Genomic Adaptations to the Loss of a Conserved Bacterial DNA Methyltransferase

DNA methylation by CcrM contributes to genome maintenance in the Agrobacterium tumefaciens plant pathogen

The cell cycle-regulated DNA methyltransferase CcrM is conserved in most Alphaproteobacteria, but its role in bacteria with complex or multicentric genomes remains unexplored. Here, we compare the methylome, the transcriptome and the phenotypes of wild-type and CcrM-depleted Agrobacterium tumefaciens cells with a dicentric chromosome with two essential replication origins. We find that DNA methylation has a pleiotropic impact on motility, biofilm formation and viability. Remarkably, CcrM promotes the expression of the repABCCh2 operon, encoding proteins required for replication initiation/partitioning at ori2, and represses gcrA, encoding a conserved global cell cycle regulator. Imaging ori1 and ori2 in live cells, we show that replication from ori2 is often delayed in cells with a hypo-methylated genome, while ori2 over-initiates in cells with a hyper-methylated genome. Further analyses show that GcrA promotes the expression of the RepCCh2 initiator, most likely through the repression of a RepECh2 anti-sense RNA. Altogether, we propose that replication at ori1 leads to a transient hemi-methylation and activation of the gcrA promoter, allowing repCCh2 activation by GcrA and contributing to initiation at ori2. This study then uncovers a novel and original connection between CcrM-dependent DNA methylation, a conserved epigenetic regulator and genome maintenance in an Alphaproteobacterial pathogen.

Moving toward the Inclusion of Epigenomics in Bacterial Genome Evolution: Perspectives and Challenges

The universality of DNA methylation as an epigenetic regulatory mechanism belongs to all biological kingdoms. However, while eukaryotic systems have been the primary focus of DNA methylation studies, the molecular mechanisms in prokaryotes are less known. Nevertheless, DNA methylation in prokaryotes plays a pivotal role in many cellular processes such as defense systems against exogenous DNA, cell cycle dynamics, and gene expression, including virulence. Thanks to single-molecule DNA sequencing technologies, genome-wide identification of methylated DNA is becoming feasible on a large scale, providing the possibility to investigate more deeply the presence, variability, and roles of DNA methylation. Here, we present an overview of the multifaceted roles of DNA methylation in prokaryotes and suggest research directions and tools which can enable us to better understand the contribution of DNA methylation to prokaryotic genome evolution and adaptation. In particular, we emphasize the need to understand the presence and role of transgenerational inheritance, as well as the impact of epigenomic signatures on adaptation and genome evolution. Research directions and the importance of novel computational tools are underlined.

Computational modeling of unphosphorylated CtrA:Cori binding in the Caulobacter cell cycle

In the alphaproteobacterium, Caulobacter crescentus, phosphorylated CtrA (CtrA∼P), a master regulatory protein, binds directly to the chromosome origin (Cori) to inhibit DNA replication. Using a mathematical model of CtrA binding at Cori site [d], we provide computational evidence that CtrA_U can displace CtrA∼P from Cori at the G1-S transition. Investigation of this interaction within a detailed model of the C. crescentus cell cycle suggests that CckA phosphatase may clear Cori of CtrA∼P by altering the [CtrA_U]/[CtrA∼P] ratio rather than by completely depleting CtrA∼P. Model analysis reveals that the mechanism allows for a speedier transition into S phase, stabilizes the timing of chromosome replication under fluctuating rates of CtrA proteolysis, and may contribute to the viability of numerous mutant strains. Overall, these results suggest that CtrA_U enhances the robustness of chromosome replication. More generally, our proposed regulation of CtrA:Cori dynamics may represent a novel motif for molecular signaling in cell physiology.

Deletion of bglC triggers a genetic compensation response by awakening the expression of alternative beta-glucosidase

In the plant pathogen Streptomyces scabies, the gene bglC encodes a GH1 family cellobiose beta-glucosidase that is both required for primary metabolism and for inducing virulence of the bacterium. Deletion of bglC (strain ΔbglC) surprisingly resulted in the augmentation of the global beta-glucosidase activity of S. scabies. This paradoxical phenotype is highly robust as it has been observed in all bglC deletion mutants independently generated, thereby highlighting a phenomenon of genetic compensation. Comparative proteomics allowed to identify two glycosyl hydrolases - named BcpE1 and BcpE2 - of which peptide levels were significantly increased in strain ΔbglC. Quantitative RT-PCR revealed that the higher abundance of BcpE1 and BcpE2 is triggered at the transcriptional level, the expression of their respective gene being 100 and 15 times upregulated. Enzymatic studies with pure BcpE proteins showed that they both possess beta-glucosidase activity thereby explaining the genotypic-phenotypic discrepancy of the bglC deletion mutant. The GH1 family BcpE1 could hydrolyze cellobiose and generate glucose similarly to BglC itself thereby mainly contributing to the survival of strain ΔbglC when cellobiose is provided as sole nutrient source. The low affinity of BcpE2 for cellobiose suggests that this GH3 family beta-glucosidase would instead primarily target another and yet unknown glucose-beta-1,4-linked substrate. These results make S. scabies a new model system to study genetic compensation. Discovering how, either the bglC DNA locus, its mRNA, the BglC protein, or either its enzymatic activity controls bcpE genes' expression, will unveil new mechanisms directing transcriptional repression.

The conserved transcriptional regulator CdnL is required for metabolic homeostasis and morphogenesis in Caulobacter

Bacterial growth and division require regulated synthesis of the macromolecules used to expand and replicate components of the cell. Transcription of housekeeping genes required for metabolic homeostasis and cell proliferation is guided by the sigma factor σ70. The conserved CarD-like transcriptional regulator, CdnL, associates with promoter regions where σ70 localizes and stabilizes the open promoter complex. However, the contributions of CdnL to metabolic homeostasis and bacterial physiology are not well understood. Here, we show that Caulobacter crescentus cells lacking CdnL have severe morphological and growth defects. Specifically, ΔcdnL cells grow slowly in both rich and defined media, and are wider, more curved, and have shorter stalks than WT cells. These defects arise from transcriptional downregulation of most major classes of biosynthetic genes, leading to significant decreases in the levels of critical metabolites, including pyruvate, α-ketoglutarate, ATP, NAD+, UDP-N-acetyl-glucosamine, lipid II, and purine and pyrimidine precursors. Notably, we find that ΔcdnL cells are glutamate auxotrophs, and ΔcdnL is synthetic lethal with other genetic perturbations that limit glutamate synthesis and lipid II production. Our findings implicate CdnL as a direct and indirect regulator of genes required for metabolic homeostasis that impacts morphogenesis through availability of lipid II and other metabolites.

Control of proline utilization by the Lrp-like regulator PutR in Caulobacter crescentus

Cellular metabolism recently emerged as a central player modulating the bacterial cell cycle. The Alphaproteobacterium Caulobacter crescentus appears as one of the best models to study these connections, but its metabolism is still poorly characterized. Considering that it lives in oligotrophic environments, its capacity to use amino-acids is often critical for its growth. Here, we characterized the C. crescentus PutA bi-functional enzyme and showed that it is required for the utilization of proline as a carbon source. We also found that putA transcription and proline utilization by PutA are strictly dependent on the Lrp-like PutR activator. The activation of putA by PutR needs proline, which most likely acts as an effector molecule for PutR. Surprisingly, we also observed that an over-production of PutR leads to cell elongation in liquid medium containing proline, while it inhibits colony formation even in the absence of proline on solid medium. These cell division and growth defects were equally pronounced in a ΔputA mutant background, indicating that PutR can play other roles beyond the control of proline catabolism. Altogether, these findings suggest that PutR might connect central metabolism with cell cycle processes.

The impact of DNA methylation in Alphaproteobacteria

Alphaproteobacteria include bacteria with very different modes of life, from free-living to host-associated and pathogenic bacteria. Their genomes vary in size and organization from single circular chromosomes to multipartite genomes and are often methylated by one or more adenine or cytosine methyltransferases (MTases). These include MTases that are part of restriction/modification systems and so-called orphan MTases. The development of novel technologies accelerated the analysis of methylomes and revealed the existence of epigenetic patterns in several Alphaproteobacteria. This review describes the known functions of DNA methylation in Alphaproteobacteria and also discusses its potential drawbacks through the accidental deamination of methylated cytosines. Particular emphasis is given to the strong connection between the cell cycle-regulated orphan MTase CcrM and the complex network that controls gene expression and cell cycle progression in Alphaproteobacteria.

Cell division control in Caulobacter crescentus

Caulobacter crescentus is a free-living Alphaproteobacterium that thrives in oligotrophic environments. This review focuses on the regulatory network used by this bacterium to control the levels of cell division proteins, their organization inside the cell and their activity as a function of the cell cycle. Strikingly, C. crescentus makes frequent use of master transcriptional regulators and epigenetic signals, most likely to synchronize cell division with other events of the cell cycle. In addition, cellular metabolism and DNA damage sensors emerge as central players regulating cell division in response to changing environmental conditions.

Hit the right spots: cell cycle control by phosphorylated guanosines in alphaproteobacteria

The class Alphaproteobacteria includes Gram-negative free-living, symbiotic and obligate intracellular bacteria, as well as important plant, animal and human pathogens. Recent work has established the key antagonistic roles that phosphorylated guanosines, cyclic-di-GMP (c-di-GMP) and the alarmones guanosine tetraphosphate and guanosine pentaphosphate (collectively referred to as (p)ppGpp), have in the regulation of the cell cycle in these bacteria. In this Review, we discuss the insights that have been gained into the regulation of the initiation of DNA replication and cytokinesis by these second messengers, with a particular focus on the cell cycle of Caulobacter crescentus. We explore how the fluctuating levels of c-di-GMP and (p)ppGpp during the progression of the cell cycle and under conditions of stress control the synthesis and proteolysis of key regulators of the cell cycle. As these signals also promote bacterial interactions with host cells, the enzymes that control (p)ppGpp and c-di-GMP are attractive antibacterial targets.

An intracellular compass spatially coordinates cell cycle modules in Caulobacter crescentus

Cellular functions in Bacteria, such as chromosome segregation and cytokinesis, result from cascades of molecular events operating largely as self-contained modules. Regulated timing of these cellular modules stems from global genetic circuits that allow precise temporal activation with respect to cell cycle progression and cell differentiation. Critically, many of these functions occur at defined locations within the cell, and therefore regulators of each module must communicate to remain coordinated in space. In this perspective, we highlight recent discoveries in Caulobacter crescentus asymmetric cell division to illuminate diverse mechanisms by which a cellular compass, composed of scaffolding and signaling proteins, directs cell cycle modules to their exact cellular addresses.

Cell cycle control in Alphaproteobacteria

Alphaproteobacteria include many medically and environmentally important organisms. Despite the diversity of their niches and lifestyles, from free-living to host-associated, they usually rely on very similar mechanisms to control their cell cycles. Studies on Caulobacter crescentus still lay the foundation for understanding the molecular details of pathways regulating DNA replication and cell division and coordinating these two processes with other events of the cell cycle. This review highlights recent discoveries on the regulation and the mode of action of conserved global regulators and small molecules like c-di-GMP and (p)ppGpp, which play key roles in cell cycle control. It also describes several newly identified mechanisms that modulate cell cycle progression in response to stresses or environmental conditions.

Convergence of alarmone and cell cycle signaling from trans-encoded sensory domains

Despite the myriad of different sensory domains encoded in bacterial genomes, only a few are known to control the cell cycle. Here, suppressor genetics was used to unveil the regulatory interplay between the PAS (Per-Arnt-Sim) domain protein MopJ and the uncharacterized GAF (cyclic GMP-phosphodiesterase-adenylyl cyclase-FhlA) domain protein PtsP, which resembles an alternative component of the phosphoenolpyruvate (PEP) transferase system. Both of these systems indirectly target the Caulobacter crescentus cell cycle master regulator CtrA, but in different ways. While MopJ acts on CtrA via the cell cycle kinases DivJ and DivL, which control the removal of CtrA at the G1-S transition, our data show that PtsP signals through the conserved alarmone (p)ppGpp, which prevents CtrA cycling under nutritional stress and in stationary phase. We found that PtsP interacts genetically and physically with the (p)ppGpp synthase/hydrolase SpoT and that it modulates several promoters that are directly activated by the cell cycle transcriptional regulator GcrA. Thus, parallel systems integrate nutritional and systemic signals within the cell cycle transcriptional network, converging on the essential alphaproteobacterial regulator CtrA while also affecting global cell cycle transcription in other ways.

IMPORTANCE

Many alphaproteobacteria divide asymmetrically, and their cell cycle progression is carefully regulated. How these bacteria control the cell cycle in response to nutrient limitation is not well understood. Here, we identify a multicomponent signaling pathway that acts on the cell cycle when nutrients become scarce in stationary phase. We show that efficient accumulation of the master cell cycle regulator CtrA in stationary-phase Caulobacter crescentus cells requires the previously identified stationary-phase/cell cycle regulator MopJ as well as the phosphoenolpyruvate protein phosphotransferase PtsP, which acts via the conserved (p)ppGpp synthase SpoT. We identify cell cycle-regulated promoters that are affected by this pathway, providing an explanation of how (p)ppGpp-signaling might couple starvation to control cell cycle progression in Caulobacter spp. and likely other Alphaproteobacteria. This pathway has the potential to integrate carbon fluctuation into cell cycle control, since in phosphotransferase systems it is the glycolytic product phosphenolpyruvate (PEP) rather than ATP that is used as the phosphor donor for phosphorylation.