Archaeal Cell Cycle Research Articles

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.

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.

The proteasome controls ESCRT-III-mediated cell division in an archaeon.

Sulfolobus acidocaldarius is the closest experimentally tractable archaeal relative of eukaryotes and, despite lacking obvious cyclin-dependent kinase and cyclin homologs, has an ordered eukaryote-like cell cycle with distinct phases of DNA replication and division. Here, in exploring the mechanism of cell division in S. acidocaldarius, we identify a role for the archaeal proteasome in regulating the transition from the end of one cell cycle to the beginning of the next. Further, we identify the archaeal ESCRT-III homolog, CdvB, as a key target of the proteasome and show that its degradation triggers division by allowing constriction of the CdvB1:CdvB2 ESCRT-III division ring. These findings offer a minimal mechanism for ESCRT-III-mediated membrane remodeling and point to a conserved role for the proteasome in eukaryotic and archaeal cell cycle control.

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.

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.

Comparative and functional analysis of the archaeal cell cycle

The temporal and spatial organization of the chromosome replication, genome segregation and cell division processes is less well understood in species belonging to the Archaea, than in those from the Bacteria and Eukarya domains. Novel insights into the regulation and key components of the Sulfolobus acidocaldarius cell cycle have been obtained through genome-wide analysis of cell-cycle-specific gene expression, followed by cloning and characterization of gene products expressed at different cell cycle stages. Here, the results of the transcript profiling are further explored, and potential key players in archaeal cell cycle progression are highlighted in an evolutionary context, by comparing gene expression patterns and gene conservation between three selected microbial species from different domains of life. We draw attention to novel putative nucleases and helicases implicated in DNA replication, recombination and repair, as well as to potential genome segregation factors. Focus is also placed upon regulatory features, including transcription factors and protein kinases inferred to be involved in the execution of specific cell cycle stages, and regulation through metabolic coupling is discussed.

The cell cycle of Sulfolobus

Much of the current information about the archaeal cell cycle has been generated through studies of the genus Sulfolobus. The overall organization of the cell cycle in these species is well understood, and information about the regulatory principles that govern cell cycle progression is rapidly accumulating. Exciting progress regarding the control and molecular details of the chromosome replication process is evident, and the first insights into the elusive crenarchaeal mitosis and cytokinesis machineries are within reach.

Genetic and Physical Mapping of DNA Replication Origins in Haloferax volcanii

The halophilic archaeon Haloferax volcanii has a multireplicon genome, consisting of a main chromosome, three secondary chromosomes, and a plasmid. Genes for the initiator protein Cdc6/Orc1, which are commonly located adjacent to archaeal origins of DNA replication, are found on all replicons except plasmid pHV2. However, prediction of DNA replication origins in H. volcanii is complicated by the fact that this species has no less than 14 cdc6/orc1 genes. We have used a combination of genetic, biochemical, and bioinformatic approaches to map DNA replication origins in H. volcanii. Five autonomously replicating sequences were found adjacent to cdc6/orc1 genes and replication initiation point mapping was used to confirm that these sequences function as bidirectional DNA replication origins in vivo. Pulsed field gel analyses revealed that cdc6/orc1-associated replication origins are distributed not only on the main chromosome (2.9 Mb) but also on pHV1 (86 kb), pHV3 (442 kb), and pHV4 (690 kb) replicons. Gene inactivation studies indicate that linkage of the initiator gene to the origin is not required for replication initiation, and genetic tests with autonomously replicating plasmids suggest that the origin located on pHV1 and pHV4 may be dominant to the principal chromosomal origin. The replication origins we have identified appear to show a functional hierarchy or differential usage, which might reflect the different replication requirements of their respective chromosomes. We propose that duplication of H. volcanii replication origins was a prerequisite for the multireplicon structure of this genome, and that this might provide a means for chromosome-specific replication control under certain growth conditions. Our observations also suggest that H. volcanii is an ideal organism for studying how replication of four replicons is regulated in the context of the archaeal cell cycle.

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.

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.

Cell cycle arrest in archaea by the hypusination inhibitor N(1)-guanyl-1,7-diaminoheptane.

Hypusination is an essential posttranslational modification unique to archaeal and eukaryotic protein synthesis initiation factor 5A (aIF5A and eIF5A, respectively). We have investigated the effect of the efficient hypusination inhibitor N(1)-guanyl-1,7-diaminoheptane (GC(7)) on four archaeal and one bacterial species. We found that (i) archaea are sensitive to GC(7), whereas the bacterium Escherichia coli is not, (ii) GC(7) causes rapid and reversible arrest of growth of the archaeon Sulfolobus acidocaldarius, and (iii) the growth arrest is accompanied by a specific reversible arrest of the cell cycle prior to cell division. Our findings establish a link between hypusination and sustained growth of archaea and thereby provide the framework to study molecular details of archaeal cell cycle in connection with in vivo functions of hypusine and of aIF5A and eIF5A.