Casposons: a new superfamily of self-synthesizing DNA transposons at the origin of prokaryotic CRISPR-Cas immunity

Overview of CRISPR-Cas9 Biology

Prokaryotes use diverse strategies to improve fitness in the face of different environmental threats and stresses, including those posed by mobile genetic elements (e.g., bacteriophages and plasmids). To defend against these elements, many bacteria and archaea use elegant, RNA-directed, nucleic acid-targeting adaptive restriction machineries called CRISPR -: Cas (CRISPR-associated) systems. While providing an effective defense against foreign genetic elements, these systems have also been observed to play critical roles in regulating bacterial physiology during environmental stress. Increasingly, CRISPR-Cas systems, in particular the Type II systems containing the Cas9 endonuclease, have been exploited for their ability to bind desired nucleic acid sequences, as well as direct sequence-specific cleavage of their targets. Cas9-mediated genome engineering is transcending biological research as a versatile and portable platform for manipulating genetic content in myriad systems. Here, we present a systematic overview of CRISPR-Cas history and biology, highlighting the revolutionary tools derived from these systems, which greatly expand the molecular biologists' toolkit.

Bipartite Network Analysis of the Archaeal Virosphere: Evolutionary Connections between Viruses and Capsidless Mobile Elements

Archaea and particularly hyperthermophilic crenarchaea are hosts to many unusual viruses with diverse virion shapes and distinct gene compositions. As is typical of viruses in general, there are no universal genes in the archaeal virosphere. Therefore, to obtain a comprehensive picture of the evolutionary relationships between viruses, network analysis methods are more productive than traditional phylogenetic approaches. Here we present a comprehensive comparative analysis of genomes and proteomes from all currently known taxonomically classified and unclassified, cultivated and uncultivated archaeal viruses. We constructed a bipartite network of archaeal viruses that includes two classes of nodes, the genomes and gene families that connect them. Dissection of this network using formal community detection methods reveals strong modularity, with 10 distinct modules and 3 putative supermodules. However, compared to similar previously analyzed networks of eukaryotic and bacterial viruses, the archaeal virus network is sparsely connected. With the exception of the tailed viruses related to bacteriophages of the order Caudovirales and the families Turriviridae and Sphaerolipoviridae that are linked to a distinct supermodule of eukaryotic and bacterial viruses, there are few connector genes shared by different archaeal virus modules. In contrast, most of these modules include, in addition to viruses, capsidless mobile elements, emphasizing tight evolutionary connections between the two types of entities in archaea. The relative contributions of distinct evolutionary origins, in particular from nonviral elements, and insufficient sampling to the sparsity of the archaeal virus network remain to be determined by further exploration of the archaeal virosphere.

IMPORTANCE

Viruses infecting archaea are among the most mysterious denizens of the virosphere. Many of these viruses display no genetic or even morphological relationship to viruses of bacteria and eukaryotes, raising questions regarding their origins and position in the global virosphere. Analysis of 5,740 protein sequences from 116 genomes allowed dissection of the archaeal virus network and showed that most groups of archaeal viruses are evolutionarily connected to capsidless mobile genetic elements, including various plasmids and transposons. This finding could reflect actual independent origins of the distinct groups of archaeal viruses from different nonviral elements, providing important insights into the emergence and evolution of the archaeal virome.

The Evolution of Antiviral Defense Systems

Self-replicating genetic material presumably provided the architecture necessary for generating the last universal ancestor of all nucleic-acid-based life. As biological complexity increased in the billions of years that followed, the same genetic material also morphed into a wide spectrum of viruses and other parasitic genetic elements. The resulting struggle for existence drove the evolution of host defenses, giving rise to a perpetual arms race. This Perspective summarizes the antiviral mechanisms evident across the tree of life, discussing each in their evolutionary context to postulate how the coevolution of host and pathogen shaped the cellular antiviral defenses we know today.

DNA Transposition at Work

DNA transposons are defined segments of DNA that are able to move from one genomic location to another. Movement is facilitated by one or more proteins, called the transposase, typically encoded by the mobile element itself. Here, we first provide an overview of the classification of such mobile elements in a variety of organisms. From a mechanistic perspective, we have focused on one particular group of DNA transposons that encode a transposase with a DD(E/D) catalytic domain that is topologically similar to RNase H. For these, a number of three-dimensional structures of transpososomes (transposase-nucleic acid complexes) are available, and we use these to describe the basics of their mechanisms. The DD(E/D) group, in addition to being the largest and most common among all DNA transposases, is the one whose members have been used for a wide variety of genomic applications. Therefore, a second focus of the article is to provide a nonexhaustive overview of transposon applications. Although several non-transposon-based approaches to site-directed genome modifications have emerged in the past decade, transposon-based applications are highly relevant when integration specificity is not sought. In fact, for many applications, the almost-perfect randomness and high frequency of integration make transposon-based approaches indispensable.

Diversity of small, single-stranded DNA viruses of invertebrates and their chaotic evolutionary past

A wide spectrum of invertebrates is susceptible to various single-stranded DNA viruses. Their relative simplicity of replication and dependence on actively dividing cells makes them highly pathogenic for many invertebrates (Hexapoda, Decapoda, etc.). We present their taxonomical classification and describe the evolutionary relationships between various groups of invertebrate-infecting viruses, their high degree of recombination, and their relationship to viruses infecting mammals or other vertebrates. They share characteristics of the viruses within the various families, including structure of the virus particle, genome properties, and gene expression strategy.

Casposon integration shows strong target site preference and recapitulates protospacer integration by CRISPR-Cas systems

Casposons are a recently discovered group of large DNA transposons present in diverse bacterial and archaeal genomes. For integration into the host chromosome, casposons employ an endonuclease that is homologous to the Cas1 protein involved in protospacer integration by the CRISPR-Cas adaptive immune system. Here we describe the site-preference of integration by the Cas1 integrase (casposase) encoded by the casposon of the archaeon Aciduliprofundum boonei. Oligonucleotide duplexes derived from the terminal inverted repeats (TIR) of the A. boonei casposon as well as mini-casposons flanked by the TIR inserted preferentially at a site reconstituting the original A. boonei target site. As in the A. boonei genome, the insertion was accompanied by a 15-bp direct target site duplication (TSD). The minimal functional target consisted of the 15-bp TSD segment and the adjacent 18-bp sequence which comprises the 3′ end of the tRNA-Pro gene corresponding to the TΨC loop. The functional casposase target site bears clear resemblance to the leader sequence-repeat junction which is the target for protospacer integration catalyzed by the Cas1–Cas2 adaptation module of CRISPR-Cas. These findings reinforce the mechanistic similarities and evolutionary connection between the casposons and the adaptation module of the prokaryotic adaptive immunity systems.

Protecting genome integrity during CRISPR immune adaptation

Bacterial CRISPR-Cas systems include genomic arrays of short repeats flanking foreign DNA sequences and provide adaptive immunity against viruses. Integration of foreign DNA must occur specifically to avoid damaging the genome or the CRISPR array, but surprisingly promiscuous activity occurs in vitro. Here we reconstituted full-site DNA integration and show that the Streptococcus pyogenes type II-A Cas1-Cas2 integrase maintains specificity in part through limitations on the second integration step. At non-CRISPR sites, integration stalls at the half-site intermediate, thereby enabling reaction reversal. S. pyogenes Cas1-Cas2 is highly specific for the leader-proximal repeat and recognizes the repeat's palindromic ends, thus fitting a model of independent recognition by distal Cas1 active sites. These findings suggest that DNA-insertion sites are less common than suggested by previous work, thereby preventing toxicity during CRISPR immune adaptation and maintaining host genome integrity.

Diverse evolutionary roots and mechanistic variations of the CRISPR-Cas systems

Adaptive immunity had been long thought of as an exclusive feature of animals. However, the discovery of the CRISPR-Cas defense system, present in almost half of prokaryotic genomes, proves otherwise. Because of the everlasting parasite-host arms race, CRISPR-Cas has rapidly evolved through horizontal transfer of complete loci or individual modules, resulting in extreme structural and functional diversity. CRISPR-Cas systems are divided into two distinct classes that each consist of three types and multiple subtypes. We discuss recent advances in CRISPR-Cas research that reveal elaborate molecular mechanisms and provide for a plausible scenario of CRISPR-Cas evolution. We also briefly describe the latest developments of a wide range of CRISPR-based applications.

[The CRISPR case, « ready-made » mutations and Lamarckian evolution of an adaptive immunity system]

Since genetics has shown that mutation predates selection, biology has developed within the Darwinian paradigm framework. However, a mechanism that produces favorable mutations preferentially in response to adaptive constraints has been recently identified. This mechanism, the CRISPR-Cas adaptive immunity system, is considered as a bona fide example of Lamarckian evolution, even if it only reflects loosely Lamarck's ideas. This unusual evolutionary process is made possible by two prokaryotic properties: i) somatic and germinal cells are not distinct sets of cells; ii) Archae and Bacteria very frequently integrate DNA fragments from the environment, and they therefore have access to a source of "ready-made" useful genetic information. The CRISPR-Cas is a defense system against viruses and plasmids that is based on the integration of genomic fragments of these infectious agents into the host genome, and that protects the host against subsequent infections. Therefore, this mechanism does produce advantageous mutations by integrating DNA from the environment and allowing its transmission to descendants. In conclusion, most of the time evolution relies on purely Darwinian processes, i.e. mutations occurring at random, but in a small minority of cases the occurrence of mutations is more or less biased, and is therefore more or less Lamarckian. Although they are rare, such processes are nevertheless important to our understanding of the plurality of modes of evolution.

Footprintless disruption of prosurvival genes in aneuploid cancer cells using CRISPR/Cas9 technology

CRISPR/Cas9 has emerged as a powerful methodology for the targeted editing of genomic DNA sequences. Nevertheless, the intrinsic inefficiency of transfection methods required to use this technique with cultured cells requires the selection and isolation of successfully modified cells, which invariably subjects the cells to stress. Here we report a workflow that allows the isolation of genomically modified cells, even where loss of functional alleles constitutes a selective disadvantage owing to impaired ability to survive stress. Using targeted disruption of the Id1 and Id3 genes in murine B16-F10 and Ret melanoma cell lines as an example, we show that the method allows for the footprintless isolation of CRISPR/Cas9-modified aneuploid cancer cells. We also provide evidence that serial CRISPR/Cas9 modifications can occur, for example when initial homologous recombination events introduce cryptic PAM sequences, and demonstrate that multiple alleles can be successfully targeted in aneuploid cancer cells. By sequencing individual alleles we also found evidence for CRISPR/Cas9-induced transposable element insertion, albeit at a low frequency. This workflow should have broad application in the functional analysis of prosurvival gene function in cultured cells.

A bend, flip and trap mechanism for transposon integration

Cut-and-paste DNA transposons of the mariner/Tc1 family are useful tools for genome engineering and are inserted specifically at TA target sites. A crystal structure of the mariner transposase Mos1 (derived from Drosophila mauritiana), in complex with transposon ends covalently joined to target DNA, portrays the transposition machinery after DNA integration. It reveals severe distortion of target DNA and flipping of the target adenines into extra-helical positions. Fluorescence experiments confirm dynamic base flipping in solution. Transposase residues W159, R186, F187 and K190 stabilise the target DNA distortions and are required for efficient transposon integration and transposition in vitro. Transposase recognises the flipped target adenines via base-specific interactions with backbone atoms, offering a molecular basis for TA target sequence selection. Our results will provide a template for re-designing mariner/Tc1 transposases with modified target specificities.

DOI:http://dx.doi.org/10.7554/eLife.15537.001

The complete set of DNA in a cell is referred to as its genome. Most genomes contain short fragments of DNA called transposons that can jump from one place to another. Transposons carry sections of DNA with them when they move, which creates diversity and can influence the evolution of a species. Transposons are also being exploited to develop tools for biotechnology and medical applications. One family of transposons – the Mariner/Tc1 family – has proved particularly useful in these endeavours because it is widespread in nature and can jump around the genomes of a broad range of species, including mammals.

DNA transposons are cut out of their position and then pasted at a new site by an enzyme called transposase, which is encoded by some of the DNA within the transposon. DNA is made up of strings of molecules called bases and Mariner/Tc1-family transposons can only insert into a new position in the genome at sites that have a specific sequence of two bases. However, it was not known how this target sequence is chosen and how the transposon inserts into it.

Morris et al. have now used a technique called X-ray crystallography to build a three-dimensional model of a Mariner/Tc1-family transposon as it inserts into a new position. The model shows that, as the transposon is pasted into its new site, the surrounding DNA bends. This causes two DNA bases in the surrounding DNA to flip out from their normal position in the DNA molecule, which enables them to be recognised by the transposase. Further experiments showed that this base-flipping is dynamic, that is, the two bases continuously flip in and out of position. Furthermore, Morris et al. identified which parts of the transposase enzyme are required for the transposon to be efficiently pasted into the genome.

Together these findings may help researchers to alter the transposase so that it can insert the transposon into different locations in a genome. This will hopefully lead to new tools for biotechnology and medical applications.

DOI:http://dx.doi.org/10.7554/eLife.15537.002

Horizontal transfer - imperative mission of acellular life forms, Acytota

Acytota is a kingdom of life covering satellites, plasmids, transposable elements, viroids and viruses, all outside the conventional tree of life but satisfying most life definitions. This review focuses on some aspects of Acytota, their "genomes" and life styles, the dominance of transposable elements and their evolutionary influence on other life forms in order to vindicate the Acytota as a life kingdom no more polyphyletic than other kingdoms and its members no more parasitic than other life forms.

The Bacteriophage Carrier State of Campylobacter jejuni Features Changes in Host Non-coding RNAs and the Acquisition of New Host-derived CRISPR Spacer Sequences

Incorporation of self-derived CRISPR DNA protospacers in Campylobacter jejuni PT14 occurs in the presence of bacteriophages encoding a CRISPR-like Cas4 protein. This phenomenon was evident in carrier state infections where both bacteriophages and host are maintained for seemingly indefinite periods as stable populations following serial passage. Carrier state cultures of C. jejuni PT14 have greater aerotolerance in nutrient limited conditions, and may have arisen as an evolutionary response to selective pressures imposed during periods in the extra-intestinal environment. A consequence of this is that bacteriophage and host remain associated and able to survive transition periods where the chances of replicative success are greatly diminished. The majority of the bacteriophage population do not commit to lytic infection, and conversely the bacterial population tolerates low-level bacteriophage replication. We recently examined the effects of Campylobacter bacteriophage/C. jejuni PT14 CRISPR spacer acquisition using deep sequencing strategies of DNA and RNA-Seq to analyze carrier state cultures. This approach identified de novo spacer acquisition in C. jejuni PT14 associated with Class III Campylobacter phages CP8/CP30A but spacer acquisition was oriented toward the capture of host DNA. In the absence of bacteriophage predation the CRISPR spacers in uninfected C. jejuni PT14 cultures remain unchanged. A distinct preference was observed for incorporation of self-derived protospacers into the third spacer position of the C. jejuni PT14 CRISPR array, with the first and second spacers remaining fixed. RNA-Seq also revealed the variation in the synthesis of non-coding RNAs with the potential to bind bacteriophage genes and/or transcript sequences.

Annotation and Classification of CRISPR-Cas Systems

The clustered regularly interspaced short palindromic repeats (CRISPR)-Cas (CRISPR-associated proteins) is a prokaryotic adaptive immune system that is represented in most archaea and many bacteria. Among the currently known prokaryotic defense systems, the CRISPR-Cas genomic loci show unprecedented complexity and diversity. Classification of CRISPR-Cas variants that would capture their evolutionary relationships to the maximum possible extent is essential for comparative genomic and functional characterization of this theoretically and practically important system of adaptive immunity. To this end, a multipronged approach has been developed that combines phylogenetic analysis of the conserved Cas proteins with comparison of gene repertoires and arrangements in CRISPR-Cas loci. This approach led to the current classification of CRISPR-Cas systems into three distinct types and ten subtypes for each of which signature genes have been identified. Comparative genomic analysis of the CRISPR-Cas systems in new archaeal and bacterial genomes performed over the 3 years elapsed since the development of this classification makes it clear that new types and subtypes of CRISPR-Cas need to be introduced. Moreover, this classification system captures only part of the complexity of CRISPR-Cas organization and evolution, due to the intrinsic modularity and evolutionary mobility of these immunity systems, resulting in numerous recombinant variants. Moreover, most of the cas genes evolve rapidly, complicating the family assignment for many Cas proteins and the use of family profiles for the recognition of CRISPR-Cas subtype signatures. Further progress in the comparative analysis of CRISPR-Cas systems requires integration of the most sensitive sequence comparison tools, protein structure comparison, and refined approaches for comparison of gene neighborhoods.

Self-synthesizing transposons: unexpected key players in the evolution of viruses and defense systems

Self-synthesizing transposons are the largest known transposable elements that encode their own DNA polymerases (DNAP). The Polinton/Maverick family of self-synthesizing transposons is widespread in eukaryotes and abundant in the genomes of some protists. In addition to the DNAP and a retrovirus-like integrase, most of the polintons encode homologs of the major and minor jelly-roll capsid proteins, DNA-packaging ATPase and capsid maturation protease. Therefore, polintons are predicted to alternate between the transposon and viral lifestyles although virion formation remains to be demonstrated. Polintons are related to a group of eukaryotic viruses known as virophages that parasitize on giant viruses of the family Mimiviridae and another recently identified putative family of polinton-like viruses (PLV) predicted to lead a similar, dual life style. Comparative genomic analysis of polintons, virophages, PLV and the other viruses with double-stranded (ds)DNA genomes infecting eukaryotes and prokaryotes suggests that the polintons evolved from bacterial tectiviruses and could have been the ancestors of a broad range of eukaryotic viruses including adenoviruses and members of the proposed order 'Megavirales' as well as linear cytoplasmic plasmids. Recently, a group of predicted self-synthesizing transposons was discovered also in prokaryotes. These elements, denoted casposons, encode a DNAP and a homolog of the CRISPR-associated Cas1 endonuclease that has an integrase activity but no capsid proteins. Thus, unlike polintons, casposons appear to be limited to the transposon life style although they could have evolved from viruses. The casposons are thought to have played a pivotal role in the origin of the prokaryotic adaptive immunity, giving rise to the adaptation module of the CRISPR-Cas systems.

Mobile genetic elements: in silico, in vitro, in vivo

Mobile genetic elements (MGEs), also called transposable elements (TEs), represent universal components of most genomes and are intimately involved in nearly all aspects of genome organization, function and evolution. However, there is currently a gap between the fast pace of TE discovery in silico, driven by the exponential growth of comparative genomic studies, and a limited number of experimental models amenable to more traditional in vitro and in vivo studies of structural, mechanistic and regulatory properties of diverse MGEs. Experimental and computational scientists came together to bridge this gap at a recent conference, 'Mobile Genetic Elements: in silico, in vitro, in vivo', held at the Marine Biological Laboratory (MBL) in Woods Hole, MA, USA.

Recent Mobility of Casposons, Self-Synthesizing Transposons at the Origin of the CRISPR-Cas Immunity

Casposons are a superfamily of putative self-synthesizing transposable elements that are predicted to employ a homolog of Cas1 protein as a recombinase and could have contributed to the origin of the CRISPR-Cas adaptive immunity systems in archaea and bacteria. Casposons remain uncharacterized experimentally, except for the recent demonstration of the integrase activity of the Cas1 homolog, and given their relative rarity in archaea and bacteria, original comparative genomic analysis has not provided direct indications of their mobility. Here, we report evidence of casposon mobility obtained by comparison of the genomes of 62 strains of the archaeon Methanosarcina mazei. In these genomes, casposons are variably inserted in three distinct sites indicative of multiple, recent gains, and losses. Some casposons are inserted into other mobile genetic elements that might provide vehicles for horizontal transfer of the casposons. Additionally, many M. mazei genomes contain previously undetected solo terminal inverted repeats that apparently are derived from casposons and could resemble intermediates in CRISPR evolution. We further demonstrate the sequence specificity of casposon insertion and note clear parallels with the adaptation mechanism of CRISPR-Cas. Finally, besides identifying additional representatives in each of the three originally defined families, we describe a new, fourth, family of casposons.

CRISPR-Cas adaptation: insights into the mechanism of action

Since the first demonstration that CRISPR-Cas systems provide bacteria and archaea with adaptive immunity against phages and plasmids, numerous studies have yielded key insights into the molecular mechanisms governing how these systems attack and degrade foreign DNA. However, the molecular mechanisms underlying the adaptation stage, in which new immunological memory is formed, have until recently represented a major unresolved question. In this Progress article, we discuss recent discoveries that have shown both how foreign DNA is identified by the CRISPR-Cas adaptation machinery and the molecular basis for its integration into the chromosome to form an immunological memory. Furthermore, we describe the roles of each of the specific CRISPR-Cas components that are involved in memory formation, and consider current models for their evolutionary origin.

The casposon-encoded Cas1 protein from Aciduliprofundum boonei is a DNA integrase that generates target site duplications

Many archaea and bacteria have an adaptive immune system known as CRISPR which allows them to recognize and destroy foreign nucleic acid that they have previously encountered. Two CRISPR-associated proteins, Cas1 and Cas2, are required for the acquisition step of adaptation, in which fragments of foreign DNA are incorporated into the host CRISPR locus. Cas1 genes have also been found scattered in several archaeal and bacterial genomes, unassociated with CRISPR loci or other cas proteins. Rather, they are flanked by nearly identical inverted repeats and enclosed within direct repeats, suggesting that these genetic regions might be mobile elements (‘casposons’). To investigate this possibility, we have characterized the in vitro activities of the putative Cas1 transposase (‘casposase’) from Aciduliprofundum boonei. The purified Cas1 casposase can integrate both short oligonucleotides with inverted repeat sequences and a 2.8 kb excised mini-casposon into target DNA. Casposon integration occurs without target specificity and generates 14–15 basepair target site duplications, consistent with those found in casposon host genomes. Thus, Cas1 casposases carry out similar biochemical reactions as the CRISPR Cas1-Cas2 complex but with opposite substrate specificities: casposases integrate specific sequences into random target sites, whereas CRISPR Cas1-Cas2 integrates essentially random sequences into a specific site in the CRISPR locus.

Evolution of an archaeal virus nucleocapsid protein from the CRISPR-associated Cas4 nuclease

Abstract

Many proteins of viruses infecting hyperthermophilic Crenarchaeota have no detectable homologs in current databases, hampering our understanding of viral evolution. We used sensitive database search methods and structural modeling to show that a nucleocapsid protein (TP1) of Thermoproteus tenax virus 1 (TTV1) is a derivative of the Cas4 nuclease, a component of the CRISPR-Cas adaptive immunity system that is encoded also by several archaeal viruses. In TTV1, the Cas4 gene was split into two, with the N-terminal portion becoming TP1, and lost some of the catalytic amino acid residues, apparently resulting in the inactivation of the nuclease. To our knowledge, this is the first described case of exaptation of an enzyme for a virus capsid protein function.

Reviewers

This article was reviewed by Vivek Anantharaman, Christine Orengo and Mircea Podar. For complete reviews, see the Reviewers’ reports section.

Electronic supplementary material

The online version of this article (doi:10.1186/s13062-015-0093-2) contains supplementary material, which is available to authorized users.

Discovery and Functional Characterization of Diverse Class 2 CRISPR-Cas Systems

Microbial CRISPR-Cas systems are divided into Class 1, with multisubunit effector complexes, and Class 2, with single protein effectors. Currently, only two Class 2 effectors, Cas9 and Cpf1, are known. We describe here three distinct Class 2 CRISPR-Cas systems. The effectors of two of the identified systems, C2c1 and C2c3, contain RuvC-like endonuclease domains distantly related to Cpf1. The third system, C2c2, contains an effector with two predicted HEPN RNase domains. Whereas production of mature CRISPR RNA (crRNA) by C2c1 depends on tracrRNA, C2c2 crRNA maturation is tracrRNA independent. We found that C2c1 systems can mediate DNA interference in a 5'-PAM-dependent fashion analogous to Cpf1. However, unlike Cpf1, which is a single-RNA-guided nuclease, C2c1 depends on both crRNA and tracrRNA for DNA cleavage. Finally, comparative analysis indicates that Class 2 CRISPR-Cas systems evolved on multiple occasions through recombination of Class 1 adaptation modules with effector proteins acquired from distinct mobile elements.

A strange family, or how a new pleolipovirus reveals its friends and relatives

A new virus of halophilic Archaea is reported by Liu et al., and is remarkable in many ways. SNJ2 is the first temperate, pleomorphic virus (pleolipovirus) that integrates into the genome of its host. Analyses of the virus structure and its genome have provided an unexpected puzzle while at the same time solving another. On the one hand, the study shows a curious relationship exists between SNJ2 and an unrelated provirus (SNJ1) found as a plasmid in the same cell. The presence of SNJ1 appears to allow much higher levels of SNJ2 virus to be produced, although the mechanism involved remains unclear. On the other hand, the curious occurrence of a conserved cluster of pleolipovirus-related genes found widely distributed among haloarchaeal genomes and known for almost 10 years, now appears to correspond to SNJ2-related proviruses.

Pleolipoviridae, a newly proposed family comprising archaeal pleomorphic viruses with single-stranded or double-stranded DNA genomes

Viruses infecting archaea show a variety of virion morphotypes, and they are currently classified into more than ten viral families or corresponding groups. A pleomorphic virus morphotype is very common among haloarchaeal viruses, and to date, several such viruses have been isolated. Here, we propose the classification of eight such viruses and formation of a new family, Pleolipoviridae (from the Greek pleo for more or many and lipos for lipid), containing three genera, Alpha-, Beta-, and Gammapleolipovirus. The proposal is currently under review by the International Committee on Taxonomy of Viruses (ICTV). The members of the proposed family Pleolipoviridae infect halophilic archaea and are nonlytic. They share structural and genomic features and differ from any other classified virus. The virion of pleolipoviruses is composed of a pleomorphic membrane vesicle enclosing the genome. All pleolipoviruses have two major structural protein species, internal membrane and spike proteins. Although the genomes of the pleolipoviruses are single- or double-stranded, linear or circular DNA molecules, they share the same genome organization and gene synteny and show significant similarity at the amino acid level. The canonical features common to all members of the proposed family Pleolipoviridae show that they are closely related and thus form a new viral family.

An updated evolutionary classification of CRISPR-Cas systems

The evolution of CRISPR-cas loci, which encode adaptive immune systems in archaea and bacteria, involves rapid changes, in particular numerous rearrangements of the locus architecture and horizontal transfer of complete loci or individual modules. These dynamics complicate straightforward phylogenetic classification, but here we present an approach combining the analysis of signature protein families and features of the architecture of cas loci that unambiguously partitions most CRISPR-cas loci into distinct classes, types and subtypes. The new classification retains the overall structure of the previous version but is expanded to now encompass two classes, five types and 16 subtypes. The relative stability of the classification suggests that the most prevalent variants of CRISPR-Cas systems are already known. However, the existence of rare, currently unclassifiable variants implies that additional types and subtypes remain to be characterized.

An updated evolutionary classification of CRISPR-Cas systems

The evolution of CRISPR-cas loci, which encode adaptive immune systems in archaea and bacteria, involves rapid changes, in particular numerous rearrangements of the locus architecture and horizontal transfer of complete loci or individual modules. These dynamics complicate straightforward phylogenetic classification, but here we present an approach combining the analysis of signature protein families and features of the architecture of cas loci that unambiguously partitions most CRISPR-cas loci into distinct classes, types and subtypes. The new classification retains the overall structure of the previous version but is expanded to now encompass two classes, five types and 16 subtypes. The relative stability of the classification suggests that the most prevalent variants of CRISPR-Cas systems are already known. However, the existence of rare, currently unclassifiable variants implies that additional types and subtypes remain to be characterized.

Virus-host arms race at the joint origin of multicellularity and programmed cell death

Unicellular eukaryotes and most prokaryotes possess distinct mechanisms of programmed cell death (PCD). How an “altruistic” trait, such as PCD, could evolve in unicellular organisms? To address this question, we developed a mathematical model of the virus-host co-evolution that involves interaction between immunity, PCD and cellular aggregation. Analysis of the parameter space of this model shows that under high virus load and imperfect immunity, joint evolution of cell aggregation and PCD is the optimal evolutionary strategy. Given the abundance of viruses in diverse habitats and the wide spread of PCD in most organisms, these findings imply that multiple instances of the emergence of multicellularity and its essential attribute, PCD, could have been driven, at least in part, by the virus-host arms race.

Mechanism of spacer integration links the CRISPR/Cas system to transposition as a form of mobile DNA

It has recently become clear that many bacterial and archaeal species possess adaptive immune systems. These are typified by multiple copies of DNA sequences known as clustered regularly interspaced short palindromic repeats (CRISPRs). These CRISPR repeats are the sites at which short spacers containing sequences of previously encountered foreign DNA are integrated, and the spacers serve as the molecular memory of previous invaders. In vivo work has demonstrated that two CRISPR-associated proteins - Cas1 and Cas2 - are required for spacer integration, but the mechanism by which this is accomplished remained unclear. Here we review a recent paper describing the in vitro reconstitution of CRISPR spacer integration using purified Cas1 and Cas2 and place the results in context of similar DNA transposition reactions and the crystal structure of the Cas1/Cas2 complex.

Exogenous enzymes upgrade transgenesis and genetic engineering of farm animals

Transgenic farm animals are attractive alternative mammalian models to rodents for the study of developmental, genetic, reproductive and disease-related biological questions, as well for the production of recombinant proteins, or the assessment of xenotransplants for human patients. Until recently, the ability to generate transgenic farm animals relied on methods of passive transgenesis. In recent years, significant improvements have been made to introduce and apply active techniques of transgenesis and genetic engineering in these species. These new approaches dramatically enhance the ease and speed with which livestock species can be genetically modified, and allow to performing precise genetic modifications. This paper provides a synopsis of enzyme-mediated genetic engineering in livestock species covering the early attempts employing naturally occurring DNA-modifying proteins to recent approaches working with tailored enzymatic systems.

The reverse transcriptase-RNase H: from viruses to antiviral defense

Ubiquitous, reverse transcriptase may have contributed to the transition from the RNA to the DNA world, a transition that also involved RNase H-like activities. Both enzymes shaped various genomes and antiviral defense systems as endogenous retroviruses (ERVs) and transposable elements (TEs). A close relationship between a dozen components of retroviruses and the small interfering RNA (siRNA) antiviral-defense machinery has been characterized. Most antiviral-defense systems involve RNase H-like enzymes destroying invading nucleic acids, RNA, or DNA. Such enzymes include RNases H, Argonaute, Dicer, Cas9, transposases, integrases, and enzymes for immunoglobulin rearrangement and splicing. Even in mammalian cells, where protein-based defense dominates, the siRNA machinery remains active, demonstrated by increased virus production and apoptosis after Dicer knockdown. We have noticed a surprising homology between the siRNA silencing system and the interferon response, as well as to siDNA and the CRISPR system. Further, ERVs serve in defense, in addition to having roles in gene regulation and cancer.

Polintons: a hotbed of eukaryotic virus, transposon and plasmid evolution

Polintons (also known as Mavericks) are large DNA transposons that are widespread in the genomes of eukaryotes. We have recently shown that Polintons encode virus capsid proteins, which suggests that these transposons might form virions, at least under some conditions. In this Opinion article, we delineate the evolutionary relationships among bacterial tectiviruses, Polintons, adenoviruses, virophages, large and giant DNA viruses of eukaryotes of the proposed order 'Megavirales', and linear mitochondrial and cytoplasmic plasmids. We hypothesize that Polintons were the first group of eukaryotic double-stranded DNA viruses to evolve from bacteriophages and that they gave rise to most large DNA viruses of eukaryotes and various other selfish genetic elements.

Evolution of adaptive immunity from transposable elements combined with innate immune systems

Adaptive immune systems in prokaryotes and animals give rise to long-term memory through modification of specific genomic loci, such as by insertion of foreign (viral or plasmid) DNA fragments into clustered regularly interspaced short palindromic repeat (CRISPR) loci in prokaryotes and by V(D)J recombination of immunoglobulin genes in vertebrates. Strikingly, recombinases derived from unrelated mobile genetic elements have essential roles in both prokaryotic and vertebrate adaptive immune systems. Mobile elements, which are ubiquitous in cellular life forms, provide the only known, naturally evolved tools for genome engineering that are successfully adopted by both innate immune systems and genome-editing technologies. In this Opinion article, we present a general scenario for the origin of adaptive immunity from mobile elements and innate immune systems.

CRISPR-Cas immunity and mobile DNA: a new superfamily of DNA transposons encoding a Cas1 endonuclease

Mobile genetic elements such as DNA transposons are a feature of most genomes. The existence of novel DNA transposons can be inferred when whole genome sequencing reveals the presence of hallmarks of mobile elements such as terminal inverted repeats (TIRs) flanked by target site duplications (TSDs). A recent report describes a new superfamily of DNA transposons in the genomes of a few bacteria and archaea that possess TIRs and TSDs, and encode several conserved genes including a cas1 endonuclease gene, previously associated only with CRISPR-Cas adaptive immune systems. The data strongly suggests that these elements, designated ‘casposons’, are likely to be bona fide DNA transposons and that their Cas1 nucleases act as transposases and are possibly still active.

Evolution of replicative DNA polymerases in archaea and their contributions to the eukaryotic replication machinery

The elaborate eukaryotic DNA replication machinery evolved from the archaeal ancestors that themselves show considerable complexity. Here we discuss the comparative genomic and phylogenetic analysis of the core replication enzymes, the DNA polymerases, in archaea and their relationships with the eukaryotic polymerases. In archaea, there are three groups of family B DNA polymerases, historically known as PolB1, PolB2 and PolB3. All three groups appear to descend from the last common ancestors of the extant archaea but their subsequent evolutionary trajectories seem to have been widely different. Although PolB3 is present in all archaea, with the exception of Thaumarchaeota, and appears to be directly involved in lagging strand replication, the evolution of this gene does not follow the archaeal phylogeny, conceivably due to multiple horizontal transfers and/or dramatic differences in evolutionary rates. In contrast, PolB1 is missing in Euryarchaeota but otherwise seems to have evolved vertically. The third archaeal group of family B polymerases, PolB2, includes primarily proteins in which the catalytic centers of the polymerase and exonuclease domains are disrupted and accordingly the enzymes appear to be inactivated. The members of the PolB2 group are scattered across archaea and might be involved in repair or regulation of replication along with inactivated members of the RadA family ATPases and an additional, uncharacterized protein that are encoded within the same predicted operon. In addition to the family B polymerases, all archaea, with the exception of the Crenarchaeota, encode enzymes of a distinct family D the origin of which is unclear. We examine multiple considerations that appear compatible with the possibility that family D polymerases are highly derived homologs of family B. The eukaryotic DNA polymerases show a highly complex relationship with their archaeal ancestors including contributions of proteins and domains from both the family B and the family D archaeal polymerases.