Cell cycle-dependent expression of an essential SMC-like protein and dynamic chromosome localization in the archaeon Halobacterium salinarum

The Archaeal Cell Cycle

Since first identified as a separate domain of life in the 1970s, it has become clear that archaea differ profoundly from both eukaryotes and bacteria. In this review, we look across the archaeal domain and discuss the diverse mechanisms by which archaea control cell cycle progression, DNA replication, and cell division. While the molecular and cellular processes archaea use to govern these critical cell biological processes often differ markedly from those described in bacteria and eukaryotes, there are also striking similarities that highlight both unique and common principles of cell cycle control across the different domains of life. Since much of the eukaryotic cell cycle machinery has its origins in archaea, exploration of the mechanisms of archaeal cell division also promises to illuminate the evolution of the eukaryotic cell cycle.

Pangenome-level analysis of nucleoid-associated proteins in the Acidithiobacillia class: insights into their functional roles in mobile genetic elements biology

Mobile genetic elements (MGEs) are relevant agents in bacterial adaptation and evolutionary diversification. Stable appropriation of these DNA elements depends on host factors, among which are the nucleoid-associated proteins (NAPs). NAPs are highly abundant proteins that bind and bend DNA, altering its topology and folding, thus affecting all known cellular DNA processes from replication to expression. Even though NAP coding genes are found in most prokaryotic genomes, their functions in host chromosome biology and xenogeneic silencing are only known for a few NAP families. Less is known about the occurrence, abundance, and roles of MGE-encoded NAPs in foreign elements establishment and mobility. In this study, we used a combination of comparative genomics and phylogenetic strategies to gain insights into the diversity, distribution, and functional roles of NAPs within the class Acidithiobacillia with a special focus on their role in MGE biology. Acidithiobacillia class members are aerobic, chemolithoautotrophic, acidophilic sulfur-oxidizers, encompassing substantial genotypic diversity attributable to MGEs. Our search for NAP protein families (PFs) in more than 90 genomes of the different species that conform the class, revealed the presence of 1,197 proteins pertaining to 12 different NAP families, with differential occurrence and conservation across species. Pangenome-level analysis revealed 6 core NAP PFs that were highly conserved across the class, some of which also existed as variant forms of scattered occurrence, in addition to NAPs of taxa-restricted distribution. Core NAPs identified are reckoned as essential based on the conservation of genomic context and phylogenetic signals. In turn, various highly diversified NAPs pertaining to the flexible gene complement of the class, were found to be encoded in known plasmids or, larger integrated MGEs or, present in genomic loci associated with MGE-hallmark genes, pointing to their role in the stabilization/maintenance of these elements in strains and species with larger genomes. Both core and flexible NAPs identified proved valuable as markers, the former accurately recapitulating the phylogeny of the class, and the later, as seed in the bioinformatic identification of novel episomal and integrated mobile elements.

Form and function of archaeal genomes

A key maxim in modernist architecture is that 'form follows function'. While modernist buildings are hopefully the product of intelligent design, the architectures of chromosomes have been sculpted by the forces of evolution over many thousands of generations. In the following, I will describe recent advances in our understanding of chromosome architecture in the archaeal domain of life. Although much remains to be learned about the mechanistic details of archaeal chromosome organization, some general principles have emerged. At the 10-100 kb level, archaeal chromosomes have a conserved local organization reminiscent of bacterial genomes. In contrast, lineage-specific innovations appear to have imposed distinct large-scale architectural features. The ultimate functions of genomes are to store and to express genetic information. Gene expression profiles have been shown to influence chromosome architecture, thus their form follows function. However, local changes to chromosome conformation can also influence gene expression and therefore, in these instances, function follows form.

A novel structural maintenance of chromosomes (SMC)-related protein family specific to Archaea

The ATPases belonging to the structural maintenance of chromosomes (SMC) superfamily are involved in the maintenance of chromosome organization and dynamics, as well as DNA repair. The major proteins in this superfamily recognized to date are either conserved among the three domains of Life (i.e., SMC and Rad50) or specific to Bacteria (i.e., RecF, RecN, and MukB). In Archaea, no protein related to SMC (SMC-related protein) with a broad taxonomic distribution has been reported. Nevertheless, two SMC-related proteins, namely coalescin and Sph, have been identified in crenarchaea Sulfolobus spp. and the euryarchaeon Halobacterium salinarum, respectively, hinting that the diversity of SMC-related proteins has been overlooked in Archaea. In this study, we report a novel SMC-related protein that is distributed among broad archaeal lineages and termed “Archaea-specific SMC-related proteins” or “ASRPs.” We further demonstrate that the ASRP family encloses both coalescin and Sph but the two proteins represent only a tip of the diversity of this family.

The Ribbon-Helix-Helix Domain Protein CdrS Regulates the Tubulin Homolog ftsZ2 To Control Cell Division in Archaea

Precise control of the cell cycle is central to the physiology of all cells. In prior work we demonstrated that archaeal cells maintain a constant size; however, the regulatory mechanisms underlying the cell cycle remain unexplored in this domain of life. Here, we use genetics, functional genomics, and quantitative imaging to identify and characterize the novel CdrSL gene regulatory network in a model species of archaea. We demonstrate the central role of these ribbon-helix-helix family transcription factors in the regulation of cell division through specific transcriptional control of the gene encoding FtsZ2, a putative tubulin homolog. Using time-lapse fluorescence microscopy in live cells cultivated in microfluidics devices, we further demonstrate that FtsZ2 is required for cell division but not elongation. The cdrS-ftsZ2 locus is highly conserved throughout the archaeal domain, and the central function of CdrS in regulating cell division is conserved across hypersaline adapted archaea. We propose that the CdrSL-FtsZ2 transcriptional network coordinates cell division timing with cell growth in archaea.IMPORTANCE Healthy cell growth and division are critical for individual organism survival and species long-term viability. However, it remains unknown how cells of the domain Archaea maintain a healthy cell cycle. Understanding the archaeal cell cycle is of paramount evolutionary importance given that an archaeal cell was the host of the endosymbiotic event that gave rise to eukaryotes. Here, we identify and characterize novel molecular players needed for regulating cell division in archaea. These molecules dictate the timing of cell septation but are dispensable for growth between divisions. Timing is accomplished through transcriptional control of the cell division ring. Our results shed light on mechanisms underlying the archaeal cell cycle, which has thus far remained elusive.

Live Imaging of a Hyperthermophilic Archaeon Reveals Distinct Roles for Two ESCRT-III Homologs in Ensuring a Robust and Symmetric Division

Live-cell imaging has revolutionized our understanding of dynamic cellular processes in bacteria and eukaryotes. Although similar techniques have been applied to the study of halophilic archaea [1-5], our ability to explore the cell biology of thermophilic archaea has been limited by the technical challenges of imaging at high temperatures. Sulfolobus are the most intensively studied members of TACK archaea and have well-established molecular genetics [6-9]. Additionally, studies using Sulfolobus were among the first to reveal striking similarities between the cell biology of eukaryotes and archaea [10-15]. However, to date, it has not been possible to image Sulfolobus cells as they grow and divide. Here, we report the construction of the Sulfoscope, a heated chamber on an inverted fluorescent microscope that enables live-cell imaging of thermophiles. By using thermostable fluorescent probes together with this system, we were able to image Sulfolobus acidocaldarius cells live to reveal tight coupling between changes in DNA condensation, segregation, and cell division. Furthermore, by imaging deletion mutants, we observed functional differences between the two ESCRT-III proteins implicated in cytokinesis, CdvB1 and CdvB2. The deletion of cdvB1 compromised cell division, causing occasional division failures, whereas the ΔcdvB2 exhibited a profound loss of division symmetry, generating daughter cells that vary widely in size and eventually generating ghost cells. These data indicate that DNA separation and cytokinesis are coordinated in Sulfolobus, as is the case in eukaryotes, and that two contractile ESCRT-III polymers perform distinct roles to ensure that Sulfolobus cells undergo a robust and symmetrical division.

Emerging views of genome organization in Archaea

Over the past decade, advances in methodologies for the determination of chromosome conformation have provided remarkable insight into the local and higher-order organization of bacterial and eukaryotic chromosomes. Locally folded domains are found in both bacterial and eukaryotic genomes, although they vary in size. Importantly, genomes of metazoans also possess higher-order organization into A- and B-type compartments, regions of transcriptionally active and inactive chromatin, respectively. Until recently, nothing was known about the organization of genomes of organisms in the third domain of life - the archaea. However, despite archaea possessing simple circular genomes that are morphologically reminiscent of those seen in many bacteria, a recent study of archaea of the genus Sulfolobus has revealed that it organizes its genome into large-scale domains. These domains further interact to form defined A- and B-type compartments. The interplay of transcription and localization of a novel structural maintenance of chromosomes (SMC) superfamily protein, termed coalescin, defines compartment identity. In this Review, we discuss the mechanistic and evolutionary implications of these findings.

Essentiality of the glnA gene in Haloferax mediterranei: gene conversion and transcriptional analysis

Glutamine synthetase is an essential enzyme in ammonium assimilation and glutamine biosynthesis. The Haloferax mediterranei genome has two other glnA-type genes (glnA2 and glnA3) in addition to the glutamine synthetase gene glnA. To determine whether the glnA2 and glnA3 genes can replace glnA in nitrogen metabolism, we generated deletion mutants of glnA. The glnA deletion mutants could not be generated in a medium without glutamine, and thus, glnA is an essential gene in H. mediterranei. The glnA deletion mutant was achieved by adding 40 mM glutamine to the selective medium. This conditional HM26-ΔglnA mutant was characterised with different approaches in the presence of distinct nitrogen sources and nitrogen starvation. Transcriptomic analysis was performed to compare the expression profiles of the strains HM26-ΔglnA and HM26 under different growth conditions. The glnA deletion did not affect the expression of glnA2, glnA3 and nitrogen assimilation genes under nitrogen starvation. Moreover, the results showed that glnA, glnA2 and glnA3 were not expressed under the same conditions. These results indicated that glnA is an essential gene for H. mediterranei and, therefore, glnA2 and glnA3 cannot replace glnA in the conditions analysed.

A Haloarchaeal Small Regulatory RNA (sRNA) Is Essential for Rapid Adaptation to Phosphate Starvation Conditions

The haloarchaeon Haloferax volcanii contains nearly 2800 small non-coding RNAs (sRNAs). One intergenic sRNA, sRNA132, was chosen for a detailed characterization. A deletion mutant had a growth defect and thus underscored the importance of sRNA132. A microarray analysis identified the transcript of an operon for a phosphate-specific ABC transporter as a putative target of sRNA132. Both the sRNA132 and the operon transcript accumulated under low phosphate concentrations, indicating a positive regulatory role of sRNA132. A kinetic analysis revealed that sRNA132 is essential shortly after the onset of phosphate starvation, while other regulatory processes take over after several hours. Comparison of the transcriptomes of wild-type and the sRNA132 gene deletion mutant 30 min after the onset of phosphate starvation revealed that sRNA132 controls a regulon of about 40 genes. Remarkably, the regulon included a second operon for a phosphate-specific ABC transporter, which also depended on sRNA132 for rapid induction in the absence of phosphate. Competitive growth experiments of the wild-type and ABC transporter operon deletion mutants underscored the importance of both transporters for growth at low phosphate concentrations. Northern blot analyses of four additional members of the sRNA132 regulon verified that all four transcripts depended on sRNA132 for rapid regulation after the onset of phosphate starvation. Importantly, this is the first example for the transient importance of a sRNA for any archaeal and bacterial species. In addition, this study unraveled the first sRNA regulon for haloarchaea.

Characterization of the transcriptome of Haloferax volcanii, grown under four different conditions, with mixed RNA-Seq

Haloferax volcanii is a well-established model species for haloarchaea. Small scale RNomics and bioinformatics predictions were used to identify small non-coding RNAs (sRNAs), and deletion mutants revealed that sRNAs have important regulatory functions. A recent dRNA-Seq study was used to characterize the primary transcriptome. Unexpectedly, it was revealed that, under optimal conditions, H. volcanii contains more non-coding sRNAs than protein-encoding mRNAs. However, the dRNA-Seq approach did not contain any length information. Therefore, a mixed RNA-Seq approach was used to determine transcript length and to identify additional transcripts, which are not present under optimal conditions. In total, 50 million paired end reads of 150 nt length were obtained. 1861 protein-coding RNAs (cdRNAs) were detected, which encoded 3092 proteins. This nearly doubled the coverage of cdRNAs, compared to the previous dRNA-Seq study. About 2/3 of the cdRNAs were monocistronic, and 1/3 covered more than one gene. In addition, 1635 non-coding sRNAs were identified. The highest fraction of non-coding RNAs were cis antisense RNAs (asRNAs). Analysis of the length distribution revealed that sRNAs have a median length of about 150 nt. Based on the RNA-Seq and dRNA-Seq results, genes were chosen to exemplify characteristics of the H. volcanii transcriptome by Northern blot analyses, e.g. 1) the transcript patterns of gene clusters can be straightforward, but also very complex, 2) many transcripts differ in expression level under the four analyzed conditions, 3) some genes are transcribed into RNA isoforms of different length, which can be differentially regulated, 4) transcripts with very long 5'-UTRs and with very long 3'-UTRs exist, and 5) about 30% of all cdRNAs have overlapping 3'-ends, which indicates, together with the asRNAs, that H. volcanii makes ample use of sense-antisense interactions. Taken together, this RNA-Seq study, together with a previous dRNA-Seq study, enabled an unprecedented view on the H. volcanii transcriptome.

Efficient non-cytotoxic fluorescent staining of halophiles

Research on halophilic microorganisms is important due to their relation to fundamental questions of survival of living organisms in a hostile environment. Here we introduce a novel method to stain halophiles with MitoTracker fluorescent dyes in their growth medium. The method is based on membrane-potential sensitive dyes, which were originally used to label mitochondria in eukaryotic cells. We demonstrate that these fluorescent dyes provide high staining efficiency and are beneficial for multi-staining purposes due to the spectral range covered (from orange to deep red). In contrast with other fluorescent dyes used so far, MitoTracker does not affect growth rate, and remains in cells after several washing steps and several generations in cell culture. The suggested dyes were tested on three archaeal (Hbt. salinarum, Haloferax sp., Halorubrum sp.) and two bacterial (Salicola sp., Halomonas sp.) strains of halophilic microorganisms. The new staining approach provides new insights into biology of Hbt. salinarum. We demonstrated the interconversion of rod-shaped cells of Hbt. salinarium to spheroplasts and submicron-sized spheres, as well as the cytoplasmic integrity of giant rod Hbt. salinarum species. By expanding the variety of tools available for halophile detection, MitoTracker dyes overcome long-standing limitations in fluorescence microscopy studies of halophiles.

Archaeal cells share common size control with bacteria despite noisier growth and division

In nature, microorganisms exhibit different volumes spanning six orders of magnitude ¹ . Despite their capability to create different sizes, a clonal population in a given environment maintains a uniform size across individual cells. Recent studies in eukaryotic and bacterial organisms showed that this homogeneity in cell size can be accomplished by growing a constant size between two cell cycle events (that is, the adder model ^2-6 ). Demonstration of the adder model led to the hypothesis that this phenomenon is a consequence of convergent evolution. Given that archaeal cells share characteristics with both bacteria and eukaryotes, we investigated whether and how archaeal cells exhibit control over cell size. To this end, we developed a soft-lithography method of growing the archaeal cells to enable quantitative time-lapse imaging and single-cell analysis, which would be useful for other microorganisms. Using this method, we demonstrated that Halobacterium salinarum, a hypersaline-adapted archaeal organism, grows exponentially at the single-cell level and maintains a narrow-size distribution by adding a constant length between cell division events. Interestingly, the archaeal cells exhibited greater variability in cell division placement and exponential growth rate across individual cells in a population relative to those observed in Escherichia coli ^6-9 . Here, we present a theoretical framework that explains how these larger fluctuations in archaeal cell cycle events contribute to cell size variability and control.

Coupling transcriptional activation of CRISPR-Cas system and DNA repair genes by Csa3a in Sulfolobus islandicus

CRISPR-Cas system provides the adaptive immunity against invading genetic elements in prokaryotes. Recently, we demonstrated that Csa3a regulator mediates spacer acquisition in Sulfolobus islandicus by activating the expression of Type I-A adaptation cas genes. However, links between the activation of spacer adaptation and CRISPR transcription/processing, and the requirement for DNA repair genes during spacer acquisition remained poorly understood. Here, we demonstrated that de novo spacer acquisition required Csa1, Cas1, Cas2 and Cas4 proteins of the Sulfolobus Type I-A system. Disruption of genes implicated in crRNA maturation or DNA interference led to a significant accumulation of acquired spacers, mainly derived from host genomic DNA. Transcriptome and proteome analyses showed that Csa3a activated expression of adaptation cas genes, CRISPR RNAs, and DNA repair genes, including herA helicase, nurA nuclease and DNA polymerase II genes. Importantly, Csa3a specifically bound the promoters of the above DNA repair genes, suggesting that they were directly activated by Csa3a for adaptation. The Csa3a regulator also specifically bound to the leader sequence to activate CRISPR transcription in vivo. Our data indicated that the Csa3a regulator couples transcriptional activation of the CRISPR-Cas system and DNA repair genes for spacer adaptation and efficient interference of invading genetic elements.

Driving Apart and Segregating Genomes in Archaea

Genome segregation is a fundamental biological process in organisms from all domains of life. How this stage of the cell cycle unfolds in Eukarya has been clearly defined and considerable progress has been made to unravel chromosome partition in Bacteria. The picture is still elusive in Archaea. The lineages of this domain exhibit different cell-cycle lifestyles and wide-ranging chromosome copy numbers, fluctuating from 1 up to 55. This plurality of patterns suggests that a variety of mechanisms might underpin disentangling and delivery of DNA molecules to daughter cells. Here I describe recent developments in archaeal genome maintenance, including investigations of novel genome segregation machines that point to unforeseen bacterial and eukaryotic connections.

Global role of the membrane protease LonB in Archaea: Potential protease targets revealed by quantitative proteome analysis of a lonB mutant in Haloferax volcanii

The membrane-associated LonB protease is essential for viability in Haloferax volcanii, however, the cellular processes affected by this protease in archaea are unknown. In this study, the impact of a lon conditional mutation (down-regulation) on H. volcanii physiology was examined by comparing proteomes of parental and mutant cells using shotgun proteomics. A total of 1778 proteins were identified (44% of H. volcanii predicted proteome) and 142 changed significantly in amount (≥2 fold). Of these, 66 were augmented in response to Lon deficiency suggesting they could be Lon substrates. The "Lon subproteome" included soluble and predicted membrane proteins expected to participate in diverse cellular processes. The dramatic stabilization of phytoene synthase (57 fold) in concert with overpigmentation of lon mutant cells suggests that Lon controls carotenogenesis in H. volcanii. Several hypothetical proteins, which may reveal novel functions and/or be involved in adaptation to extreme environments, were notably increased (300 fold). This study, which represents the first proteome examination of a Lon deficient archaeal cell, shows that Lon has a strong impact on H. volcanii physiology evidencing the cellular processes controlled by this protease in Archaea. Additionally, this work provides a platform for the discovery of novel targets of Lon proteases.

BIOLOGICAL SIGNIFICANCE

The proteome of a Lon-deficient archaeal cell was examined for the first time showing that Lon has a strong impact on H. volcanii physiology and evidencing the proteins and cellular processes controlled by this protease in Archaea. This work will facilitate future investigations aiming to address Lon function in archaea and provides a platform for the discovery of endogenous targets of the archaeal-type Lon as well as novel targets/processes regulated by Lon proteases. This knowledge will advance the understanding on archaeal physiology and the biological function of membrane proteases in microorganisms.

Polyploidy in haloarchaea: advantages for growth and survival

The investigated haloarchaeal species, Halobacterium salinarum, Haloferax mediterranei, and H. volcanii, have all been shown to be polyploid. They contain several replicons that have independent copy number regulation, and most have a higher copy number during exponential growth phase than in stationary phase. The possible evolutionary advantages of polyploidy for haloarchaea, most of which have experimental support for at least one species, are discussed. These advantages include a low mutation rate and high resistance toward X-ray irradiation and desiccation, which depend on homologous recombination. For H. volcanii, it has been shown that gene conversion operates in the absence of selection, which leads to the equalization of genome copies. On the other hand, selective forces might lead to heterozygous cells, which have been verified in the laboratory. Additional advantages of polyploidy are survival over geological times in halite deposits as well as at extreme conditions on earth and at simulated Mars conditions. Recently, it was found that H. volcanii uses genomic DNA as genetic material and as a storage polymer for phosphate. In the absence of phosphate, H. volcanii dramatically decreases its genome copy number, thereby enabling cell multiplication, but diminishing the genetic advantages of polyploidy. Stable storage of phosphate is proposed as an alternative driving force for the emergence of DNA in early evolution. Several additional potential advantages of polyploidy are discussed that have not been addressed experimentally for haloarchaea. An outlook summarizes selected current trends and possible future developments.

DNA as a phosphate storage polymer and the alternative advantages of polyploidy for growth or survival

Haloferax volcanii uses extracellular DNA as a source for carbon, nitrogen, and phosphorous. However, it can also grow to a limited extend in the absence of added phosphorous, indicating that it contains an intracellular phosphate storage molecule. As Hfx. volcanii is polyploid, it was investigated whether DNA might be used as storage polymer, in addition to its role as genetic material. It could be verified that during phosphate starvation cells multiply by distributing as well as by degrading their chromosomes. In contrast, the number of ribosomes stayed constant, revealing that ribosomes are distributed to descendant cells, but not degraded. These results suggest that the phosphate of phosphate-containing biomolecules (other than DNA and RNA) originates from that stored in DNA, not in rRNA. Adding phosphate to chromosome depleted cells rapidly restores polyploidy. Quantification of desiccation survival of cells with different ploidy levels showed that under phosphate starvation Hfx. volcanii diminishes genetic advantages of polyploidy in favor of cell multiplication. The consequences of the usage of genomic DNA as phosphate storage polymer are discussed as well as the hypothesis that DNA might have initially evolved in evolution as a storage polymer, and the various genetic benefits evolved later.

The cell cycle of archaea

Growth and proliferation of all cell types require intricate regulation and coordination of chromosome replication, genome segregation, cell division and the systems that determine cell shape. Recent findings have provided insight into the cell cycle of archaea, including the multiple-origin mode of DNA replication, the initial characterization of a genome segregation machinery and the discovery of a novel cell division system. The first archaeal cytoskeletal protein, crenactin, was also recently described and shown to function in cell shape determination. Here, we outline the current understanding of the archaeal cell cycle and cytoskeleton, with an emphasis on species in the genus Sulfolobus, and consider the major outstanding questions in the field.

The archaeal Lsm protein binds to small RNAs

Proteins of the Lsm family, including eukaryotic Sm proteins and bacterial Hfq, are key players in RNA metabolism. Little is known about the archaeal homologues of these proteins. Therefore, we characterized the Lsm protein from the haloarchaeon Haloferax volcanii using in vitro and in vivo approaches. H. volcanii encodes a single Lsm protein, which belongs to the Lsm1 subfamily. The lsm gene is co-transcribed and overlaps with the gene for the ribosomal protein L37e. Northern blot analysis shows that the lsm gene is differentially transcribed. The Lsm protein forms homoheptameric complexes and has a copy number of 4000 molecules/cell. In vitro analyses using electrophoretic mobility shift assays and ultrasoft mass spectrometry (laser-induced liquid bead ion desorption) showed a complex formation of the recombinant Lsm protein with oligo(U)-RNA, tRNAs, and an small RNA. Co-immunoprecipitation with a FLAG-tagged Lsm protein produced in vivo confirmed that the protein binds to small RNAs. Furthermore, the co-immunoprecipitation revealed several protein interaction partners, suggesting its involvement in different cellular pathways. The deletion of the lsm gene is viable, resulting in a pleiotropic phenotype, indicating that the haloarchaeal Lsm is involved in many cellular processes, which is in congruence with the number of protein interaction partners.

Bacterial nucleoid-associated proteins, nucleoid structure and gene expression

Emerging models of the bacterial nucleoid show that nucleoid-associated proteins (NAPs) and transcription contribute in combination to the dynamic nature of nucleoid structure. NAPs and other DNA-binding proteins that display gene-silencing and anti-silencing activities are emerging as key antagonistic regulators of nucleoid structure. Furthermore, it is becoming clear that the boundary between NAPs and conventional transcriptional regulators is quite blurred and that NAPs facilitate the evolution of novel gene regulatory circuits. Here, NAP biology is considered from the standpoints of both gene regulation and nucleoid structure.

A 2-oxoacid dehydrogenase complex of Haloferax volcanii is essential for growth on isoleucine but not on other branched-chain amino acids

The halophilic archaeon Haloferax volcanii contains three operons encoding 2-oxoacid dehydrogenase complexes (OADHCs) OADHC1-OADHC3. However, the biological role of these OADHCs is not known as previous studies have demonstrated that they cannot use any of the known OADHC substrates. Even the construction of single mutants in all three oadhc operons, reported recently, could not identify a substrate. Therefore, all three possible double mutants and a triple mutant were generated, and single, double and triple mutants were compared to the wild-type. The four mutants devoid of a functional OADHC1 had a reduced growth yield during nitrate-respirative growth on tryptone. A metabolome analysis of the medium after growth of the triple mutant in comparison to the wild-type revealed that the mutant was unable to degrade isoleucine and leucine, in contrast to the wild-type. It was shown that oadhc1 mutants were unable to grow in synthetic medium on isoleucine, in contrast to the other mutants and the isogenic parent strain. However, all strains grew indistinguishably on valine and leucine. The transcript of the oadhc1 operon was highly induced during growth on isoleucine. However, attempts to detect enzymic activity were unsuccessful, while the branched-chain OADHC (BCDHC) of Pseudomonas putida could be measured easily. Therefore, the growth capability of the triple mutant and the wild-type on the two first degradation intermediates of isoleucine was tested and provided further evidence that OADHC is involved in isoleucine degradation. Taken together, the results indicate that OADHC1 is a specialized BCDHC that uses only one (or maximally two) of the three branched-chain 2-oxoacids, in contrast to BCDHCs from other species.

Dynamics of the bacterial SMC complex and SMC-like proteins involved in DNA repair

Bacteria and archaea possess several different SMC-like proteins, which perform essential functions in a variety of chromosome dynamics, such as chromosome compaction, segregation, and DNA repair. SMC-like proteins localize to distinct sites within the cells at different time points in the cell cycle, or are recruited to sites of DNA breaks and damage. The bacterial SMC (MukB) complex appears to perform a condensin-like function, while SbcC and RecN act early during DNA repair, but apparently at different sites within the cells. Thus, bacterial SMC-like proteins have dynamic functions in chromosome segregation and maintenance of genetic stability.

Characterization of a Haloferax volcanii member of the enolase superfamily: deletion mutant construction, expression analysis, and transcriptome comparison

The enolase superfamily (COG4948) contains proteins with very different biological functions including regulators like the Escherichia coli RspA and metabolic enzymes like enolase. To unravel the biological function of an archaeal family member, an in frame deletion mutant of a gene encoding a COG4948 protein of Haloferax volcanii was generated. The mutant had a lag phase of 3 days after transition from a richer to a poorer medium, in contrast to the wild-type, and the gene was therefore named "important for transition" (iftA). After inoculation of fresh casamino acids or complex medium with stationary phase wild-type cells, the transcript level of iftA was transiently induced at the onset of growth. In contrast, in minimal (or "poor") glucose medium, both transcript and protein were present throughout growth, even in late stationary phase. A comparison of the transcriptomes of deletion mutant and wild-type revealed that transcript levels of a very restricted set of genes were differentially regulated, including genes encoding proteins involved in phosphate metabolism, regulators and stress response proteins. Taken together, the results indicate that IftA might have a dual function, i.e., transiently after transition to fresh medium and permanently during growth in glucose medium.

Genomics and functional genomics with haloarchaea

The first haloarchaeal genome was published in 2000 and today five genome sequences are available. Transcriptome and proteome analyses have been established for two and three haloarchaeal species, respectively, and more than 20 studies using these functional genomic approaches have been published in the last two years. These studies gave global overviews of metabolic regulation (aerobic and anaerobic respiration, phototrophy, carbon source usage), stress response (UV, X-rays, transition metals, osmotic and temperature stress), cell cycle-dependent transcript level regulation, and transcript half-lives. The only translatome analysis available for any prokaryotic species revealed that 10 and 20% of all transcripts are translationally regulated in Haloferax volcanii and Halobacterium salinarum, respectively. Very effective methods for the construction of in frame deletion mutants have been established recently for haloarchaea and are intensively used to unravel the biological roles of genes in this group. Bioinformatic analyses include both cross-genome comparisons as well as integration of genomic data with experimental results. The first systems biology approaches have been performed that used experimental data to construct predictive models of gene expression and metabolism, respectively. In this contribution the current status of genomics, functional genomics, and molecular genetics of haloarchaea is summarized and selected examples are discussed.

Synchronisation of Giardia lamblia: identification of cell cycle stage-specific genes and a differentiation restriction point

The intestinal parasite Giardia lamblia undergoes cell differentiations that entail entry into and departure from the replicative cell cycle. The pathophysiology of giardiasis depends directly upon the ability of the trophozoite form to replicate in the host upper small intestine. Thus, cell proliferation is tightly linked to disease. However, studies of cell cycle regulation in Giardia have been hampered by the inability to synchronise cultures. Here we report that Giardia isolates of the major human genotypes A and B can be synchronised using aphidicolin, a mycotoxin that reversibly inhibits replicative DNA polymerases in eukaryotic cells. Aphidicolin arrests Giardia trophozoites in the early DNA synthesis (S) phase of the cell cycle. We identified a set of cell cycle orthologues in the Giardia genome using bioinformatic analyses and showed that synchronised parasites express these genes in a cell cycle stage-specific manner. The synchronisation method also showed that during encystation, exit from the ordinary cell cycle occurs preferentially in G(2) and defines a restriction point for differentiation. Synchronisation opens up possibilities for further molecular and cell biological studies of chromosome replication, mitosis and segregation of the complex cytoskeleton in Giardia.

Three 2-oxoacid dehydrogenase operons in Haloferax volcanii: expression, deletion mutants and evolution

Two unrelated protein families catalyse the oxidative decarboxylation of 2-oxoacids, i.e. the 2-oxoacid dehydrogenase complexes (OADHCs) and the 2-oxoacid ferredoxin oxidoreductases (OAFORs). In halophilic archaea, OAFORs were found to be responsible for decarboxylation of pyruvate and 2-oxoglutarate. Nevertheless, two gene clusters encoding OADHCs were found previously in Haloferax volcanii, but their biological function remained obscure. Here a third oadhc gene cluster of H. volcanii is presented. To characterize the function, the genes encoding the E1 subunit were inactivated in all three gene clusters by in-frame deletions. Under aerobic conditions none of the three mutants showed any phenotypic difference from the wild-type in various media. However, growth yields of two mutants were considerably lower than that of wild-type under nitrate-respirative conditions in complex medium. Northern blot analyses revealed (1) that polycistronic transcripts are formed and all three gene clusters are bona fide operons and (2) that transcription of all three operons is induced under anaerobic conditions compared to aerobic conditions. Taken together, the three H. volcanii enzymes do not fulfil one of the 'usual' aerobic functions of typical OADHCs, but decarboxylate an as-yet-unidentified novel substrate under anaerobic conditions. A survey of all 28 fully sequenced archaeal genomes revealed that nearly all archaea contain several OAFORs (three to four on average), suggesting that this protein family was already present in their last common ancestor. In contrast, only nine archaea encode one or two OADHCs, indicating that this protein family entered archaea by lateral transfer of the cognate genes from bacteria. This view is underscored by a phylogenetic tree of 33 archaeal and bacterial OADHCs.

Genome-wide analysis of growth phase-dependent translational and transcriptional regulation in halophilic archaea

BACKGROUND

Differential expression of genes can be regulated on many different levels. Most global studies of gene regulation concentrate on transcript level regulation, and very few global analyses of differential translational efficiencies exist. The studies have revealed that in Saccharomyces cerevisiae, Arabidopsis thaliana, and human cell lines translational regulation plays a significant role. Additional species have not been investigated yet. Particularly, until now no global study of translational control with any prokaryotic species was available.

RESULTS

A global analysis of translational control was performed with two haloarchaeal model species, Halobacterium salinarum and Haloferax volcanii. To identify differentially regulated genes, exponentially growing and stationary phase cells were compared. More than 20% of H. salinarum transcripts are translated with non-average efficiencies. By far the largest group is comprised of genes that are translated with above-average efficiency specifically in exponential phase, including genes for many ribosomal proteins, RNA polymerase subunits, enzymes, and chemotaxis proteins. Translation of 1% of all genes is specifically repressed in either of the two growth phases. For comparison, DNA microarrays were also used to identify differential transcriptional regulation in H. salinarum, and 17% of all genes were found to have non-average transcript levels in exponential versus stationary phase. In H. volcanii, 12% of all genes are translated with non-average efficiencies. The overlap with H. salinarum is negligible. In contrast to H. salinarum, 4.6% of genes have non-average translational efficiency in both growth phases, and thus they might be regulated by other stimuli than growth phase.

CONCLUSION

For the first time in any prokaryotic species it was shown that a significant fraction of genes is under differential translational control. Groups of genes with different regulatory patterns were discovered. However, neither the fractions nor the identity of regulated genes are conserved between H. salinarum and H. volcanii, indicating that prokaryotes as well as eukaryotes use differential translational control for the regulation of gene expression, but that the identity of regulated genes is not conserved. For 70 H. salinarum genes potentiation of regulation was observed, but for the majority of regulated genes either transcriptional or translational regulation is employed.

Transcriptome changes and cAMP oscillations in an archaeal cell cycle

BACKGROUND

The cell cycle of all organisms includes mass increase by a factor of two, replication of the genetic material, segregation of the genome to different parts of the cell, and cell division into two daughter cells. It is tightly regulated and typically includes cell cycle-specific oscillations of the levels of transcripts, proteins, protein modifications, and signaling molecules. Until now cell cycle-specific transcriptome changes have been described for four eukaryotic species ranging from yeast to human, but only for two prokaryotic species. Similarly, oscillations of small signaling molecules have been identified in very few eukaryotic species, but not in any prokaryote.

RESULTS

A synchronization procedure for the archaeon Halobacterium salinarum was optimized, so that nearly 100% of all cells divide in a time interval that is 1/4th of the generation time of exponentially growing cells. The method was used to characterize cell cycle-dependent transcriptome changes using a genome-wide DNA microarray. The transcript levels of 87 genes were found to be cell cycle-regulated, corresponding to 3% of all genes. They could be clustered into seven groups with different transcript level profiles. Cluster-specific sequence motifs were detected around the start of the genes that are predicted to be involved in cell cycle-specific transcriptional regulation. Notably, many cell cycle genes that have oscillating transcript levels in eukaryotes are not regulated on the transcriptional level in H. salinarum. Synchronized cultures were also used to identify putative small signaling molecules. H. salinarum was found to contain a basal cAMP concentration of 200 microM, considerably higher than that of yeast. The cAMP concentration is shortly induced directly prior to and after cell division, and thus cAMP probably is an important signal for cell cycle progression.

CONCLUSION

The analysis of cell cycle-specific transcriptome changes of H. salinarum allowed to identify a strategy of transcript level regulation that is different from all previously characterized species. The transcript levels of only 3% of all genes are regulated, a fraction that is considerably lower than has been reported for four eukaryotic species (6%-28%) and for the bacterium C. crescentus (19%). It was shown that cAMP is present in significant concentrations in an archaeon, and the phylogenetic profile of the adenylate cyclase indicates that this signaling molecule is widely distributed in archaea. The occurrence of cell cycle-dependent oscillations of the cAMP concentration in an archaeon and in several eukaryotic species indicates that cAMP level changes might be a phylogenetically old signal for cell cycle progression.

Genome-wide transcription map of an archaeal cell cycle

Relative RNA abundance was measured at different cell-cycle stages in synchronized cultures of the hyperthermophilic archaeon Sulfolobus acidocaldarius. Cyclic induction was observed for >160 genes, demonstrating central roles for transcriptional regulation and cell-cycle-specific gene expression in archaeal cell-cycle progression. Many replication genes were induced in a cell-cycle-specific manner, and novel replisome components are likely to be among the genes of unknown function with similar induction patterns. Candidate genes for the unknown genome segregation and cell division machineries were also identified, as well as seven transcription factors likely to be involved in cell-cycle control. Two serine-threonine protein kinases showed distinct cell-cycle-specific induction, suggesting regulation of the archaeal cell cycle also through protein modification. Two candidate recognition elements, CCR boxes, for transcription factors in control of cell-cycle regulons were identified among gene sets with similar induction kinetics. The results allow detailed characterization of the genome segregation, division, and replication processes and may, because of the extensive homologies between the archaeal and eukaryotic information machineries, also be applicable to core features of the eukaryotic cell cycle.

Regulated polyploidy in halophilic archaea

Polyploidy is common in higher eukaryotes, especially in plants, but it is generally assumed that most prokaryotes contain a single copy of a circular chromosome and are therefore monoploid. We have used two independent methods to determine the genome copy number in halophilic archaea, 1) cell lysis in agarose blocks and Southern blot analysis, and 2) Real-Time quantitative PCR. Fast growing H. salinarum cells contain on average about 25 copies of the chromosome in exponential phase, and their ploidy is downregulated to 15 copies in early stationary phase. The chromosome copy number is identical in cultures with a twofold lower growth rate, in contrast to the results reported for several other prokaryotic species. Of three additional replicons of H. salinarum, two have a low copy number that is not growth-phase regulated, while one replicon even shows a higher degree of growth phase-dependent regulation than the main replicon. The genome copy number of H. volcanii is similarly high during exponential phase (on average 18 copies/cell), and it is also downregulated (to 10 copies) as the cells enter stationary phase. The variation of genome copy numbers in the population was addressed by fluorescence microscopy and by FACS analysis. These methods allowed us to verify the growth phase-dependent regulation of ploidy in H. salinarum, and they revealed that there is a wide variation in genome copy numbers in individual cells that is much larger in exponential than in stationary phase. Our results indicate that polyploidy might be more widespread in archaea (or even prokaryotes in general) than previously assumed. Moreover, the presence of so many genome copies in a prokaryote raises questions about the evolutionary significance of this strategy.

From replication to cultivation: hot news from Haloarchaea

Haloarchaea have developed into model organisms that are utilized to study many biological processes. Examples are the mechanisms of chromosome maintenance, gene expression and its regulation, protein export and degradation, and motility and sensing. In addition to the analysis of model species like Halobacterium salinarum and Haloferax volcanii, natural communities have been characterized. Halophilic Archaea were found in low-salt environments and are thus more widespread than previously thought.

Anucleate and titan cell phenotypes caused by insertional inactivation of the structural maintenance of chromosomes (smc) gene in the archaeon Methanococcus voltae

SMC (structural maintenance of chromosomes) proteins are highly conserved and present in eukaryotes, bacteria and archaea. They function in chromosome condensation and segregation and in DNA repair. Using an insertion vector containing the pac gene for resistance to puromycin, we have created an insertion in the smc gene of Methanococcus voltae. We used epifluorescence microscopy to examine the cell and nucleoid morphology, DNA content and metabolic activity. This insertion causes gross defects in chromosome segregation and cell morphology. Approximately 20% of mutant cells contain little or no DNA, and a subset of cells ( approximately 2%) IS abnormally large (three to four times their normal diameter) titan cells. We believe that these titan cells indicate cell division arrest at a cell cycle checkpoint. The results confirm that SMC in archaea is an important player in chromosome dynamics (as it is in bacteria and eukaryotes).

Construction and usage of a onefold-coverage shotgun DNA microarray to characterize the metabolism of the archaeon Haloferax volcanii

Haloferax volcanii is a moderately halophilic archaeon that can grow aerobically and anaerobically with a variety of substrates. We undertook a novel approach for the characterization of metabolic adaptations, i.e. transcriptome analysis with a onefold-coverage shotgun DNA microarray. A genomic library was constructed and converted into a polymerase chain reaction (PCR) product library, which was used to print two DNA microarrays, a 960-spot test array used for optimization of microarray analysis and a 2880-spot onefold-coverage array. H. volcanii cultures were shifted from casamino acid-based metabolism to glucose-based metabolism, and the transcriptome changes were analysed with the onefold-coverage array at five time points covering the transition phase and the onset of exponential growth with the new carbon source. About 10% of all genes were found to be more than 2.5-fold regulated at at least one time point. The genes fall into five clusters of kinetically co-regulated genes. For members of all five clusters, the results were verified by Northern blot analyses. The identity of the regulated genes was determined by sequencing. Many co-regulated genes encode proteins of common functions. Expected as well as a variety of unexpected findings allowed predictions about the central metabolism, the transport capacity and the cellular composition of H. volcanii growing on casamino acids and on glucose. The microarray analyses are in accordance with the growth rates and ribosome contents of H. volcanii growing on the two carbon sources. Analysis of the results revealed that onefold-coverage shotgun DNA microarrays are well suited to characterize the regulation of metabolic pathways as well as protein complexes in response to changes in environmental conditions.

The archaeal cell cycle: current issues

The recently discovered structural similarities between the archaeal Orc1/Cdc6 and bacterial DnaA initiator proteins for chromosome replication have exciting implications for cell cycle regulation. Together with current attempts to identify archaeal chromosome replication origins, the information is likely to yield fundamental insights into replication control in both archaea and eukaryotes within the near future. Several proteins that affect, or are likely to affect, chromatin structure and genome segregation in archaea have been described recently, including Sph1 and 2, ScpA and B, Sir2, Alba and Rio1p. Important insights into the properties of the MinD and FtsZ cell division proteins, and of putative cytoskeletal elements, have recently been gained in bacteria. As these proteins also are present among archaea, it is likely that the new information will also be essential for understanding archaeal genome segregation and cell division. A series of interesting cell cycle issues has been brought to light through the discovery of the novel Nanoarchaeota phylum, and these are outlined briefly. Exciting areas for extended cell cycle investigations of archaea are identified, including termination of chromosome replication, application of in situ cytological techniques for localization of cell cycle proteins and the regulatory roles of GTP-binding proteins and small RNAs.