DNA Replication in Engineered Escherichia coli Genomes with Extra Replication Origins

An additional replication origin causes cell cycle specific DNA replication fork speed

Replication fork speed (RFS) in Escherichia coli has long been considered constant throughout the replication and cell cycles. In wild-type cells, the circular chromosome is duplicated bidirectionally from oriC, yielding two replication forks that converge at the ter region. Under slow-growth conditions, cells are smaller at initiation than at termination, so DNA replication consumes a larger fraction of cellular resources early in the cell cycle. To challenge this paradigm, we analyzed an E. coli strain with an additional ectopic copy of oriC-designated oriX-inserted midway along the left replichore. In this mutant, replication initiates simultaneously from both oriC and oriX, resulting in four active replication forks early in the cycle. Specifically, the rightward-moving fork from oriX and the leftward-moving fork from oriC converge first, while the leftward-moving fork from oriX is halted at the terA site until the arrival of the rightward-moving oriC fork. Consequently, the number of active replication forks varies dynamically-from zero to four, then two, then one, and finally zero-compared to the fixed zero-two-zero pattern observed in wild-type cells. RFS was calculated using marker frequency analysis of deep sequencing data. Our analysis revealed that RFS is reduced by approximately one third when four replication forks are active and increases by about one fourth when only one fork is active, resulting in a 2-fold variation in RFS during the replication cycle. Moreover, delaying replication initiation or increasing the available dNTP pool normalized these variations, indicating that nucleotide supply is the primary constraint on replication speed. These findings demonstrate that RFS is not inherently constant within a replication cycle and provide a basis for further studies into the factors that regulate replication kinetics.

Modular Assembly of Synthetic Secondary Chromosomes

The development of novel DNA assembly methods in recent years has paved the way for the construction of synthetic replicons to be used for basic research and biotechnological applications. A learning-by-building approach can now answer questions about how chromosomes must be constructed to maintain genetic information. Here we describe an efficient pipeline for the design and assembly of synthetic, secondary chromosomes in Escherichia coli based on the popular modular cloning (MoClo) system.

Transcriptional Potential Determines the Adaptability of Escherichia coli Strains with Different Fitness Backgrounds

Adaptation through the fitness landscape may be influenced by the gene pool or expression network. However, genetic factors that determine the contribution of beneficial mutations during adaptive evolution are poorly understood. In this study, we experimentally evolved wild-type Escherichia coli K-12 MG1655 and its isogenic derivative that has two additional replication origins and shows higher background fitness. During the short time of experimental evolution, the fitness gains of the two E. coli strains with different fitness backgrounds converged. Populational genome sequencing revealed various mutations with different allele frequencies in evolved populations. Several mutations occurred in genes affecting transcriptional regulation (e.g., RNA polymerase subunit, RNase, ppGpp synthetase, and transcription termination/antitermination factor genes). When we introduced mutations into the ancestral E. coli strains, beneficial effects tended to be lower in the ancestor with higher initial fitness. Replication rate analysis showed that the various replication indices do not correlate with the growth rate. Transcriptome profiling showed that gene expression and gene ontology are markedly enriched in populations with lower background fitness after experimental evolution. Further, the degree of transcriptional change was proportional to the fitness gain. Thus, the evolutionary trajectories of bacteria with different fitness backgrounds can be complex and counterintuitive. Notably, transcriptional change is a major contributor to adaptability. IMPORTANCE Predicting the adaptive potential of bacterial populations can be difficult due to their complexity and dynamic environmental conditions. Also, epistatic interaction between mutations affects the adaptive trajectory. Nevertheless, next-generation sequencing sheds light on understanding evolutionary dynamics through high-throughput genome and transcriptome information. Experimental evolution of two E. coli strains with different background fitness showed that the trajectories of fitness gain, which slowed down during the later stages of evolution, became convergent. This suggests that the adaptability of bacteria can be counterintuitive and that predicting the evolutionary path of bacteria can be difficult even in a constant environment. In addition, transcriptional change is associated with fitness gain during the evolutionary process. Thus, the adaptability of cells depends on their intrinsic genetic capacity for a given evolutionary period. This should be considered when genetically engineered bacteria are optimized through adaptive evolution.

Too Much of a Good Thing: How Ectopic DNA Replication Affects Bacterial Replication Dynamics

Each cell division requires the complete and accurate duplication of the entire genome. In bacteria, the duplication process of the often-circular chromosomes is initiated at a single origin per chromosome, resulting in two replication forks that traverse the chromosome in opposite directions. DNA synthesis is completed once the two forks fuse in a region diametrically opposite the origin. In some bacteria, such as Escherichia coli, the region where forks fuse forms a specialized termination area. Polar replication fork pause sites flanking this area can pause the progression of replication forks, thereby allowing forks to enter but not to leave. Transcription of all required genes has to take place simultaneously with genome duplication. As both of these genome trafficking processes share the same template, conflicts are unavoidable. In this review, we focus on recent attempts to add additional origins into various ectopic chromosomal locations of the E. coli chromosome. As ectopic origins disturb the native replichore arrangements, the problems resulting from such perturbations can give important insights into how genome trafficking processes are coordinated and the problems that arise if this coordination is disturbed. The data from these studies highlight that head-on replication–transcription conflicts are indeed highly problematic and multiple repair pathways are required to restart replication forks arrested at obstacles. In addition, the existing data also demonstrate that the replication fork trap in E. coli imposes significant constraints to genome duplication if ectopic origins are active. We describe the current models of how replication fork fusion events can cause serious problems for genome duplication, as well as models of how such problems might be alleviated both by a number of repair pathways as well as the replication fork trap system. Considering the problems associated both with head-on replication-transcription conflicts as well as head-on replication fork fusion events might provide clues of how these genome trafficking issues have contributed to shape the distinct architecture of bacterial chromosomes.

Exception to the exception rule: synthetic and naturally occurring single chromosome Vibrio cholerae

The genome of Vibrio cholerae, the etiological agent of cholera, is an exception to the single chromosome rule found in the vast majority of bacteria and has its genome partitioned between two unequally sized chromosomes. This unusual two-chromosome arrangement in V. cholerae has sparked considerable research interest since its discovery. It was demonstrated that the two chromosomes could be fused by deliberate genome engineering or forced to fuse spontaneously by blocking the replication of Chr2, the secondary chromosome. Recently, natural isolates of V. cholerae with chromosomal fusion have been found. Here, we summarize the pertinent findings on this exception to the exception rule and discuss the potential utility of single-chromosome V. cholerae to address fundamental questions on chromosome biology in general and DNA replication in particular.

Replication termination without a replication fork trap

Bacterial chromosomes harbour a unique origin of bidirectional replication, oriC. They are almost always circular, with replication terminating in a region diametrically opposite to oriC, the terminus. The oriC-terminus organisation is reflected by the orientation of the genes and by the disposition of DNA-binding protein motifs implicated in the coordination of chromosome replication and segregation with cell division. Correspondingly, the E. coli and B. subtilis model bacteria possess a replication fork trap system, Tus/ter and RTP/ter, respectively, which enforces replication termination in the terminus region. Here, we show that tus and rtp are restricted to four clades of bacteria, suggesting that tus was recently domesticated from a plasmid gene. We further demonstrate that there is no replication fork system in Vibrio cholerae, a bacterium closely related to E. coli. Marker frequency analysis showed that replication forks originating from ectopic origins were not blocked in the terminus region of either of the two V. cholerae chromosomes, but progressed normally until they encountered an opposite fork. As expected, termination synchrony of the two chromosomes is disrupted by these ectopic origins. Finally, we show that premature completion of the primary chromosome replication did not modify the choreography of segregation of its terminus region.

Functionality of Two Origins of Replication in Vibrio cholerae Strains With a Single Chromosome

Chromosomal inheritance in bacteria usually entails bidirectional replication of a single chromosome from a single origin into two copies and subsequent partitioning of one copy each into daughter cells upon cell division. However, the human pathogen Vibrio cholerae and other Vibrionaceae harbor two chromosomes, a large Chr1 and a small Chr2. Chr1 and Chr2 have different origins, an oriC-type origin and a P1 plasmid-type origin, respectively, driving the replication of respective chromosomes. Recently, we described naturally occurring exceptions to the two-chromosome rule of Vibrionaceae: i.e., Chr1 and Chr2 fused single chromosome V. cholerae strains, NSCV1 and NSCV2, in which both origins of replication are present. Using NSCV1 and NSCV2, here we tested whether two types of origins of replication can function simultaneously on the same chromosome or one or the other origin is silenced. We found that in NSCV1, both origins are active whereas in NSCV2 ori2 is silenced despite the fact that it is functional in an isolated context. The ori2 activity appears to be primarily determined by the copy number of the triggering site, crtS, which in turn is determined by its location with respect to ori1 and ori2 on the fused chromosome.

Origins Left, Right, and Centre: Increasing the Number of Initiation Sites in the Escherichia coli Chromosome

The bacterium Escherichia coli contains a single circular chromosome with a defined architecture. DNA replication initiates at a single origin called oriC. Two replication forks are assembled and proceed in opposite directions until they fuse in a specialised zone opposite the origin. This termination area is flanked by polar replication fork pause sites that allow forks to enter, but not to leave. Thus, the chromosome is divided into two replichores, each replicated by a single replication fork. Recently, we analysed the replication parameters in E. coli cells, in which an ectopic origin termed oriZ was integrated in the right-hand replichore. Two major obstacles to replication were identified: (1) head-on replication⁻transcription conflicts at highly transcribed rrn operons, and (2) the replication fork trap. Here, we describe replication parameters in cells with ectopic origins, termed oriX and oriY, integrated into the left-hand replichore, and a triple origin construct with oriX integrated in the left-hand and oriZ in the right-hand replichore. Our data again highlight both replication⁻transcription conflicts and the replication fork trap as important obstacles to DNA replication, and we describe a number of spontaneous large genomic rearrangements which successfully alleviate some of the problems arising from having an additional origin in an ectopic location. However, our data reveal additional factors that impact efficient chromosome duplication, highlighting the complexity of chromosomal architecture.

Synchronous termination of replication of the two chromosomes is an evolutionary selected feature in Vibrionaceae

Vibrio cholerae, the causative agent of the cholera disease, is commonly used as a model organism for the study of bacteria with multipartite genomes. Its two chromosomes of different sizes initiate their DNA replication at distinct time points in the cell cycle and terminate in synchrony. In this study, the time-delayed start of Chr2 was verified in a synchronized cell population. This replication pattern suggests two possible regulation mechanisms for other Vibrio species with different sized secondary chromosomes: Either all Chr2 start DNA replication with a fixed delay after Chr1 initiation, or the timepoint at which Chr2 initiates varies such that termination of chromosomal replication occurs in synchrony. We investigated these two models and revealed that the two chromosomes of various Vibrionaceae species terminate in synchrony while Chr2-initiation timing relative to Chr1 is variable. Moreover, the sequence and function of the Chr2-triggering crtS site recently discovered in V. cholerae were found to be conserved, explaining the observed timing mechanism. Our results suggest that it is beneficial for bacterial cells with multiple chromosomes to synchronize their replication termination, potentially to optimize chromosome related processes as dimer resolution or segregation.

Establishing a System for Testing Replication Inhibition of the Vibrio cholerae Secondary Chromosome in Escherichia coli

Regulators of DNA replication in bacteria are an attractive target for new antibiotics, as not only is replication essential for cell viability, but its underlying mechanisms also differ from those operating in eukaryotes. The genetic information of most bacteria is encoded on a single chromosome, but about 10% of species carry a split genome spanning multiple chromosomes. The best studied bacterium in this context is the human pathogen Vibrio cholerae, with a primary chromosome (Chr1) of 3 M bps, and a secondary one (Chr2) of about 1 M bps. Replication of Chr2 is under control of a unique mechanism, presenting a potential target in the development of V. cholerae-specific antibiotics. A common challenge in such endeavors is whether the effects of candidate chemicals can be focused on specific mechanisms, such as DNA replication. To test the specificity of antimicrobial substances independent of other features of the V. cholerae cell for the replication mechanism of the V. cholerae secondary chromosome, we establish the replication machinery in the heterologous E. coli system. We characterize an E. coli strain in which chromosomal replication is driven by the replication origin of V. cholerae Chr2. Surprisingly, the E. coli ori2 strain was not inhibited by vibrepin, previously found to inhibit ori2-based replication.

A Family of Single Copy repABC-Type Shuttle Vectors Stably Maintained in the Alpha-Proteobacterium Sinorhizobium meliloti

A considerable share of bacterial species maintains segmented genomes. Plant symbiotic α-proteobacterial rhizobia contain up to six repABC-type replicons in addition to the primary chromosome. These low or unit-copy replicons, classified as secondary chromosomes, chromids, or megaplasmids, are exclusively found in α-proteobacteria. Replication and faithful partitioning of these replicons to the daughter cells is mediated by the repABC region. The importance of α-rhizobial symbiotic nitrogen fixation for sustainable agriculture and Agrobacterium-mediated plant transformation as a tool in plant sciences has increasingly moved biological engineering of these organisms into focus. Plasmids are ideal DNA-carrying vectors for these engineering efforts. On the basis of repABC regions collected from α-rhizobial secondary replicons, and origins of replication derived from traditional cloning vectors, we devised the versatile family of pABC shuttle vectors propagating in Sinorhizobium meliloti, related members of the Rhizobiales, and Escherichia coli. A modular plasmid library providing the elemental parts for pABC vector assembly was founded. The standardized design of these vectors involves five basic modules: (1) repABC cassette, (2) plasmid-derived origin of replication, (3) RK2/RP4 mobilization site (optional), (4) antibiotic resistance gene, and (5) multiple cloning site flanked by transcription terminators. In S. meliloti, pABC vectors showed high propagation stability and unit-copy number. We demonstrated stable coexistence of three pABC vectors in addition to the two indigenous megaplasmids in S. meliloti, suggesting combinability of multiple compatible pABC plasmids. We further devised an in vivo cloning strategy involving Cre/lox-mediated translocation of large DNA fragments to an autonomously replicating repABC-based vector, followed by conjugation-mediated transfer either to compatible rhizobia or E. coli.

BiFCROS: A Low-Background Fluorescence Repressor Operator System for Labeling of Genomic Loci

Fluorescence-based methods are widely used to analyze elementary cell processes such as DNA replication or chromosomal folding and segregation. Labeling DNA with a fluorescent protein allows the visualization of its temporal and spatial organization. One popular approach is FROS (fluorescence repressor operator system). This method specifically labels DNA in vivo through binding of a fusion of a fluorescent protein and a repressor protein to an operator array, which contains numerous copies of the repressor binding site integrated into the genomic site of interest. Bound fluorescent proteins are then visible as foci in microscopic analyses and can be distinguished from the background fluorescence caused by unbound fusion proteins. Even though this method is widely used, no attempt has been made so far to decrease the background fluorescence to facilitate analysis of the actual signal of interest. Here, we present a new method that greatly reduces the background signal of FROS. BiFCROS (Bimolecular Fluorescence Complementation and Repressor Operator System) is based on fusions of repressor proteins to halves of a split fluorescent protein. Binding to a hybrid FROS array results in fluorescence signals due to bimolecular fluorescence complementation. Only proteins bound to the hybrid FROS array fluoresce, greatly improving the signal to noise ratio compared to conventional FROS. We present the development of BiFCROS and discuss its potential to be used as a fast and single-cell readout for copy numbers of genetic loci.

Optimization and Characterization of the Synthetic Secondary Chromosome synVicII in Escherichia coli

Learning by building is one of the core ideas of synthetic biology research. Consequently, building synthetic chromosomes is the way to fully understand chromosome characteristics. The last years have seen exciting synthetic chromosome studies. We had previously introduced the synthetic secondary chromosome synVicII in Escherichia coli. It is based on the replication mechanism of the secondary chromosome in Vibrio cholerae. Here, we present a detailed analysis of its genetic characteristics and a selection approach to optimize replicon stability. We probe the origin diversity of secondary chromosomes from Vibrionaceae by construction of several new respective replicons. Finally, we present a synVicII version 2.0 with several innovations including its full compatibility with the popular modular cloning (MoClo) assembly system.