DNA polymerase V and RecA protein, a minimal mutasome

Regulation of bacterial RecA protein function

The RecA protein is a recombinase functioning in recombinational DNA repair in bacteria. RecA is regulated at many levels. The expression of the recA gene is regulated within the SOS response. The activity of the RecA protein itself is autoregulated by its own C-terminus. RecA is also regulated by the action of other proteins. To date, these include the RecF, RecO, RecR, DinI, RecX, RdgC, PsiB, and UvrD proteins. The SSB protein also indirectly affects RecA function by competing for ssDNA binding sites. The RecO and RecR, and possibly the RecF proteins, all facilitate RecA loading onto SSB-coated ssDNA. The RecX protein blocks RecA filament extension, and may have other effects on RecA activity. The DinI protein stabilizes RecA filaments. The RdgC protein binds to dsDNA and blocks RecA access to dsDNA. The PsiB protein, encoded by F plasmids, is uncharacterized, but may inhibit RecA in some manner. The UvrD helicase removes RecA filaments from RecA. All of these proteins function in a network that determines where and how RecA functions. Additional regulatory proteins may remain to be discovered. The elaborate regulatory pattern is likely to be reprised for RecA homologues in archaeans and eukaryotes.

A novel role for RecA under non-stress: promotion of swarming motility in Escherichia coli K-12

Background

Bacterial motility is a crucial factor in the colonization of natural environments. Escherichia coli has two flagella-driven motility types: swimming and swarming. Swimming motility consists of individual cell movement in liquid medium or soft semisolid agar, whereas swarming is a coordinated cellular behaviour leading to a collective movement on semisolid surfaces. It is known that swimming motility can be influenced by several types of environmental stress. In nature, environmentally induced DNA damage (e.g. UV irradiation) is one of the most common types of stress. One of the key proteins involved in the response to DNA damage is RecA, a multifunctional protein required for maintaining genome integrity and the generation of genetic variation.

Results

The ability of E. coli cells to develop swarming migration on semisolid surfaces was suppressed in the absence of RecA. However, swimming motility was not affected. The swarming defect of a ΔrecA strain was fully complemented by a plasmid-borne recA gene. Although the ΔrecA cells grown on semisolidsurfaces exhibited flagellar production, they also presented impaired individual movement as well as a fully inactive collective swarming migration. Both the comparative analysis of gene expression profiles in wild-type and ΔrecA cells grown on a semisolid surface and the motility of lexA1 [Ind-] mutant cells demonstrated that the RecA effect on swarming does not require induction of the SOS response. By using a RecA-GFP fusion protein we were able to segregate the effect of RecA on swarming from its other functions. This protein fusion failed to regulate the induction of the SOS response, the recombinational DNA repair of UV-treated cells and the genetic recombination, however, it was efficient in rescuing the swarming motility defect of the ΔrecA mutant. The RecA-GFP protein retains a residual ssDNA-dependent ATPase activity but does not perform DNA strand exchange.

Conclusion

The experimental evidence presented in this work supports a novel role for RecA: the promotion of swarming motility. The defective swarming migration of ΔrecA cells does not appear to be associated with defective flagellar production; rather, it seems to be associated with an abnormal flagellar propulsion function. Our results strongly suggest that the RecA effect on swarming motility does not require an extensive canonical RecA nucleofilament formation. RecA is the first reported cellular factor specifically affecting swarming but not swimming motility in E. coli. The integration of two apparently disconnected biologically important processes, such as the maintenance of genome integrity and motility in a unique protein, may have important evolutive consequences.

Purification and characterization of Escherichia coli DNA polymerase V

Cell survival and genome rescue after UV irradiation in Escherichia coli depends on DNA repair mechanisms induced in response to DNA damage as part of the SOS regulon. SOS occurs in two phases. The first phase is dominated by accurate repair processes such as excision and recombinational DNA repair, while the second phase is characterized by a large approximately 100-fold increase in mutations caused by an error-prone replication of damaged DNA templates. SOS mutagenesis occurs as a direct result of the action of the UmuDC gene-products, which form the low fidelity Escherichia coli DNA polymerase V, a heterotrimeric complex composed of UmuD'(2)C. This chapter describes the preparation of highly purified native pol V that is suitable for a wide range of biochemical studies of protein-protein, protein-DNA interactions and translesion-synthesis (TLS) mechanisms.

Y-family DNA polymerases in Escherichia coli

The observation that mutations in the Escherichia coli genes umuC+ and umuD+ abolish mutagenesis induced by UV light strongly supported the counterintuitive notion that such mutagenesis is an active rather than passive process. Genetic and biochemical studies have revealed that umuC+ and its homolog dinB+ encode novel DNA polymerases with the ability to catalyze synthesis past DNA lesions that otherwise stall replication--a process termed translesion synthesis (TLS). Similar polymerases have been identified in nearly all organisms, constituting a new enzyme superfamily. Although typically viewed as unfaithful copiers of DNA, recent studies suggest that certain TLS polymerases can perform proficient and moderately accurate bypass of particular types of DNA damage. Moreover, various cellular factors can modulate their activity and mutagenic potential.

Motoring along with the bacterial RecA protein

The recombinases of the RecA family are often viewed only as DNA-pairing proteins - they bind to one DNA segment, align it with homologous sequences in another DNA segment, promote an exchange of DNA strands and then dissociate. To a first approximation, this description seems to fit the eukaryotic (Rad51 and Dmc1) and archaeal (RadA) RecA homologues. However, the bacterial RecA protein does much more, coupling ATP hydrolysis with DNA-strand exchange in a manner that greatly expands its repertoire of activities. This article explores the protein activities and experimental results that have identified RecA as a motor protein.

Environmental stress and lesion-bypass DNA polymerases

In nature, microbes live under a variety of harsh conditions, such as excess DNA damage, starvation, pH shift, or high temperatures. Microbial cells respond to such stressful conditions mostly by switching global patterns of gene expression to relieve the environmental stress. The SOS response, which is induced by DNA damage, is one such global network of gene expression that plays a crucial role in balancing the genomic stability and flexibility that are necessary to adapt to harsh environments. Here, I review the roles of SOS-inducible and noninducible lesion-bypass DNA polymerases in mutagenesis induced by environmental stress, and discuss how these polymerases are coordinated for the replication of damaged chromosomes. Possible contributions of lesion-bypass DNA polymerase in hyperthermophilic archaea, e.g., Sulfolobus solfataricus, to genome maintenance are also discussed.

RecFOR proteins are essential for Pol V-mediated translesion synthesis and mutagenesis

When the replication fork moves through the template DNA containing lesions, daughter-strand gaps are formed opposite lesion sites. These gaps are subsequently filled-in either by translesion synthesis (TLS) or by homologous recombination. RecA filaments formed within these gaps are key intermediates for both of the gap-filling pathways. For instance, Pol V, the major lesion bypass polymerase in Escherichia coli, requires a functional interaction with the tip of the RecA filament. Here, we show that all three recombination mediator proteins RecFOR are needed to build a functionally competent RecA filament that supports efficient Pol V-mediated TLS in the presence of ssDNA-binding protein (SSB). A positive contribution of RecF protein to Pol V lesion bypass is demonstrated. When Pol III and Pol V are both present, Pol III imparts a negative effect on Pol V-mediated lesion bypass that is counteracted by the combined action of RecFOR and SSB. Mutations in recF, recO or recR gene abolish induced mutagenesis in E. coli.

RecA acts in trans to allow replication of damaged DNA by DNA polymerase V

The DNA polymerase V (pol V) and RecA proteins are essential components of a mutagenic translesion synthesis pathway in Escherichia coli designed to cope with DNA damage. Previously, it has been assumed that RecA binds to the DNA template strand being copied. Here we show, however, that pol-V-catalysed translesion synthesis, in the presence or absence of the beta-processivity-clamp, occurs only when RecA nucleoprotein filaments assemble or RecA protomers bind on separate single-stranded (ss)DNA molecules in trans. A 3'-proximal RecA filament end on trans DNA is essential for stimulation; however, synthesis is strengthened by further pol V-RecA interactions occurring elsewhere along a trans nucleoprotein filament. We suggest that trans-stimulation of pol V by RecA bound to ssDNA reflects a distinctive regulatory mechanism of mutation that resolves the paradox of RecA filaments assembled in cis on a damaged template strand obstructing translesion DNA synthesis despite the absolute requirement of RecA for SOS mutagenesis.

Role of DNA polymerase IV in Escherichia coli SOS mutator activity

Constitutive expression of the SOS regulon in Escherichia coli recA730 strains leads to a mutator phenotype (SOS mutator) that is dependent on DNA polymerase V (umuDC gene product). Here we show that a significant fraction of this effect also requires DNA polymerase IV (dinB gene product).

Mechanism of homologous recombination: mediators and helicases take on regulatory functions

Homologous recombination (HR) is an important mechanism for the repair of damaged chromosomes, for preventing the demise of damaged replication forks, and for several other aspects of chromosome maintenance. As such, HR is indispensable for genome integrity, but it must be regulated to avoid deleterious events. Mutations in the tumour-suppressor protein BRCA2, which has a mediator function in HR, lead to cancer formation. DNA helicases, such as Bloom's syndrome protein (BLM), regulate HR at several levels, in attenuating unwanted HR events and in determining the outcome of HR. Defects in BLM are also associated with the cancer phenotype. The past several years have witnessed dramatic advances in our understanding of the mechanism and regulation of HR.

Multiple interlinked mechanisms to circumvent DNA replication roadblocks

DNA replication is a fragile process, since unavoidable lesions in the template DNA cause replicative polymerases to stall, posing a serious threat to genome integrity. Homologous recombination, translesion DNA synthesis and de novo reinitiation of DNA synthesis ensure robust replication by navigating it passed damaged DNA. In this review, we highlight the relationship between these three processes.

The Rad51/RadA N-Terminal Domain Activates Nucleoprotein Filament ATPase Activity

Proteins in the RecA/RadA/Rad51 family form helical filaments on DNA that function in homologous recombination. While these proteins all have the same highly conserved ATP binding core, the RadA/Rad51 proteins have an N-terminal domain that shows no homology with the C-terminal domain found in RecA. Both the Rad51 N-terminal and RecA C-terminal domains have been shown to bind DNA, but no role for these domains has been established. We show that RadA filaments can be trapped in either an inactive or active conformation with respect to the ATPase and that activation involves a large rotation of the subunit aided by the N-terminal domain. The G103E mutation within the yeast Rad51 N-terminal domain inactivates the filament by failing to make proper contacts between the N-terminal domain and the core. These results show that the N-terminal domains play a regulatory role in filament activation and highlight the modular architecture of the recombination proteins.

Two processivity clamp interactions differentially alter the dual activities of UmuC

DNA polymerases of the Y family promote survival by their ability to synthesize past lesions in the DNA template. One Escherichia coli member of this family, DNA pol V (UmuC), which is primarily responsible for UV-induced and chemically induced mutagenesis, possesses a canonical beta processivity clamp-binding motif. A detailed analysis of this motif in DNA pol V (UmuC) showed that mutation of only two residues in UmuC is sufficient to result in a loss of UV-induced mutagenesis. Increased levels of wild-type beta can partially rescue this loss of mutagenesis. Alterations in this motif of UmuC also cause loss of the cold-sensitive and beta-dependent synthetic lethal phenotypes associated with increased levels of UmuD and UmuC that are thought to represent an exaggeration of a DNA damage checkpoint. By designing compensatory mutations in the cleft between domains II and III in beta, we restored UV-induced mutagenesis by a UmuC beta-binding motif variant. A recent co-crystal structure of the 'little finger' domain of E. coli pol IV (DinB) with beta suggests that, in addition to the canonical beta-binding motif, a second site of pol IV ((303)VWP(305)) interacts with beta at the outer rim of the dimer interface. Mutational analysis of the corresponding motif in UmuC showed that it is dispensable for induced mutagenesis, but that alterations in this motif result in loss of the cold-sensitive phenotype. These two beta interaction sites of UmuC affect the dual functions of UmuC differentially and indicate subtle and sophisticated polymerase management by the beta clamp.

Complementation of one RecA protein point mutation by another. Evidence for trans catalysis of ATP hydrolysis

The RecA residues Lys248 and Glu96 are closely opposed across the RecA subunit-subunit interface in some recent models of the RecA nucleoprotein filament. The K248R and E96D single mutant proteins of the Escherichia coli RecA protein each bind to DNA and form nucleoprotein filaments but do not hydrolyze ATP or dATP. A mixture of K248R and E96D single mutant proteins restores dATP hydrolysis to 25% of the wild type rate, with maximum restoration seen when the proteins are present in a 1:1 ratio. The K248R/E96D double mutant RecA protein also hydrolyzes ATP and dATP at rates up to 10-fold higher than either single mutant, although at a reduced rate compared with the wild type protein. Thus, the K248R mutation partially complements the inactive E96D mutation and vice versa. The complementation is not sufficient to allow DNA strand exchange. The K248R and E96D mutations originate from opposite sides of the subunit-subunit interface. The functional complementation suggests that Lys248 plays a significant role in ATP hydrolysis in trans across the subunit-subunit interface in the RecA nucleoprotein filament. This could be part of a mechanism for the long range coordination of hydrolytic cycles between subunits within the RecA filament.

Human DNA polymerase eta promotes DNA synthesis from strand invasion intermediates of homologous recombination

Stalled replication forks pose a serious threat to genome integrity. To overcome the catastrophic consequences associated with fork demise, translesion synthesis (TLS) polymerases such as poleta promote DNA synthesis past lesions. Alternatively, a stalled fork may collapse and undergo repair by homologous recombination. By using fractionated cell extracts and purified recombinant proteins, we show that poleta extends DNA synthesis from D loop recombination intermediates in which an invading strand serves as the primer. Extracts from XP-V cells, which are defective in poleta, exhibit severely reduced D loop extension activity. The D loop extension activity of poleta is unusual, as this reaction cannot be promoted by the replicative DNA polymerase delta or by other TLS polymerases such as poliota. Moreover, we find that poleta interacts with RAD51 recombinase and RAD51 stimulates poleta-mediated D loop extension. Our results indicate a dual function for poleta at stalled replication forks: the promotion of translesion synthesis and the reinitiation of DNA synthesis by homologous recombination repair.

Characterization of Escherichia coli translesion synthesis polymerases and their accessory factors

Members of the Y family of DNA polymerases are specialized to replicate lesion-containing DNA. However, they lack 3'-5' exonuclease activity and have reduced fidelity compared to replicative polymerases when copying undamaged templates, and thus are potentially mutagenic. Y family polymerases must be tightly regulated to prevent aberrant mutations on undamaged DNA while permitting replication only under conditions of DNA damage. These polymerases provide a mechanism of DNA damage tolerance, confer cellular resistance to a variety of DNA-damaging agents, and have been implicated in bacterial persistence. The Y family polymerases are represented in all domains of life. Escherichia coli possesses two members of the Y family, DNA pol IV (DinB) and DNA pol V (UmuD'(2)C), and several regulatory factors, including those encoded by the umuD gene that influence the activity of UmuC. This chapter outlines procedures for in vivo and in vitro analysis of these proteins. Study of the E. coli Y family polymerases and their accessory factors is important for understanding the broad principles of DNA damage tolerance and mechanisms of mutagenesis throughout evolution. Furthermore, study of these enzymes and their role in stress-induced mutagenesis may also give insight into a variety of phenomena, including the growing problem of bacterial antibiotic resistance.

Roles of DNA Polymerases in Replication, Repair, and Recombination in Eukaryotes

The functioning of the eukaryotic genome depends on efficient and accurate DNA replication and repair. The process of replication is complicated by the ongoing decomposition of DNA and damage of the genome by endogenous and exogenous factors. DNA damage can alter base coding potential resulting in mutations, or block DNA replication, which can lead to double-strand breaks (DSB) and to subsequent chromosome loss. Replication is coordinated with DNA repair systems that operate in cells to remove or tolerate DNA lesions. DNA polymerases can serve as sensors in the cell cycle checkpoint pathways that delay cell division until damaged DNA is repaired and replication is completed. Eukaryotic DNA template-dependent DNA polymerases have different properties adapted to perform an amazingly wide spectrum of DNA transactions. In this review, we discuss the structure, the mechanism, and the evolutionary relationships of DNA polymerases and their possible functions in the replication of intact and damaged chromosomes, DNA damage repair, and recombination.

Structure and mechanism of Escherichia coli RecA ATPase

RecA protein catalyses an ATP-dependent DNA strand-exchange reaction that is the central step in the repair of dsDNA breaks by homologous recombination. Although much is known about the structure of RecA protein itself, we do not at present have a detailed picture of how RecA binds to ssDNA and dsDNA substrates, and how these interactions are controlled by the binding and hydrolysis of the ATP cofactor. Recent studies from electron microscopy and X-ray crystallography have revealed important ATP-mediated conformational changes that occur within the protein, providing new insights into how RecA catalyses DNA strand-exchange. A unifying theme is emerging for RecA and related ATPase enzymes in which the binding of ATP at a subunit interface results in large conformational changes that are coupled to interactions with the substrates in such a way as to promote the desired reactions.

Roles of the Escherichia coli RecA protein and the global SOS response in effecting DNA polymerase selection in vivo

The Escherichia coli beta sliding clamp protein is proposed to play an important role in effecting switches between different DNA polymerases during replication, repair, and translesion DNA synthesis. We recently described how strains bearing the dnaN159 allele, which encodes a mutant form of the beta clamp (beta159), display a UV-sensitive phenotype that is suppressed by inactivation of DNA polymerase IV (M. D. Sutton, J. Bacteriol. 186:6738-6748, 2004). As part of an ongoing effort to understand mechanisms of DNA polymerase management in E. coli, we have further characterized effects of the dnaN159 allele on polymerase usage. Three of the five E.coli DNA polymerases (II, IV, and V) are regulated as part of the global SOS response. Our results indicate that elevated expression of the dinB-encoded polymerase IV is sufficient to result in conditional lethality of the dnaN159 strain. In contrast, chronically activated RecA protein, expressed from the recA730 allele, is lethal to the dnaN159 strain, and this lethality is suppressed by mutations that either mitigate RecA730 activity (i.e., DeltarecR), or impair the activities of DNA polymerase II or DNA polymerase V (i.e., DeltapolB or DeltaumuDC). Thus, we have identified distinct genetic requirements whereby each of the three different SOS-regulated DNA polymerases are able to confer lethality upon the dnaN159 strain, suggesting the presence of multiple mechanisms by which the actions of the cell's different DNA polymerases are managed in vivo.

Suffering in silence: the tolerance of DNA damage

When cells that are actively replicating DNA encounter sites of base damage or strand breaks, replication might stall or arrest. In this situation, cells rely on DNA-damage-tolerance mechanisms to bypass the damage effectively. One of these mechanisms, known as translesion DNA synthesis, is supported by specialized DNA polymerases that are able to catalyse nucleotide incorporation opposite lesions that cannot be negotiated by high-fidelity replicative polymerases. A second category of tolerance mechanism involves alternative replication strategies that obviate the need to replicate directly across sites of template-strand damage.

Lyase activities intrinsic to Escherichia coli polymerases IV and V

Escherichia coli DNA polymerase IV and V (pol IV and pol V) are error-prone DNA polymerases that are induced as part of the SOS regulon in response to DNA damage. Both are members of the Y-family of DNA polymerases. Their principal biological roles appear to involve translesion synthesis (TLS) and the generation of mutational diversity to cope with stress. Although neither enzyme is known to be involved in base excision repair (BER), we have nevertheless observed apurinic/apyrimidinic 5'-deoxyribose phosphate (AP/5'-dRP) lyase activities intrinsic to each polymerase. Pols IV and V catalyze cleavage of the phosphodiester backbone at the 3'-side of an apurinic/apyrimidinic (AP) site as well as the removal of a 5'-deoxyribose phosphate (dRP) at a preincised AP site. The specific activities of the two error-prone polymerase-associated lyases are approximately 80-fold less than the associated lyase activity of human DNA polymerase beta, which is a key enzyme used in short patch BER. Pol IV forms a covalent Schiff's base intermediate with substrate DNA that is trapped by sodium borohydride, as proscribed by a beta-elimination mechanism. In contrast, a NaBH(4) trapped intermediate is not observed for pol V, even though the lyase specific activity of pol V is slightly higher than that of pol IV. Incubation of pol V (UmuD'(2)C) with a molar excess of UmuD drives an exchange of subunits to form UmuD'D+insoluble UmuC causing inactivation of polymerase and lyase activities. The concomitant loss of both activities is strong evidence that pol V contains a bona fide lyase activity.

Trading places: how do DNA polymerases switch during translesion DNA synthesis?

The replicative bypass of base damage in DNA (translesion DNA synthesis [TLS]) is a ubiquitous mechanism for relieving arrested DNA replication. The process requires multiple polymerase switching events during which the high-fidelity DNA polymerase in the replication machinery arrested at the primer terminus is replaced by one or more polymerases that are specialized for TLS. When replicative bypass is fully completed, the primer terminus is once again occupied by high-fidelity polymerases in the replicative machinery. This review addresses recent advances in our understanding of DNA polymerase switching during TLS in bacteria such as E. coli and in lower and higher eukaryotes.