Influence of single-stranded DNA-binding protein on recA induction in Escherichia coli

Seventeen Sxy-dependent cyclic AMP receptor protein site-regulated genes are needed for natural transformation in Haemophilus influenzae

Natural competence is the ability of bacteria to actively take up extracellular DNA. This DNA can recombine with the host chromosome, transforming the host cell and altering its genotype. In Haemophilus influenzae, natural competence is induced by energy starvation and the depletion of nucleotide pools. This induces a 26-gene competence regulon (Sxy-dependent cyclic AMP receptor protein [CRP-S] regulon) whose expression is controlled by two regulators, CRP and Sxy. The role of most of the CRP-S genes in DNA uptake and transformation is not known. We have therefore created in-frame deletions of each CRP-S gene and studied their competence phenotypes. All but one gene (ssb) could be deleted. Although none of the remaining CRP-S genes were required for growth in rich medium or survival under starvation conditions, DNA uptake and transformation were abolished or reduced in most of the mutants. Seventeen genes were absolutely required for transformation, with 14 of these genes being specifically required for the assembly and function of the type IV pilus DNA uptake machinery. Only five genes were dispensable for both competence and transformation. This is the first competence regulon for which all genes have been mutationally characterized.

Recombinational repair of DNA damage in Escherichia coli and bacteriophage lambda

Although homologous recombination and DNA repair phenomena in bacteria were initially extensively studied without regard to any relationship between the two, it is now appreciated that DNA repair and homologous recombination are related through DNA replication. In Escherichia coli, two-strand DNA damage, generated mostly during replication on a template DNA containing one-strand damage, is repaired by recombination with a homologous intact duplex, usually the sister chromosome. The two major types of two-strand DNA lesions are channeled into two distinct pathways of recombinational repair: daughter-strand gaps are closed by the RecF pathway, while disintegrated replication forks are reestablished by the RecBCD pathway. The phage lambda recombination system is simpler in that its major reaction is to link two double-stranded DNA ends by using overlapping homologous sequences. The remarkable progress in understanding the mechanisms of recombinational repair in E. coli over the last decade is due to the in vitro characterization of the activities of individual recombination proteins. Putting our knowledge about recombinational repair in the broader context of DNA replication will guide future experimentation.

Enhancement of the uvrA gene dosage reduces pyrimidine dimer excision in UV-irradiated Escherichia coli

E. coli possesses an efficient repair mechanism able to remove pyrimidine dimers from UV-irradiated DNA, which is catalyzed by UvrABC endonuclease. In E. coli B/r Hcr+ cells transformed with a multicopy plasmid harboring a gene coding for UvrA, the excision capacity was greatly reduced. The course of thymine dimer excision was investigated using the enzymatic as well as the radiochromatographic method and the results are discussed in term of nonspecific interaction between the excess of UvrA protein and undamaged DNA duplex.

Interaction of the heat shock protein GroEL of Escherichia coli with single-stranded DNA-binding protein: suppression of ssb-113 by groEL46

Previous studies from our laboratory have shown that an allele of the heat shock protein GroEL (groEL411) is able to specifically suppress some of the physiological defects of the single-stranded DNA-binding protein mutation ssb-1. A search for additional alleles of the groE genes which may act as suppressors for ssb mutations has led to the identification of groEL46 as a specific suppressor of ssb-113. It has very little or no effect on ssb-1 or ssb-3. All of the physiological defects of ssb-113, including temperature-sensitive growth, temperature-sensitive DNA synthesis, sensitivity to UV irradiation, methyl methanesulfonate, and bleomycin, and reduced recombinational capacity, are restored to wild-type levels. The ssb-113 allele, however, is unable to restore sensitivity of groEL46 cells to phage lambda. The mechanism of suppression of ssb-113 by groEL46 appears to differ from that of ssb-1 by groEL411. The data suggest that GroEL may interact with single-stranded DNA-binding protein in more than one domain.

Overproduction of the RecD polypeptide sensitizes Escherichia coli cells to γ-radiation

We investigated DNA metabolism in Escherichia coli cells carrying the multicopy recD+ plasmid (pKI13). In the presence of pKI13, the cellular level of the recD gene product (RecD polypeptide) is amplified at least 60-fold. Overproduction of the RecD polypeptide has no effect on UV repair and conjugational recombination. In contrast, high cellular levels of this polypeptide sensitize wild-type cells to gamma-radiation; also, they increase the rate of radiation-induced DNA degradation. A possible mechanism for the enhancement of gamma-ray-induced killing by large amounts of the RecD polypeptide is discussed.

The single-stranded DNA-binding protein of Escherichia coli

The single-stranded DNA-binding protein (SSB) of Escherichia coli is involved in all aspects of DNA metabolism: replication, repair, and recombination. In solution, the protein exists as a homotetramer of 18,843-kilodalton subunits. As it binds tightly and cooperatively to single-stranded DNA, it has become a prototypic model protein for studying protein-nucleic acid interactions. The sequences of the gene and protein are known, and the functional domains of subunit interaction, DNA binding, and protein-protein interactions have been probed by structure-function analyses of various mutations. The ssb gene has three promoters, one of which is inducible because it lies only two nucleotides from the LexA-binding site of the adjacent uvrA gene. Induction of the SOS response, however, does not lead to significant increases in SSB levels. The binding protein has several functions in DNA replication, including enhancement of helix destabilization by DNA helicases, prevention of reannealing of the single strands and protection from nuclease digestion, organization and stabilization of replication origins, primosome assembly, priming specificity, enhancement of replication fidelity, enhancement of polymerase processivity, and promotion of polymerase binding to the template. E. coli SSB is required for methyl-directed mismatch repair, induction of the SOS response, and recombinational repair. During recombination, SSB interacts with the RecBCD enzyme to find Chi sites, promotes binding of RecA protein, and promotes strand uptake.

Post-ultraviolet DNA synthesis in the absence of repair: role of the single-strand DNA-binding protein

Post-ultraviolet DNA synthesis kinetics were investigated in the Escherichia coli uvrA recA strain and its isogenic counterpart, overproducing single-strand DNA-binding protein (SSB). It was demonstrated that large quantities of SSB enhance the capacity of the unmodified replisome to use the UV-damaged template for DNA synthesis. DNA thus synthesized is of low molecular weight, as shown by sedimentation in alkaline sucrose gradients. It is therefore suggested that SSB actively participates in the replisome translocation past dimers and/or the initiation of new DNA chains downstream of these lesions.

Overproduction of SSB protein enhances the capacity for photorepair in Escherichia coli recA cells

We studied photoreactivation in cells carrying the multicopy ssb+ plasmid. These cells overproduce single-stranded DNA-binding protein (SSB). Overproduction of SSB enhances the capacity for photoreactivation in recA bacteria but not in the recA+ background. It is suggested that, in recA cells, SSB binds to the dimer region of DNA and that this binding stimulates the process of photoreactivation. In recA+ cells, the same stimulation might be achieved by RecA protein.

PsiB polypeptide prevents activation of RecA protein in Escherichia coli

We further characterize a novel plasmid function preventing SOS induction called Psi (Plasmid SOS Inhibition). We show that Psi function is expressed by psiB, a gene located at coordinate 54.9 of plasmid R6-5 and near oriT, the origin of conjugal transfer. Deletions and amber mutations of the psiB gene permitted us to demonstrate that PsiB polypeptide (apparent molecular weight, 12 kDa) is responsible for Psi function. PsiB protein prevents recA730-promoted mutagenesis and intra-chromosomal recombination but not recombination following conjugation. Overproduction of PsiB protein sensitizes the host cell to UV irradiation. We propose that PsiB polypeptide has an anti-SOS action by inhibiting activation of RecA protein, thus preventing the occurrence of LexA-controlled functions.

Biochemical basis of the temperature-inducible constitutive protease activity of the RecA441 protein of Escherichia coli

We compared the biochemical properties of the RecA441 protein to those of the wild-type RecA protein in an effort to account for the constitutive protease activity observed in recA441 strains. The two RecA proteins have similar properties in the absence of single-stranded DNA binding protein (SSB protein), and the differences that do exist shed little light on the temperature-inducible phenotype observed in recA441 strains. In contrast, several biochemical differences are apparent when the two proteins are compared in the presence of SSB protein, and these are conducive to a hypothesis that explains the temperature-sensitive behavior observed in these strains. We find that both the single-stranded DNA (ssDNA)-dependent ATPase and LexA-protease activities of RecA441 protein are more resistant to inhibition by SSB protein than are the activities of the wild-type protein. Additionally, the RecA441 protein is more capable of using ssDNA that has been precoated with SSB protein as a substrate for ATPase and protease activities, implying that RecA441 protein is more proficient at displacing SSB protein from ssDNA. The enhanced SSB protein displacement ability of the RecA441 protein is dependent on elevated temperature. These observations are consistent with the hypothesis that the RecA441 protein competes more efficiently with SSB protein for limited ssDNA sites and can be activated to cleave repressors at elevated temperature by displacing SSB protein from the limited ssDNA that occurs naturally in Escherichia coli. Neither the ssDNA binding characteristics of the RecA441 protein nor the rate at which it transfers from one DNA molecule to another provides an explanation for its enhanced activities, leading us to conclude that kinetics of RecA441 protein association with DNA may be responsible for the properties of the RecA441 protein.

Overproduction of single-stranded DNA-binding protein increases UV-induced mutagenesis in Escherichia coli

UV-induced mutagenesis was investigated in the uvrB strain and its isogenic counterpart overproducing the single-stranded DNA-binding protein (SSB). It was demonstrated that overproduction of SSB significantly increases the frequency of mutation. Our results indicate that such an increase might be due to certain abnormalities in induction of the SOS response (untimely and prolonged activation of the RecA protein).

Suppression of the Escherichia coli ssb-1 mutation by an allele of groEL

A series of spontaneous suppressors to the temperature-sensitive phenotype of the single-stranded DNA-binding protein mutation ssb-1 were isolated. A genomic library of EcoRI fragments from one of these suppressor strains was prepared by using pBR325 as the cloning vector. A 10.0-kilobase class of inserts was identified as carrying the ssb-1 gene itself. A second class of 8.3-kilobase inserts was shown to contain the groE region by (i) restriction analysis, (ii) Southern hybridization of the 8.3-kilobase insert to groE+ DNA, and (iii) identification of the gene products by similar migration on polyacrylamide gels. Subcloning demonstrated that an intact mutant groEL gene was necessary for suppression and that plasmids carrying the 8.3-kilobase insert could suppress mutants carrying groES- but not groEL- genes for phage lambda growth. The suppressor, designated as groEL411, was specific for the ssb-1 allele. In ssb-1 groEL411 cells, DNA synthesis stopped after a shift to 42.5 degrees C but rapidly recovered within minutes. The data suggest a direct interaction between the single-stranded DNA-binding protein and GroEL proteins in DNA replication.

The relative rate of synthesis and levels of single-stranded DNA binding protein during induction of SOS repair in Escherichia coli

Induction of the SOS response in Escherichia coli results in an increase in the relative rate of synthesis of single-stranded DNA binding protein (SSB). In contrast to RecA protein, this increase is slow and does not lead to higher SSB levels. The significance of ssb induction to SOS repair is discussed.

Effects of overproduction of single-stranded DNA-binding protein on RecA protein-dependent processes in Escherichia coli

Overproduction of single-stranded DNA-binding protein (SSB) in Escherichia coli led to a decrease in the basal level of repressor LexA. Expression of the LexA-controlled genes was increased differentially, depending on the affinity of the LexA repressor for each promoter: expression of the recA and sfiA genes was increased 5-fold and 1.5-fold, respectively. Despite only a slight effect on expression of sfiA, which codes for an inhibitor of cell division, bacteria overproducing SSB produced elongated cells. In fact, the effect on cell shape appeared to be essentially independent of the expression of the sfiA and recA genes. Bacteria overproducing SSB were therefore phenotypically similar to bacteria partially starved of thymine, in which filamentation results from both sfiA-dependent and sfiA-recA-independent pathways. These data indicate that excess SSB acts primarily by perturbing DNA replication, thereby favoring gratuitous activation of RecA protein to promote cleavage of LexA protein. When bacteria overproducing SSB were exposed to a DNA-damaging agent such as ultraviolet light or mitomycin C, the recA and sfiA genes were fully induced. Induction of the sfiA gene occurred, however, at higher doses in bacteria overproducing SSB protein than in bacteria with normal levels of SSB. Whereas the efficiency of excision repair was apparently increased by excess SSB, the efficiency of post-replication recombinational repair was reduced as judged by a decrease in the recombination proficiency between a prophage and ultraviolet-irradiated heteroimmune infecting phage. Following induction of ssb+ bacteria with mitomycin C, the cellular content of SSB was slightly increased. These results provide evidence that SSB modulates RecA protein-dependent activities in vivo. It is proposed that SSB favors the formation of short complexes of RecA protein and single-stranded DNA that mediate cleavage of the LexA and lambda repressors, while it delays the formation of long nucleoprotein filaments, thereby slowing down RecA-promoted recombinational events in uninduced as well as in induced bacteria.

Differential suppressor effects of the ssb-1 and ssb-113 alleles on uvrD mutator of Escherichia coli in DNA repair and mutagenesis

We have constructed double mutants carrying either ssb-1 or ssb-113 alleles, which encode temperature-sensitive single strand DNA binding proteins (SSB), and the uvrD::Tn5 allele causing deficiency in DNA helicase II, and have examined sensitivity to ultraviolet light (UV), recombination and spontaneous as well as UV-induced mutagenesis. We have found in a recA+ background that (i) none of the ssb uvrD double mutants was more sensitive to UV than either single mutant; (ii) the ssb-1 allele partially suppressed the strong UV sensitivity of uvrD::Tn5 mutants; (iii) in the recA730 background with constitutive SOS expression, the ssb-1 and ssb-113 alleles suppressed the strong UV-sensitivity caused by the uvrD::Tn5 mutation; (iv) in ssb-113 mutants, the level of recombination was reduced only 10-fold but 100-fold in ssb-1 mutants, showing that there was no correlation between the DNA repair deficiency and the recombination deficiency; (v) the hyper-recombination phenotype of the uvrD::Tn5 mutant was suppressed by the addition of either the ssb-1 or the ssb-113 allele; (vi) no addition of the spontaneous mutator effects promoted by the uvrD::Tn5 and the ssb-113 alleles was observed. These results suggest a possible functional interaction between SSB and Helicase II in DNA repair and mutagenesis.

SOS induction of the gene sulA is partly inhibited in Escherichia coli K12 cells overproducing the RecA protein

A study was made of the SOS induction of the gene sulA of Escherichia coli K12 in relation to the gene dosage of the gene recA. In experiments the sulA::lacZ fusion strain PQ37 and derivatives of PQ37 with the multi-copy plasmids pDR1453 or pBR322 were used. The SOS response was induced with nitrofurantoin, SOS induction of the gene sulA was determined on the basis of the amount of beta-galactosidase synthesized, i.e. by the SOS chromotest (Quillardet et al., 1982a). It was found in this work that cells with the plasmid pDR1453, which contain the gene recA of E. coli K12 (Sancar and Rupp, 1979), have a decreased SOS induction of the gene sulA. Cells with the plasmid pBR322 do not exhibit this decrease. Inactivation of the gene recA in the plasmid pDR1453 with preservation of the functional gene recA in the chromosome leads to a restoration of 'standard' SOS induction of the gene sulA. The results show that the amount of the gene product of the gene recA affects the SOS induction of the gene sulA.

Relationship between cellular RecA protein concentration and untargeted mutagenesis in Escherichia coli

We measured the production of untargeted mutations in the cI and cII genes of untreated lambda phage undergoing a lytic cycle in UV-irradiated bacterial hosts. As previously shown, treatment with 4 micrograms/ml of rifampicin during post-irradiation incubation inhibited amplification of the RecA protein in these cells. In addition, we observed a decreased mutation rate compared to the untreated, irradiated bacteria. Treatment with 4 micrograms/ml or 8 micrograms/ml rifampicin did not prevent the UV induction of the umuDC operon, as judged by assay of beta-galactosidase activity in a umuC-lacZ fusion strain. In contrast, the UV-induction of beta-galactosidase in the sulA-lacZ fusion strain was decreased by 4 micrograms/ml rifampicin. The inhibition of untargeted mutagenesis by this drug treatment was also observed in a strain constitutive for SOS functions (lexA (Def)) as well as in a RecA-overproducing plasmid strain, suggesting the requirement of other factor(s) in wild-type recA+ cells. An htpR165-carrying strain, that blocks induction of heat-shock proteins, exhibited normal UV-promoted mutagenesis. A correlation was observed between the cellular concentration of RecA protein, increased spontaneously by a temperature shift in a lexA(Ts) strain, and the extent of UV-promoted untargeted mutagenesis. These results suggest a mechanistic role of RecA protein in this process.

Analysis of the regulatory region of the ssb gene of Escherichia coli

The regulation of the ssb gene of E. coli has been studied. We reported earlier that the SOS box of the neighbouring uvrA gene also controls the transcription of the ssb gene. Detailed analysis of the upstream region of ssb by S1 mapping reveals the existence of three in vivo functional promoters of which the most upstream one (PI) is inducible by DNA damage. Measurement of galactokinase synthesis using galK fusion plasmids indicates that the uninduced level of transcription from the PI promoter is low. Ssb multicopy plasmids lacking the PI promoter still complement the UV sensitivity of an Ssb mutant. The role of the three promoters in the regulation of the level of Ssb protein in the cell, is discussed.

A loss of uvrA function decreases the induction of the SOS functions recA and umuC by mitomycin C in Escherichia coli

We have studied the levels of recA and umuC protein synthesis in Escherichia coli as a probe for regulatory and mechanistic events involved in mitomycin C mutagenesis. Both RecA and UmuC protein induction were greatly stimulated by mitomycin C in the wild-type strain, reached a peak at about 60 min for the recA gene, and at 90 min for the umuC gene, respectively, and maintained a plateau. The induction was blocked by recA and lexA(Ind-) mutations that conferred no mutagenesis on the cell. Mutation affecting uvrA protein markedly decreased induction of the recA gene as well as the umuC gene by mitomycin C. The results established that UvrA protein is involved in the induction of recA and umuC, and account, at least in part, for the mitomycin C nonmutability of uvrA mutants.

Variability in the nucleic acid binding site size and the amount of single-stranded DNA-binding protein in Escherichia coli

The Escherichia coli single-stranded DNA binding protein (SSB), essential for DNA replication, recombination and repair, can undergo a thermally induced irreversible conformational change which does not eliminate its biological activity, but changes the number of nucleotides it covers (binding site size) when binding to a single-stranded nucleic acid lattice. The binding site size of native and conformationally changed SSB was also found to be a function of the molecular mass of the polynucleotide, an observation which is unusual for single-stranded DNA binding proteins and will greatly affect the affinity relationship of this protein for nucleic acids. A radioimmunoassay used to quantitate in SSB level in cells revealed the number of SSB tetramers to be larger than initial estimates by a factor of as much as six. All these data suggest that the biological role of SSB and its mechanism of action is by far more complex than originally assumed.

RecA protein--promoted lambda repressor cleavage: complementation between RecA441 and RecA430 proteins in vitro

Induction of prophage lambda occurs in recA441 mutant lysogens after a shift to 42 degrees C in the presence of adenine. If the synthesis of RecA441 protein is maintained at a low basal level by the presence of a second mutation in the recA441 gene, recA453, induction of prophage lambda is prevented. The ability to induce prophage lambda is restored by the introduction, on a transducing phage, of a second recA gene carrying the recA430 mutation; by itself, the RecA430 protein is devoid of activity against the lambda repressor (Rebollo et al. 1984). In order to explain how the RecA430 protein might complement the RecA441 protein to provide lambda repressor cleavage in a recA453-441 (recA430) diploid lysogen, we characterized the cleavage reaction catalysed by a mixture of these proteins in vitro. Our results suggest that, in the presence of dATP, the RecA441 and RecA430 proteins form mixed multimers on single-stranded DNA, in which the RecA441 protein molecules enhance the DNA binding affinity of RecA430 protein molecules, but RecA430 protein molecules support no cleavage of the lambda repressor. Although the effects of the RecA430 and single-strand binding (SSB) proteins are similar in vitro, we show that the SSB protein cannot substitute for the RecA430 protein in restoring lambda repressor cleavage in a recA453-441 lysogen. Comparison of the stimulatory effect of long single-stranded DNA with that of (dA)14 oligonucleotides on the RecA441 protein-directed cleavage of the lambda repressor in the presence of various nucleoside triphosphates (NTPs) indicates that the cooperative binding of the RecA441 protein to single-stranded DNA stabilizes the RecA protein-DNA complexes so that they remain intact long enough to support cleavage of the lambda repressor. We conclude that the low basal level of the RecA441 protein in a recA453-441 cell is sufficient to cleave the lambda repressor, under conditions where a normal basal level of RecA430 protein is also present allowing the formation of mixed multimers on single-stranded DNA regions normally present in the cell.

Measurement of in vivo expression of the recA gene of Escherichia coli by using lacZ gene fusions

A recA-lacZ protein fusion was constructed in vivo by using bacteriophage Mu dII301(Ap lac). The fusion contained the promoter and first 47 codons of the recA mutant, as determined by DNA sequence analysis. The fusion was cloned and used to construct a recA-lacZ operon fusion at the same site within the recA gene. These fusions were introduced into the Escherichia coli chromosome at the lambda attachment site either as complete or cryptic lambda prophages. Synthesis of beta-galactosidase from these fusions was inducible by UV radiation. As the UV dose was increased, induction became slower and persisted for a longer period of time. At low doses of UV radiation, more beta-galactosidase was produced in a uvrA mutant than in a wild-type strain; however, at high doses, no induced synthesis of beta-galactosidase occurred in a uvrA mutant. recA+ strains carrying either the protein or operon fusion on a multicopy plasmid showed reduced survival after UV irradiation. This UV sensitivity was not exhibited by strains containing a single copy of either fusion, however; hence, the fusions provide a reliable measure of recA expression.

Quantification of SSB protein in E. coli and its variation during RECA protein induction

Using a two-site immunometric assay (IRMA) we quantified the concentration of single-stranded DNA binding protein (SSB) in several E. coli strains. We found approximately 7,000 monomers of SSB present per bacterium, and this number remained constant throughout the exponential phase of growth. Two ssb- mutants (ssb-1 and ssb-113) are defective in the induction of the S.O.S. pathway. One of the first functions expressed upon induction of the S.O.S. pathway is the amplification of recA protein (RECA), which we monitored by an IRMA assay similar to the one used for SSB quantification. By combining the two assays we determined the level of SSB and RECA in ssb- mutants or in SSB and RECA overproducer strains. We found: a) a normal induction of RECA following UV irradiation of E. coli bacteria overproducing SSB, b) a normal level of SSB in wild type and ssb-1 and ssb-113 mutants either in the absence or in the presence of S.O.S. inducing agents. We confirmed a severe impairment in the induction of RECA in these two mutants after nalidixic acid treatment. Our results suggest that the concentrations of RECA and SSB protein in E. coli are regulated by independent biochemical pathways.

Evolution of cellular ATP concentration after UV-mediated induction of SOS system in Escherichia coli

UV-irradiation of E. coli induces a two fold increase in ATP pool in the first 20 min. Afterwards, in RecA+ strains ATP level drops quickly below values of non irradiated cells. Mutants of E. coli defective in RecA protein or with either RecA protease activity deficient or protease resistant LexA repressor do not present this decrease, showing that it is due to cleavage of LexA repressor by RecA protease. The ATP increase produced in the first 20 min is dependent on RecBC exonuclease activity and it must be due to substrate level phosphorylation since an uncoupler such as dinitrophenol does not affect it.

A common regulatory region shared by divergently transcribed genes of the Escherichia coli SOS system

The Escherichia coli single-stranded DNA binding protein (SSB) is implicated in DNA replication, recombination and repair. On the chromosome, the ssb gene is located adjacent to the excision repair gene uvrA, but the two genes are transcribed in opposite directions. uvrA has been shown to be part of the E. coli SOS system by introducing Mud(Ap, lac) insertions distal to the regulatory region of the gene in the chromosome. Recent investigations suggest that SSB is also involved in the SOS response. However, because the SSB protein is essential to the cell, the inducibility of the ssb gene cannot be investigated by the insertion method. Therefore, we used plasmids harbouring the regulatory region of ssb fused to the galK structural gene, while leaving an intact ssb gene in the chromosome. We show here that expression of the ssb gene is dependent on two promoters of which one is damage inducible. Evidence is presented that the divergently transcribed ssb and uvrA genes are controlled by a common LexA binding site.

Study of the expression of UVRA and SSB proteins in vivo in lambda hybrid phages containing the uvrA and ssbA genes of Escherichia coli

A 9.3-kb Eco RI fragment obtained by partial digestion of the plasmid pDR2000 and containing the uvrA and ssbA genes was subcloned in the insertion vector lambda gt4. Two hybrid bacteriophages carrying this fragment inserted in opposite orientations were isolated and used to lysogenize a uvrA and an ssbA mutant of Escherichia coli. Both phages conferred to these host bacteria the ultraviolet resistance of the wild-type parent indicating full complementation of the uvrA and of the ssbA defect. Two polypeptides corresponding to the molecular weights of the UVRA protein (115 000 dalton) and of the SSB protein (18 500 dalton) were synthesized and amplified after infection of a UV-irradiated lambda ind- lysogen with these 2 hybrid phages. The UVRA protein was not amplified after infection of a lex A3 host while SSB was still produced in large amount. These results establish that uvrA is repressed by lexA in vivo whereas ssbA is not.

DNA degradation, UV sensitivity and SOS-mediated mutagenesis in strains of Escherichia coli deficient in single-strand DNA binding protein: effects of mutations and treatments that alter levels of Exonuclease V or recA protein

Certain strains suppress the temperature-sensitivity caused by ssb-1, which encodes a mutant ssDNA binding protein (SSB). At 42 degrees C, such strains are extremely UV-sensitive, degrade their DNA extensively after UV irradiation, and are deficient in UV mutability and UV induction of recA protein synthesis. We transduced recC22, which eliminates Exonuclease V activity, and recAo281, which causes operator-constitutive synthesis of recA protein, into such an ssb-1 strain. Both double mutants degraded their DNA extensively at 42 degrees C after UV irradiation, and both were even more UV-sensitive than the ssb-1 single mutant. We conclude that one or more nucleases other than Exonuclease V degrades DNA in the ssb recC strain, and that recA protein, even if synthesized copiously, can function efficiently in recombinational DNA repair and in control of post-UV DNA degradation only if normal SSB is also present. Pretreatment with nalidixic acid at 30 degrees C restored normal UV mutability at 42 degrees C, but did not increase UV resistance, in an ssb-1 strain. Another ssb allele, ssb-113, which blocks SOS induction at 30 degrees C, increases spontaneous mutability more than tenfold. The ssb-113 allele was transduced into the SOS-constitutive recA730 strain SC30. This double mutant expressed the same elevated spontaneous and UV-induced mutability at 30 degrees C as the ssb+ recA730 strain, and was three times more UV-resistant than its ssb-113 recA+ parent. We conclude that ssb-1 at 42 degrees C and ssb-113 at 30 degrees C block UV-induced activation of recA protease, but that neither allele interferes with subsequent steps in SOS-mediated mutagenesis.

Suppression of the ssb-1 and ssb-113 mutations of Escherichia coli by a wild-type rep gene, NaCl, and glucose

The ssb-1 mutation confers severe temperature sensitivity and UV sensitivity on many strains of Escherichia coli K-12 and C, including strain C1412. However, ssb-1 confers only slight temperature sensitivity and slight UV sensitivity on strain C1a, suggesting that strain C1a contains extragenic suppressors of ssb-1. We found that introduction of the wild-type rep gene from C1a into strain C1412 ssb-1 gave strong suppression of temperature sensitivity and moderate suppression of UV sensitivity. Also, the C1a rep+ gene mildly suppressed the temperature sensitivity conferred by the ssb-113 mutation, formerly called lexC113. Suppression of the C1412 ssb-1 growth defect by C1a rep+ rendered the cells Gro- for phi X174. In contrast to the positive suppression of ssb-1 and ssb-113 by a wild-type rep gene, mutant rep alleles enhanced the severity of the ssb-1 defect, with several C1a ssb-1 double mutants being either more temperature sensitive or more UV sensitive than C1a ssb-1, depending on which mutant rep allele was used. As a control, the same rep alleles in combination with a dnaB mutation gave an allele-independent increase in temperature sensitivity. Our results on suppression of ssb-1 by rep and on the role of the genetic background in this suppression suggested that the rep and ssb proteins interact to form a subcomplex of the total DNA replication complex and that this subcomplex has some function in repair. The effects of NaCl and glucose on suppression of both the temperature sensitivity and the UV sensitivity conferred by ssb-1 and ssb-113 are described. The degree of suppression of temperature sensitivity by salt or glucose was dependent on the source of the wild-type rep allele, as well as on the genetic background.