Is RecG a general guardian of the bacterial genome?

Prokaryotic nucleotide excision repair

Nucleotide excision repair (NER) has allowed bacteria to flourish in many different niches around the globe that inflict harsh environmental damage to their genetic material. NER is remarkable because of its diverse substrate repertoire, which differs greatly in chemical composition and structure. Recent advances in structural biology and single-molecule studies have given great insight into the structure and function of NER components. This ensemble of proteins orchestrates faithful removal of toxic DNA lesions through a multistep process. The damaged nucleotide is recognized by dynamic probing of the DNA structure that is then verified and marked for dual incisions followed by excision of the damage and surrounding nucleotides. The opposite DNA strand serves as a template for repair, which is completed after resynthesis and ligation.

wrwyrggrywrw is a single-chain functional analog of the Holliday junction-binding homodimer, (wrwycr)2

DNA repair pathways in bacteria that use homologous recombination involve the formation and subsequent resolution of Holliday junction (HJ) intermediates. We have previously identified several hexameric peptides that bind to HJs and interfere with HJ processing enzymes in vitro. The peptide WRWYCR and its D-amino acid stereoisomer wrwycr, are potent antibacterial agents. These hexapeptides must form homodimers in order to interact stably with HJs, and inhibit bacterial growth, and this represents a potential limitation. Herein we describe a disulfide bond-independent inhibitor, WRWYRGGRYWRW and its D-stereoisomer wrwyrggrywrw. We have characterized these single-chain, linear analogs of the hexapeptides, and show that in addition to effectively binding to HJs, and inhibiting the activity of DNA repair enzymes that process HJs, they have equal or greater potency against Gram-positive and Gram-negative bacterial growth. The analogs were also shown to cause DNA damage in bacteria, and disrupt the integrity of the bacterial cytoplasmic membrane. Finally, we found that they have little toxicity toward several eukaryotic cell types at concentrations needed to inhibit bacterial growth.

Structure and Mechanisms of SF2 DNA Helicases

Effective transcription, replication, and maintenance of the genome require a diverse set of molecular machines to perform the many chemical transactions that constitute these processes. Many of these machines use single-stranded nucleic acids as templates, and their actions are often regulated by the participation of nucleic acids in multimeric structures and macromolecular assemblies that restrict access to chemical information. Superfamily II (SF2) DNA helicases and translocases are a group of molecular machines that remodel nucleic acid lattices and enable essential cellular processes to use the information stored in the duplex DNA of the packaged genome. Characteristic accessory domains associated with the subgroups of the superfamily direct the activity of the common motor core and expand the repertoire of activities and substrates available to SF2 DNA helicases, translocases, and large multiprotein complexes containing SF2 motors. In recent years, single-molecule studies have contributed extensively to the characterization of this ubiquitous and essential class of enzymes.

Modulation of DNA damage tolerance in Escherichia coli recG and ruv strains by mutations affecting PriB, the ribosome and RNA polymerase

RecG is a DNA translocase that helps to maintain genomic integrity. Initial studies suggested a role in promoting recombination, a possibility consistent with synergism between recG and ruv null alleles and reinforced when the protein was shown to unwind Holliday junctions. In this article we describe novel suppressors of recG and show that the pathology seen without RecG is suppressed on reducing or eliminating PriB, a component of the PriA system for replisome assembly and replication restart. Suppression is conditional, depending on additional mutations that modify ribosomal subunit S6 or one of three subunits of RNA polymerase. The latter suppress phenotypes associated with deletion of priB, enabling the deletion to suppress recG. They include alleles likely to disrupt interactions with transcription anti-terminator, NusA. Deleting priB has a different effect in ruv strains. It provokes abortive recombination and compromises DNA repair in a manner consistent with PriB being required to limit exposure of recombinogenic ssDNA. This synergism is reduced by the RNA polymerase mutations identified. Taken together, the results reveal that RecG curbs a potentially negative effect of proteins that direct replication fork assembly at sites removed from the normal origin, a facility needed to resolve conflicts between replication and transcription.

Unwinding and rewinding: double faces of helicase?

Helicases are enzymes that use ATP-driven motor force to unwind double-stranded DNA or RNA. Recently, increasing evidence demonstrates that some helicases also possess rewinding activity—in other words, they can anneal two complementary single-stranded nucleic acids. All five members of the human RecQ helicase family, helicase PIF1, mitochondrial helicase TWINKLE, and helicase/nuclease Dna2 have been shown to possess strand-annealing activity. Moreover, two recently identified helicases—HARP and AH2 have only ATP-dependent rewinding activity. These findings not only enhance our understanding of helicase enzymes but also establish the presence of a new type of protein: annealing helicases. This paper discusses what is known about these helicases, focusing on their biochemical activity to zip and unzip double-stranded DNA and/or RNA, their possible regulation mechanisms, and biological functions.

Superfamily 2 helicases

Superfamily 2 helicases are involved in all aspects of RNA metabolism, and many steps in DNA metabolism. This review focuses on the basic mechanistic, structural and biological properties of each of the families of helicases within superfamily 2. There are ten separate families of helicases within superfamily 2, each playing specific roles in nucleic acid metabolism. The mechanisms of action are diverse, as well as the effect on the nucleic acid. Some families translocate on single-stranded nucleic acid and unwind duplexes, some unwind double-stranded nucleic acids without translocation, and some translocate on double-stranded or single-stranded nucleic acids without unwinding.

Mycobacterium tuberculosis RecG binds and unwinds model DNA substrates with a preference for Holliday junctions

The RecG enzyme, a superfamily 2 helicase, is present in nearly all bacteria. Here we report for the first time that the recG gene is also present in the genomes of most vascular plants as well as in green algae, but is not found in other eukaryotes or archaea. The precise function of RecG is poorly understood, although ample evidence shows that it plays critical roles in DNA repair, recombination and replication. We further demonstrate that Mycobacterium tuberculosis RecG (RecG_Mtb) DNA binding activity had a broad substrate specificity, whereas it only unwound branched-DNA substrates such as Holliday junctions (HJs), replication forks, D-loops and R-loops, with a strong preference for the HJ as a helicase substrate. In addition, RecG_Mtb preferentially bound relatively long (≥40 nt) ssDNA, exhibiting a higher affinity for the homopolymeric nucleotides poly(dT), poly(dG) and poly(dC) than for poly(dA). RecG_Mtb helicase activity was supported by hydrolysis of ATP or dATP in the presence of Mg²⁺, Mn²⁺, Cu²⁺ or Fe²⁺. Like its Escherichia coli orthologue, RecG_Mtb is also a strictly DNA-dependent ATPase.

ATP-dependent RecG helicase is required for the transcriptional regulator OxyR function in Pseudomonas species

The oxyR gene appears to reside in an operon with the recG helicase gene in many bacteria, including pathogenic Pseudomonas aeruginosa and Pseudomonas putida. Analysis of P. putida transcriptomes shows that many OxyR-controlled genes are regulated by the ATP-dependent RecG helicase and that RecG alone modulates the expression of many genes. We found that purified RecG binds to the promoters of many OxyR-controlled genes and that expression of these genes was not induced under conditions of oxidative stress in recG mutants of P. aeruginosa, P. putida, and Escherichia coli. In vitro data revealed that promoters containing palindromic sequences are essential for RecG binding and that single-strand binding proteins and ATP are also needed for RecG to promote transcription, whereas a magnesium ion has the opposite effect. The OxyR tetramer preferentially binds to promoters after RecG has generated linear DNA in the presence of ATP; otherwise, the OxyR dimer has higher affinity. This study provides new insights into the mechanism of bacterial transcription by demonstrating that RecG might be required for the induction of the OxyR regulon by unwinding palindromic DNA for transcription. This work describes a novel bacterial transcriptional function by RecG helicase with OxyR and may provide new targets for controlling Pseudomonas species pathogen.

Chromosome demise in the wake of ligase-deficient replication

Bacterial DNA ligases, NAD⁺-dependent enzymes, are distinct from eukaryotic ATP-dependent ligases, representing promising targets for broad-spectrum antimicrobials. Yet, the chromosomal consequences of ligase-deficient DNA replication, during which Okazaki fragments accumulate, are still unclear. Using ligA251(Ts), the strongest ligase mutant of Escherichia coli, we studied ligase-deficient DNA replication by genetic and physical approaches. Here we show that replication without ligase kills after a short resistance period. We found that double-strand break repair via RecA, RecBCD, RuvABC and RecG explains the transient resistance, whereas irreparable chromosomal fragmentation explains subsequent cell death. Remarkably, death is mostly prevented by elimination of linear DNA degradation activity of ExoV, suggesting that non-allelic double-strand breaks behind replication forks precipitate DNA degradation that enlarge them into allelic double-strand gaps. Marker frequency profiling of synchronized replication reveals stalling of ligase-deficient forks with subsequent degradation of the DNA synthesized without ligase. The mechanism that converts unsealed nicks behind replication forks first into repairable double-strand breaks and then into irreparable double-strand gaps may be behind lethality of any DNA damaging treatment.

Replication fork reversal after replication-transcription collision

Replication fork arrest is a recognized source of genetic instability, and transcription is one of the most prominent causes of replication impediment. We analyze here the requirement for recombination proteins in Escherichia coli when replication–transcription head-on collisions are induced at a specific site by the inversion of a highly expressed ribosomal operon (rrn). RecBC is the only recombination protein required for cell viability under these conditions of increased replication-transcription collisions. In its absence, fork breakage occurs at the site of collision, and the resulting linear DNA is not repaired and is slowly degraded by the RecJ exonuclease. Lethal fork breakage is also observed in cells that lack RecA and RecD, i.e. when both homologous recombination and the potent exonuclease V activity of the RecBCD complex are inactivated, with a slow degradation of the resulting linear DNA by the combined action of the RecBC helicase and the RecJ exonuclease. The sizes of the major linear fragments indicate that DNA degradation is slowed down by the encounter with another rrn operon. The amount of linear DNA decreases nearly two-fold when the Holliday junction resolvase RuvABC is inactivated in recB, as well as in recA recD mutants, indicating that part of the linear DNA is formed by resolution of a Holliday junction. Our results suggest that replication fork reversal occurs after replication–transcription head-on collision, and we propose that it promotes the action of the accessory replicative helicases that dislodge the obstacle.

Genomes are duplicated prior to cell division by DNA replication, and in all organisms replication impairment leads to chromosome instability. In bacteria, replication and transcription take place simultaneously, and in eukaryotes house-keeping genes are expressed during the S-phase; consequently, transcription is susceptible to impair replication progression. Here, we increase head-on replication–transcription collisions on the bacterial chromosome by inversion of a ribosomal operon (rrn). We show that only one recombination protein is required for growth when the rrn genes are highly expressed: the RecBCD complex, an exonuclease/recombinase that promotes degradation and RecA-dependent homologous recombination of linear DNA. In the absence of RecBCD, we observe linear DNA that ends in the collision region. This linear DNA is composed of only the origin-proximal region of the inverted rrn operon, indicating that it results from fork breakage. It is partly RuvABC-dependent (i.e. produced by the E. coli Holliday junction resolvase), indicating that blocked forks are reversed. The linear DNA ends up at the inverted rrn locus only if the RecJ exonuclease is inactivated; otherwise it is degraded, with major products ending in other upstream rrn operons, indicating that DNA degradation is slowed down by ribosomal operon sequences.

Polarity and bypass of DNA heterology during branch migration of Holliday junctions by human RAD54, BLM, and RECQ1 proteins

Several proteins have been shown to catalyze branch migration (BM) of the Holliday junction, a key intermediate in DNA repair and recombination. Here, using joint molecules made by human RAD51 or Escherichia coli RecA, we find that the polarity of the displaced ssDNA strand of the joint molecules defines the polarity of BM of RAD54, BLM, RECQ1, and RuvAB. Our results demonstrate that RAD54, BLM, and RECQ1 promote BM preferentially in the 3'→5' direction, whereas RuvAB drives it in the 5'→3' direction relative to the displaced ssDNA strand. Our data indicate that the helicase activity of BM proteins does not play a role in the heterology bypass. Thus, RAD54 that lacks helicase activity is more efficient in DNA heterology bypass than BLM or REQ1 helicases. Furthermore, we demonstrate that the BLM helicase and BM activities require different protein stoichiometries, indicating that different complexes, monomers and multimers, respectively, are responsible for these two activities. These results define BM as a mechanistically distinct activity of DNA translocating proteins, which may serve an important function in DNA repair and recombination.

Transcription as a source of genome instability

Alterations in genome sequence and structure contribute to somatic disease, affect the fitness of subsequent generations and drive evolutionary processes. The crucial roles of highly accurate replication and efficient repair in maintaining overall genome integrity are well-known, but the more localized stability costs that are associated with transcribing DNA into RNA molecules are less appreciated. Here we review the diverse ways in which the essential process of transcription alters the underlying DNA template and thereby modifies the genetic landscape.

Functional analysis of DNA replication fork reversal catalyzed by Mycobacterium tuberculosis RuvAB proteins

Initially discovered in Escherichia coli, RuvAB proteins are ubiquitous in bacteria and play a dual role as molecular motor proteins responsible for branch migration of the Holliday junction(s) and reversal of stalled replication forks. Despite mounting genetic evidence for a crucial role of RuvA and RuvB proteins in reversal of stalled replication forks, the mechanistic aspects of this process are still not fully understood. Here, we elucidate the ability of Mycobacterium tuberculosis RuvAB (MtRuvAB) complex to catalyze the reversal of replication forks using a range of DNA replication fork substrates. Our studies show that MtRuvAB, unlike E. coli RuvAB, is able to drive replication fork reversal via the formation of Holliday junction intermediates, suggesting that RuvAB-catalyzed fork reversal involves concerted unwinding and annealing of nascent leading and lagging strands. We also demonstrate the reversal of replication forks carrying hemi-replicated DNA, indicating that MtRuvAB complex-catalyzed fork reversal is independent of symmetry at the fork junction. The fork reversal reaction catalyzed by MtRuvAB is coupled to ATP hydrolysis, is processive, and culminates in the formation of an extended reverse DNA arm. Notably, we found that sequence heterology failed to impede the fork reversal activity of MtRuvAB. We discuss the implications of these results in the context of recognition and processing of varied types of replication fork structures by RuvAB proteins.

Homologous Recombination-Experimental Systems, Analysis, and Significance

Homologous recombination is the most complex of all recombination events that shape genomes and produce material for evolution. Homologous recombination events are exchanges between DNA molecules in the lengthy regions of shared identity, catalyzed by a group of dedicated enzymes. There is a variety of experimental systems in Escherichia coli and Salmonella to detect homologous recombination events of several different kinds. Genetic analysis of homologous recombination reveals three separate phases of this process: pre-synapsis (the early phase), synapsis (homologous strand exchange), and post-synapsis (the late phase). In E. coli, there are at least two independent pathway of the early phase and at least two independent pathways of the late phase. All this complexity is incongruent with the originally ascribed role of homologous recombination as accelerator of genome evolution: there is simply not enough duplication and repetition in enterobacterial genomes for homologous recombination to have a detectable evolutionary role and therefore not enough selection to maintain such a complexity. At the same time, the mechanisms of homologous recombination are uniquely suited for repair of complex DNA lesions called chromosomal lesions. In fact, the two major classes of chromosomal lesions are recognized and processed by the two individual pathways at the early phase of homologous recombination. It follows, therefore, that homologous recombination events are occasional reflections of the continual recombinational repair, made possible in cases of natural or artificial genome redundancy.

Double-strand break repair in bacteria: a view from Bacillus subtilis

In all living organisms, the response to double-strand breaks (DSBs) is critical for the maintenance of chromosome integrity. Homologous recombination (HR), which utilizes a homologous template to prime DNA synthesis and to restore genetic information lost at the DNA break site, is a complex multistep response. In Bacillus subtilis, this response can be subdivided into five general acts: (1) recognition of the break site(s) and formation of a repair center (RC), which enables cells to commit to HR; (2) end-processing of the broken end(s) by different avenues to generate a 3'-tailed duplex and RecN-mediated DSB 'coordination'; (3) loading of RecA onto single-strand DNA at the RecN-induced RC and concomitant DNA strand exchange; (4) branch migration and resolution, or dissolution, of the recombination intermediates, and replication restart, followed by (5) disassembly of the recombination apparatus formed at the dynamic RC and segregation of sister chromosomes. When HR is impaired or an intact homologous template is not available, error-prone nonhomologous end-joining directly rejoins the two broken ends by ligation. In this review, we examine the functions that are known to contribute to DNA DSB repair in B. subtilis, and compare their properties with those of other bacterial phyla.

Molecular mechanisms of the whole DNA repair system: a comparison of bacterial and eukaryotic systems

DNA is subjected to many endogenous and exogenous damages. All organisms have developed a complex network of DNA repair mechanisms. A variety of different DNA repair pathways have been reported: direct reversal, base excision repair, nucleotide excision repair, mismatch repair, and recombination repair pathways. Recent studies of the fundamental mechanisms for DNA repair processes have revealed a complexity beyond that initially expected, with inter- and intrapathway complementation as well as functional interactions between proteins involved in repair pathways. In this paper we give a broad overview of the whole DNA repair system and focus on the molecular basis of the repair machineries, particularly in Thermus thermophilus HB8.

RecG protein and single-strand DNA exonucleases avoid cell lethality associated with PriA helicase activity in Escherichia coli

Replication of the Escherichia coli chromosome usually initiates at a single origin (oriC) under control of DnaA. Two forks are established and move away in opposite directions. Replication is completed when these meet in a broadly defined terminus area half way around the circular chromosome. RecG appears to consolidate this arrangement by unwinding D-loops and R-loops that PriA might otherwise exploit to initiate replication at other sites. It has been suggested that without RecG such replication generates 3' flaps as the additional forks collide and displace nascent leading strands, providing yet more potential targets for PriA. Here we show that, to stay alive, cells must have either RecG or a 3' single-stranded DNA (ssDNA) exonuclease, which can be exonuclease I, exonuclease VII, or SbcCD. Cells lacking all three nucleases are inviable without RecG. They also need RecA recombinase and a Holliday junction resolvase to survive rapid growth, but SOS induction, although elevated, is not required. Additional requirements for Rep and UvrD are identified and linked with defects in DNA mismatch repair and with the ability to cope with conflicts between replication and transcription, respectively. Eliminating PriA helicase activity removes the requirement for RecG. The data are consistent with RecG and ssDNA exonucleases acting to limit PriA-mediated re-replication of the chromosome and the consequent generation of linear DNA branches that provoke recombination and delay chromosome segregation.