Generation and Repair of Postreplication Gaps in Escherichia coli

Dysregulated DnaB unwinding induces replisome decoupling and daughter strand gaps that are countered by RecA polymerization

The replicative helicase, DnaB, is a central component of the replisome and unwinds duplex DNA coupled with immediate template-dependent DNA synthesis by the polymerase, Pol III. The rate of helicase unwinding is dynamically regulated through structural transitions in the DnaB hexamer between dilated and constricted states. Site-specific mutations in DnaB enforce a faster more constricted conformation that dysregulates unwinding dynamics, causing replisome decoupling that generates excess ssDNA and induces severe cellular stress. This surplus ssDNA can stimulate RecA recruitment to initiate recombinational repair, restart, or activation of the transcriptional SOS response. To better understand the consequences of dysregulated unwinding, we combined targeted genomic dnaB mutations with an inducible RecA filament inhibition strategy to examine the dependencies on RecA in mitigating replisome decoupling phenotypes. Without RecA filamentation, dnaB:mut strains had reduced growth rates, decreased mutagenesis, but a greater burden from endogenous damage. Interestingly, disruption of RecA filamentation in these dnaB:mut strains also reduced cellular filamentation but increased markers of double strand breaks and ssDNA gaps as detected by in situ fluorescence microscopy and FACS assays, TUNEL and PLUG, respectively. Overall, RecA plays a critical role in strain survival by protecting and processing ssDNA gaps caused by dysregulated helicase activity in vivo.

Controlling genome topology with sequences that trigger post-replication gap formation during replisome passage: the E. coli RRS elements

We report that the Escherichia coli chromosome includes novel GC-rich genomic structural elements that trigger formation of post-replication gaps upon replisome passage. The two nearly perfect 222 bp repeats, designated Replication Risk Sequences or RRS, are each 650 kb from the terminus sequence dif and flank the Ter macrodomain. RRS sequence and positioning is highly conserved in enterobacteria. At least one RRS appears to be essential unless a 200 kb region encompassing one of them is amplified. The RRS contain a G-quadruplex on the lagging strand which impedes DNA polymerase extension producing lagging strand ssDNA gaps, $ \le$2000 bp long, upon replisome passage. Deletion of both RRS elements has substantial effects on global genome structure and topology. We hypothesize that RRS elements serve as topological relief valves during chromosome replication and segregation. There have been no screens for genomic sequences that trigger transient gap formation. Functional analogs of RRS could be widespread, possibly including some enigmatic G-quadruplexes in eukaryotes.

RecA-dependent or independent recombination of plasmid DNA generates a conflict with the host EcoKI immunity by launching restriction alleviation

Bacterial defence systems are tightly regulated to avoid autoimmunity. In Type I restriction-modification (R-M) systems, a specific mechanism called restriction alleviation (RA) controls the activity of the restriction module. In the case of the Escherichia coli Type I R-M system EcoKI, RA proceeds through ClpXP-mediated proteolysis of restriction complexes bound to non-methylated sites that appear after replication or reparation of host DNA. Here, we show that RA is also induced in the presence of plasmids carrying EcoKI recognition sites, a phenomenon we refer to as plasmid-induced RA. Further, we show that the anti-restriction behavior of plasmid-borne non-conjugative transposons such as Tn5053, previously attributed to their ardD loci, is due to plasmid-induced RA. Plasmids carrying both EcoKI and Chi sites induce RA in RecA- and RecBCD-dependent manner. However, inactivation of both RecA and RecBCD restores RA, indicating that there exists an alternative, RecA-independent, homologous recombination pathway that is blocked in the presence of RecBCD. Indeed, plasmid-induced RA in a RecBCD-deficient background does not depend on the presence of Chi sites. We propose that processing of random dsDNA breaks in plasmid DNA via homologous recombination generates non-methylated EcoKI sites, which attract EcoKI restriction complexes channeling them for ClpXP-mediated proteolysis.

The intrinsically disordered linker in the single-stranded DNA-binding protein influences DNA replication restart and recombination pathways in Escherichia coli K-12

Tetrameric single-stranded (ss) DNA-binding proteins (SSBs) stabilize ssDNA intermediates formed during genome maintenance reactions in Bacteria. SSBs also recruit proteins important for these processes through direct SSB-protein interactions, including proteins involved in DNA replication restart and recombination processes. SSBs are composed of an N-terminal oligomerization and ssDNA-binding domain, a C-terminal acidic tip that mediates SSB-protein interactions, and an internal intrinsically disordered linker (IDL). Deletions and insertions into the IDL are well tolerated with few phenotypes, although the largest deletions and insertions exhibit some sensitivity to DNA-damaging agents. To define specific DNA metabolism processes dependent on IDL length, ssb mutants that lack 16, 26, 37, or 47 residues of the 57-residue IDL were tested for synthetic phenotypes with mutations in DNA replication restart or recombination genes. We also tested the impact of integrating a fluorescent domain within the SSB IDL using an ssb::mTur2 insertion mutation. Only the largest deletion tested or the insertion mutation causes sensitivity in any of the pathways. Mutations in two replication restart pathways (PriA-B1 and PriA-C) showed synthetic lethalities or small colony phenotypes with the largest deletion or insertion mutations. Recombination gene mutations del(recBCD) and del(ruvABC) show synthetic phenotypes only when combined with the largest ssb deletion. These results suggest that a minimum IDL length is important in some genome maintenance reactions in Escherichia coli. These include pathways involving PriA-PriB1, PriA-PriC, RecFOR, and RecG. The mTur2 insertion in the IDL may also affect SSB interactions in some processes, particularly the PriA-PriB1 and PriA-PriC replication restart pathways.IMPORTANCEssb is essential in Escherichia coli due to its roles in protecting ssDNA and coordinating genome maintenance events. While the DNA-binding core and acidic tip have well-characterized functions, the purpose of the intrinsically disordered linker (IDL) is poorly understood. In vitro studies have revealed that the IDL is important for cooperative ssDNA binding and phase separation. However, single-stranded (ss) DNA-binding protein (SSB) variants with large deletions and insertions in the IDL support normal cell growth. We find that the PriA-PriB1 and PriA-C replication restart, as well as the RecFOR- and RecG-dependent recombination, pathways are sensitive to IDL length. This suggests that cooperativity, phase separation, or a longer spacer between the core and acidic tip of SSB may be important for specific cellular functions.

Controlling Genome Topology with Sequences that Trigger Post-replication Gap Formation During Replisome Passage: The E. coli RRS Elements

We report that the Escherichia coli chromosome includes novel GC-rich genomic structural elements that trigger formation of post-replication gaps upon replisome passage. The two nearly perfect 222 bp repeats, designated Replication Risk Sequences or RRS, are each 650 kb from the terminus sequence dif and flank the Ter macrodomain. RRS sequence and positioning is highly conserved in enterobacteria. At least one RRS appears to be essential unless a 200 kb region encompassing one of them is amplified. The RRS contain a G-quadruplex on the lagging strand which impedes DNA polymerase extension producing lagging strand ssDNA gaps, ≤2000 bp long, upon replisome passage. Deletion of both RRS elements has substantial effects on global genome structure and topology. We hypothesize that RRS elements serve as topological relief valves during chromosome replication and segregation. There have been no screens for genomic sequences that trigger transient gap formation. Functional analogs of RRS could be widespread, possibly including some enigmatic G-quadruplexes in eukaryotes.

Molecular insights into the prototypical single-stranded DNA-binding protein from E. coli

The SSB protein of Escherichia coli functions to bind single-stranded DNA wherever it occurs during DNA metabolism. Depending upon conditions, SSB occurs in several different binding modes. In the course of its function, SSB diffuses on ssDNA and transfers rapidly between different segments of ssDNA. SSB interacts with many other proteins involved in DNA metabolism, with 22 such SSB-interacting proteins, or SIPs, defined to date. These interactions chiefly involve the disordered and conserved C-terminal residues of SSB. When not bound to ssDNA, SSB can aggregate to form a phase-separated biomolecular condensate. Current understanding of the properties of SSB and the functional significance of its many intermolecular interactions are summarized in this review.

BRCA2 promotes genomic integrity and therapy resistance primarily through its role in homology-directed repair

Tumor suppressor BRCA2 functions in homology-directed repair (HDR), the protection of stalled replication forks, and the suppression of replicative gaps, but their relative contributions to genome integrity and chemotherapy response are under scrutiny. Here, we report that mouse and human cells require a RAD51 filament stabilization motif in BRCA2 for fork protection and gap suppression but not HDR. In mice, the loss of fork protection/gap suppression does not compromise genome stability or shorten tumor latency. By contrast, HDR deficiency increases spontaneous and replication stress-induced chromosome aberrations and tumor predisposition. Unlike with HDR, fork protection/gap suppression defects are also observed in Brca2 heterozygous cells, likely due to reduced RAD51 stabilization at stalled forks/gaps. Gaps arise from PRIMPOL activity, which is associated with 5-hydroxymethyl-2'-deoxyuridine sensitivity due to the formation of SMUG1-generated abasic sites and is exacerbated by poly(ADP-ribose) polymerase (PARP) inhibition. However, HDR proficiency has the major role in mitigating sensitivity to chemotherapeutics, including PARP inhibitors.

Processing of stalled replication forks in Bacillus subtilis

Accurate DNA replication and transcription elongation are crucial for preventing the accumulation of unreplicated DNA and genomic instability. Cells have evolved multiple mechanisms to deal with impaired replication fork progression, challenged by both intrinsic and extrinsic impediments. The bacterium Bacillus subtilis, which adopts multiple forms of differentiation and development, serves as an excellent model system for studying the pathways required to cope with replication stress to preserve genomic stability. This review focuses on the genetics, single molecule choreography, and biochemical properties of the proteins that act to circumvent the replicative arrest allowing the resumption of DNA synthesis. The RecA recombinase, its mediators (RecO, RecR, and RadA/Sms) and modulators (RecF, RecX, RarA, RecU, RecD2, and PcrA), repair licensing (DisA), fork remodelers (RuvAB, RecG, RecD2, RadA/Sms, and PriA), Holliday junction resolvase (RecU), nucleases (RnhC and DinG), and translesion synthesis DNA polymerases (PolY1 and PolY2) are key functions required to overcome a replication stress, provided that the fork does not collapse.

Identification of recG genetic interactions in Escherichia coli by transposon sequencing

IMPORTANCE

DNA damage and subsequent DNA repair processes are mutagenic in nature and an important driver of evolution in prokaryotes, including antibiotic resistance development. Genetic screening approaches, such as transposon sequencing (Tn-seq), have provided important new insights into gene function and genetic relationships. Here, we employed Tn-seq to gain insight into the function of the recG gene, which renders Escherichia coli cells moderately sensitive to a variety of DNA-damaging agents when they are absent. The reported recG genetic interactions can be used in combination with future screens to aid in a more complete reconstruction of DNA repair pathways in bacteria.