfectly or imperfectly the genomes of archaeal viruses and plasmids found in the Icelandic hot springs. Strikingly, the CRISPR_2 region of LAL14/1 carries an unusually long 1.9 kb spacer interspersed between two repeat regions and displays a high similarity to pING1-like conjugative plasmids. Finally, we have developed a genetic system for S. islandicus LAL14/1 and created ΔpyrEF and ΔCRISPR_1 mutants using double cross-over and pop-in/pop-out approaches, respectively. Thus, LAL14/1 is a promising model to study virus-host interactions and the CRISPR/Cas defence mechanism in Archaea.

Cis-regulatory logic in archaeal transcription.

For cellular fitness and survival, gene expression levels need to be regulated in response to a wealth of cellular and environmental signals. TFs (transcription factors) execute a large part of this regulation by interacting with the basal transcription machinery at promoter regions. Archaea are characterized by a simplified eukaryote-like basal transcription machinery and bacteria-type TFs, which convert sequence information into a gene expression output according to cis-regulatory rules. In the present review, we discuss the current state of knowledge about these rules in archaeal systems, ranging from DNA-binding specificities and operator architecture to regulatory mechanisms.

Solution structure of an archaeal DNA binding protein with an eukaryotic zinc finger fold.

While the basal transcription machinery in archaea is eukaryal-like, transcription factors in archaea and their viruses are usually related to bacterial transcription factors. Nevertheless, some of these organisms show predicted classical zinc fingers motifs of the C2H2 type, which are almost exclusively found in proteins of eukaryotes and most often associated with transcription regulators. In this work, we focused on the protein AFV1p06 from the hyperthermophilic archaeal virus AFV1. The sequence of the protein consists of the classical eukaryotic C2H2 motif with the fourth histidine coordinating zinc missing, as well as of N- and C-terminal extensions. We showed that the protein AFV1p06 binds zinc and solved its solution structure by NMR. AFV1p06 displays a zinc finger fold with a novel structure extension and disordered N- and C-termini. Structure calculations show that a glutamic acid residue that coordinates zinc replaces the fourth histidine of the C2H2 motif. Electromobility gel shift assays indicate that the protein binds to DNA with different affinities depending on the DNA sequence. AFV1p06 is the first experimentally characterised archaeal zinc finger protein with a DNA binding activity. The AFV1p06 protein family has homologues in diverse viruses of hyperthermophilic archaea. A phylogenetic analysis points out a common origin of archaeal and eukaryotic C2H2 zinc fingers.

Evidence for a Xer/dif system for chromosome resolution in archaea.

Homologous recombination events between circular chromosomes, occurring during or after replication, can generate dimers that need to be converted to monomers prior to their segregation at cell division. In Escherichia coli, chromosome dimers are converted to monomers by two paralogous site-specific tyrosine recombinases of the Xer family (XerC/D). The Xer recombinases act at a specific dif site located in the replication termination region, assisted by the cell division protein FtsK. This chromosome resolution system has been predicted in most Bacteria and further characterized for some species. Archaea have circular chromosomes and an active homologous recombination system and should therefore resolve chromosome dimers. Most archaea harbour a single homologue of bacterial XerC/D proteins (XerA), but not of FtsK. Therefore, the role of XerA in chromosome resolution was unclear. Here, we have identified dif-like sites in archaeal genomes by using a combination of modeling and comparative genomics approaches. These sites are systematically located in replication termination regions. We validated our in silico prediction by showing that the XerA protein of Pyrococcus abyssi specifically recombines plasmids containing the predicted dif site in vitro. In contrast to the bacterial system, XerA can recombine dif sites in the absence of protein partners. Whereas Archaea and Bacteria use a completely different set of proteins for chromosome replication, our data strongly suggest that XerA is most likely used for chromosome resolution in Archaea.

Two novel families of plasmids from hyperthermophilic archaea encoding new families of replication proteins.

Thermococcales (phylum Euryarchaeota) are model organisms for physiological and molecular studies of hyperthermophiles. Here we describe three new plasmids from Thermococcales that could provide new tools and model systems for genetic and molecular studies in Archaea. The plasmids pTN2 from Thermococcus nautilus sp. 30-1 and pP12-1 from Pyrococcus sp. 12-1 belong to the same family. They have similar size (approximately 12 kb) and share six genes, including homologues of genes encoded by the virus PAV1 from Pyrococcus abyssi. The plasmid pT26-2 from Thermococcus sp. 26-2 (21.5 kb), that corresponds to another plasmid family, encodes many proteins having homologues in virus-like elements integrated in several genomes of Thermococcales and Methanococcales. Our analyses confirm that viruses and plasmids are evolutionary related and co-evolve with their hosts. Whereas all plasmids previously isolated from Thermococcales replicate by the rolling circle mechanism, the three plasmids described here probably replicate by the theta mechanism. The plasmids pTN2 and pP12-1 encode a putative helicase of the SFI superfamily and a new family of DNA polymerase, whose activity was demonstrated in vitro, whereas pT26-2 encodes a putative new type of helicase. This strengthens the idea that plasmids and viruses are a reservoir of novel protein families involved in DNA replication.

Structure, function, and targets of the transcriptional regulator SvtR from the hyperthermophilic archaeal virus SIRV1.

We have characterized the structure and the function of the 6.6-kDa protein SvtR (formerly called gp08) from the rod-shaped virus SIRV1, which infects the hyperthermophilic archaeon Sulfolobus islandicus that thrives at 85 degrees C in hot acidic springs. The protein forms a dimer in solution. The NMR solution structure of the protein consists of a ribbon-helix-helix (RHH) fold between residues 13 and 56 and a disordered N-terminal region (residues 1-12). The structure is very similar to that of bacterial RHH proteins despite the low sequence similarity. We demonstrated that the protein binds DNA and uses its beta-sheet face for the interaction like bacterial RHH proteins. To detect all the binding sites on the 32.3-kb SIRV1 linear genome, we designed and performed a global genome-wide search of targets based on a simplified electrophoretic mobility shift assay. Four targets were recognized by the protein. The strongest binding was observed with the promoter of the gene coding for a virion structural protein. When assayed in a host reconstituted in vitro transcription system, the protein SvtR (Sulfolobus virus transcription regulator) repressed transcription from the latter promoter, as well as from the promoter of its own gene.

Escherichia coli physiology in Luria-Bertani broth.

Luria-Bertani broth supports Escherichia coli growth to an optical density at 600 nm (OD(600)) of 7. Surprisingly, however, steady-state growth ceases at an OD(600) of 0.3, when the growth rate slows down and cell mass decreases. Growth stops for lack of a utilizable carbon source. The carbon sources for E. coli in Luria-Bertani broth are catabolizable amino acids, not sugars.

A novel archaeal regulatory protein, Sta1, activates transcription from viral promoters.

While studying gene expression of the rudivirus SIRV1 in cells of its host, the hyperthermophilic crenarchaeon Sulfolobus, a novel archaeal transcriptional regulator was isolated. The 14 kDa protein, termed Sulfolobus transcription activator 1, Sta1, is encoded on the host chromosome. Its activating effect on transcription initiation from viral promoters was demonstrated in in vitro transcription experiments using a reconstituted host system containing the RNA polymerase, TATA-binding protein (TBP) and transcription factor B (TFB). Most pronounced activation was observed at low concentrations of either of the two transcription factors, TBP or TFB. Sta1 was able to bind viral promoters independently of any component of the host pre-initiation complex. Two binding sites were revealed by footprinting, one located in the core promoter region and the second approximately 30 bp upstream of it. Comparative modeling, NMR and circular dichroism of Sta1 indicated that the protein contained a winged helix-turn-helix motif, most probably involved in DNA binding. This strategy of the archaeal virus to co-opt a host cell regulator to promote transcription of its genes resembles eukaryal virus-host relationships.

Conjugal immunity of Streptomyces strains carrying the integrative element pSAM2 is due to the pif gene (pSAM2 immunity factor).

Mechanisms of conjugal immunity preventing redundant exchange between two cells harbouring the same conjugative element have been reported in diverse bacteria. Such a system does exist for pSAM2, a conjugative and integrative element of Streptomyces. The apparition of the conjugative free form of pSAM2 in the donor strain during mating can be considered as the initial step of transfer. We analysed the genes involved in transfer inhibition by mating donors harbouring pSAM2 with recipient strains containing different regions of pSAM2. The conjugal immunity was previously thought to be mediated by the transcriptional repressor KorSA. Although the transfer efficiency is reduced by its presence in the recipient, the initiation of the transfer process is not affected. In contrast, the presence in the recipient strain of a single pSAM2 gene, pif (pSAM2 immunity factor), was sufficient to abolish both transfer and initiation of transfer. Thus, the clustered genes korSA and pif act complementarily to maintain pSAM2 in a 'prophage' state under non-conjugal conditions. KorSA is involved in intracellular signalling, whereas Pif participates in intercellular signalling. The Pif nudix motif is essential for its activity. This is the first protein of the nudix family shown to be involved in bacterial conjugation.