Cytotoxicity and mutagenesis induced by singlet oxygen in wild type and DNA repair deficient Escherichia coli strains

Singlet oxygen ((1)O(2)) is a product of several biological processes and can be generated in photodynamic therapy, through a photosensitization type II mechanism. (1)O(2) is able to interact with lipids, proteins and DNA, leading to cell killing and mutagenesis, and can be directly involved with degenerative processes such as cancer and aging. In this work, we analyzed the cytotoxicity and mutagenesis induced after direct treatment of wild type and the DNA repair fpg and/or mutY deficient Escherichia coli strains with disodium 3,3'-(1,4-naphthylidene) diproprionate endoperoxide (NDPO(2)), which releases (1)O(2) by thermodissociation. The treatment induced cell killing and mutagenesis in all strains, but the mutY strain showed to be more sensitive. These results indicate that even (1)O(2) generated outside bacterial cells may lead to DNA damage that could be repaired by pathways that employ MutY protein. As (1)O(2) is highly reactive, its interaction with cell membranes may generate secondary products that could react with DNA, leading to mutagenic lesions.     

Untargeted mutagenesis induced by UV in the lacI gene of Escherichia coli

Summary Using a nonselective method, we have estimated the proportion of untargeted mutations in the lacI gene of E. coli by transferring either irradiated or unirradiated F pro lac plasmids from an excision deficient donor to an excision deficient pro lac deleted recipient that had been irradiated and allowed to induce recA dependent functions for 30 min. We find that about 10 percent of the mutations induced by either 3.5 Jm^-2 or 7 Jm^-2 UV are untargeted.     

DNA sequence context affects UV-induced mutagenesis in Escherichia coli

We investigated the effect of altering the DNA sequence surrounding a mutable target site on the production of ultraviolet light (UV) induced mutations. Site-directed base substitutions were incorporated on both sides of a TAA sequence encoding a UAA nonsense defect in the tyrA14 allele of Escherichia coli. This allele is readily revertable by UV and a total of eight different base substitution mutations can be recovered. Five different strains harboring DNA sequences allowing the formation of 5'-TT, 5'-CT and 5'-TA* photoproducts were constructed and exposed to UV. DNA sequence analysis was used to determine the spectrum of the revertants that were recovered. The results showed that changes at the 3'-base of a TT site were predominantly T to C transitions and T to A transversions. However, unlike the TT site, a 5'-CT site produced a relatively high frequency of T to G transversions. In addition, T to A transversions that could not have been targeted by a cyclobutane-type or [6-4]-type pyrimidine dimer were produced; this result suggested that these mutations may be targeted by a TA* photoproduct. Also, a distinct strand bias was noted for two mechanistically identical base substitutions in a strain having a palindromic target sequence; this result may reflect an unequal damage distribution or processing of photoproducts as a consequence of asymmetric DNA replication. Finally, our results show that DNA sequences expected to allow the greatest density of UV-induced DNA damage produce the highest mutation frequencies. Overall, these findings provide new insights regarding the role of DNA photoproducts in UV mutagenesis.     

Growth inhibition of Escherichia coli by E colicin plasmids

Plasmids were isolated from E colicinogenic strains and transformed into prototrophic Escherichia coli K 12 strain DB364. Screening of E colicinogenic transformants for growth on defined medium revealed an apparent amino acid auxotrophy mediated by E4 and, to a lesser extent, E7 colicin plasmids. The auxotrophy was further investigated in E4 colicinogenic strains. From such auxotrophic transformants, denoted Pmi+ (plasmid-mediated inhibition of growth), Pmi- variants were obtained at a frequency of 3 X 10(-4) per bacterium. Plasmid loss was not detected among Pmi- clones. Isolation of E4 colicin plasmids from Pmi- clones and retransformation of strain DB364 with these plasmids showed that 40% of the plasmids were unable to inhibit growth of DB364 and were inferred to have alterations in an E4 colicin plasmid gene termed pmi. All such plasmids were indistinguishable from native E4 colicin plasmids, with respect to colicin immunity, colicin production and excretion, and sensitivity to lysis by mitomycin C. Experiments examining the nutritional basis of the plasmid-mediated auxotrophy indicated that at least seven amino acids, isoleucine, leucine, valine, arginine, methionine, serine and glycine, were involved in the auxotrophy. However, supplementation with only these seven amino acids did not completely restore growth. Assays of the activities of enzymes involved in amino acid biosynthesis in colicinogenic and non-colicinogenic strains under repressing and derepressing growth conditions suggested that E4 colicin plasmids did not repress synthesis of the implicated amino acids.     

Frameshift mutagenesis in Escherichia coli by reversible DNA intercalators: sequence specificity

The mutagenic potency of the simple reversible intercalators isopropyl-OPC (iPr-OPC) and 9-aminoacridine (9-AA) is assessed in E. coli using reversion assays based on plasmids derived from pBR322 carrying various frameshift mutations within the tetracycline resistance gene in repetitive sequences: +/- 2 frameshift mutations within alternating GC sequences; +/- 1 frameshift mutation at runs of guanines. The results obtained show that iPr-OPC and 9-AA have a sequence specificity for mutagenesis: they revert +1 and -1 frameshift mutations within runs of monotonous G:C base pairs. The precise determination of the size of a small restriction fragment which contains the mutation allowed us to demonstrate that reversion occurred by -1 deletions for the +1 frameshift mutations and by +1 additions for the -1 frameshift mutations. The possible relations of this specific reversion with the base sequence specificity of the mutagenesis are briefly discussed.     

Preferential inhibition of plasmid replication in vivo by altered DNA gyrase activity in Escherichia coli.

The thermosensitive growth phenotype exerted by runaway-mutant plasmids was suppressed by sublethal doses of the DNA gyrase inhibitors novobiocin or nalidixic acid, although the latter drug was less efficient. A novobiocin-resistant gyrB mutant Escherichia coli strain prevented expression of the runaway phenotype at 37 to 42 degrees C in the absence of any drug.     

Dissection of functional domains in Escherichia coli DNA photolyase by linker-insertion mutagenesis.

Summary The phr gene, which encodes protein of 472 amino acid residues, is required for light-dependent photoreactivation and enhances light-independent excision repair of ultraviolet light (UV)-induced DNA damage. In this study, dodecamer HindIII linker insertions were introduced into the cloned phr gene and the functional effects of the resulting mutations on photoreactivation and light-independent dark repair in vivo were studied. Among 22 mutants obtained, 7 showed no photoreactivation as well as no enhancement of light-independent repair. Four of these were located in amino acid residues between Gln333 and Leu371 near the 3 end of the gene, two were located in a small region at Glu275 to Glu280 near the middle of the gene and the remaining one was between Pro49 and Arg50. Three mutants that had insertions located in the 42 by segment from 399 to 441 by of the phr coding sequence (corresponding to amino acid residues Ile134 to Lys149) lost the light-independent repair effect but retained photoreactivation. These results suggest that (i) Escherichia coli DNA photolyase contains several critical sites that are distributed over much of the enzyme molecule, and (ii) a functional domain required for the effect on light-independent repair is at least in part distinct from that necessary for light-dependent photoreactivation.     

Involvement of Y-family DNA polymerases in mutagenesis caused by oxidized nucleotides in Escherichia coli

Escherichia coli DNA polymerase IV incorporated 2-hydroxy-dATP opposite template guanine or thymine and 8-hydroxy-dGTP exclusively opposite adenine in vitro. Mutator phenotypes in sod/fur strains were substantially diminished by deletion of dinB and/or umuDC. DNA polymerases IV and V may be involved in mutagenesis caused by incorporation of the oxidized deoxynucleoside triphosphates.     

Site-directed mutagenesis of residues involved in G Strand DNA binding by Escherichia coli DNA topoisomerase I

Crystal structures of complexes between type IA DNA topoisomerases and single-stranded DNA suggest that the residues Ser-192, Arg-195, and Gln-197 in a conserved region of Escherichia coli topoisomerase I may be important for direct interactions with phosphates on the G strand of DNA, which is the substrate for DNA cleavage and religation (Changela A., DiGate, R. J., and Mondragón, A. (2001) Nature 411, 1077-1081; Perry, K., and Mondragón, A. (2003) Structure 11, 1349-1358). Site-directed mutagenesis experiments altering these residues to alanines and other amino acids were carried out to probe the relevance of these interactions in the catalytic activities of the enzyme. The results show that the side chains of Arg-195 and Gln-197 are required for DNA cleavage by the enzyme and are likely to be important for positioning of the G strand of DNA at the active site prior to DNA cleavage. Mutation of Ser-192 did not affect DNA binding and cleavage but nevertheless decreased the overall rate of relaxation of supercoiled DNA probably because of its participation in a later step of the reaction pathway.     

Inducible SOS response system of DNA repair and mutagenesis in Escherichia coli

Chromosomal DNA is exposed to continuous damage and repair. Cells contain a number of proteins and specific DNA repair systems that help maintain its correct structure. The SOS response was the first DNA repair system described in Escherichia coli induced upon treatment of bacteria with DNA damaging agents arrest DNA replication and cell division. Induction of the SOS response involves more than forty independent SOS genes, most of which encode proteins engaged in protection, repair, replication, mutagenesis and metabolism of DNA. Under normal growth conditions the SOS genes are expressed at a basal level, which increases distinctly upon induction of the SOS response. The SOS-response has been found in many bacterial species (e.g., Salmonella typhimurium, Caulobacter crescentus, Mycobacterium tuberculosis), but not in eukaryotic cells. However, species from all kingdoms contain some SOS-like proteins taking part in DNA repair that exhibit amino acid homology and enzymatic activities related to those found in E. coli. but are not organized in an SOS system. This paper presents a brief up-to-date review describing the discovery of the SOS system, the physiology of SOS induction, methods for its determination, and the role of some SOS-induced genes.     

Frameshift mutagenesis by chloroquine in Escherichia coli and Salmonella typhimurium

Chloroquine can be detected as a direct-acting mutagen in plate-incorporation assays using the excision-deficient Salmonella typhimurium strain TA97, but very much more effectively using the repair-proficient Escherichia coli strain DG1669 which carries the lacZ19124 marker. When tested at concentrations of 200-1000 micrograms/plate with strain DG1669, the mutagenicity of chloroquine is enhanced by the addition of Aroclor-induced rat-liver S9. Further experiments indicated that chloroquine-induced reversion frequencies were essentially identical in wild-type, recA, umuC and uvrC derivatives of DG1669, as well as in strains carrying the mutation enhancing plasmid pKM101, over a wide range of doses (0-1200 micrograms/plate). These results suggest that neither excision repair nor SOS-type repair are important in chloroquine-induced frameshift mutagenesis.     

Polymerases and UV mutagenesis in Escherichia coli

Evidence for and against the involvement of the known nucleic acid polymerases in UV mutagenesis in Escherichia coli is reviewed. There is no evidence that rules out the participation of any of them when they are present but only one, the alpha subunit of DNA polymerase III holoenzyme (polC gene product) has been shown to be essential. It is argued that the PolC protein that functions in UV mutagenesis may not be immediately recognizable as one of the normal cellular polymerases or polymerase complexes.