LexA-independent expression of a mutant mucAB operon

McNally, Karen Perry; Freitag, Nancy E.; Walker, Graham C.

doi:10.1128/jb.172.11.6223-6231.1990

Cited by 20 publications

(8 citation statements)

References 49 publications

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“…ss AFB, and AP site-containing genomes (Fig. 1) (34,35). Mutation frequencies were determined in both UV-and UV+ cells in order to examine the effect of the normal E. coli SOS mutagenic processing analog umuDC.…”

Section: Resultsmentioning

confidence: 99%

Mutational properties of the primary aflatoxin B1-DNA adduct.

Bailey

Iyer

Stone

et al. 1996

Proc. Natl. Acad. Sci. U.S.A.

159

143

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The mutagenic activity of the major DNA adduct formed by the liver carcinogen aflatoxin B1 (AFB1) was investigated in vivo. An oligonucleotide containing a single 8,9-dihydro-8-(N7-guanyl)-9-hydroxyaflatoxin B1 (AFB1-N7-Gua) adduct was inserted into the single-stranded genome of bacteriophage M13. Replication in SOS-induced Escherichia coli yielded a mutation frequency for AFBI-N7-Gua of 4%. The predominant mutation was G --T, identical to the principal mutation in human liver tumors believed to be induced by aflatoxin. The G -> T mutations of AFB,-N7-Gua, unlike those of the AFBI-N7-Gua-derived apurinic site, were much more strongly dependent on MucAB than UmuDC, a pattern matching that in intact cells treated with the toxin. It is concluded that the AFB,-N7-Gua adduct, and not the apurinic site, has genetic requirements for mutagenesis that best explain mutations in aflatoxin-treated cells. While most mutations were targeted to the site of the lesion, a significant fraction (13%) occurred at the base 5' to the modified guanine. In contrast, the apurinic site-containing genome gave rise only to targeted mutations. The mutational asymmetry observed for AFB1-N7-Gua is consistent with structural models indicating that the aflatoxin moiety of the aflatoxin guanine adduct is covalently intercalated on the 5' face ofthe guanine residue. These results suggest a molecular mechanism that could explain an important step in the carcinogenicity of aflatoxin B1.The fungal metabolite aflatoxin B1 (AFB1) contaminates the food supply in eastern Asia and sub-Saharan Africa, where it is associated with an increased incidence of hepatocellular carcinoma (HCC) (1). AFB1 requires metabolic conversion to its exo-8,9-epoxide (2, 3) in order to cause damage to DNA (4), the event that presumably initiates the genetic changes resulting in HCC (5). The AFB1 epoxide reacts with guanine to form a population of adducts (6), the principal of which both in vitro (3, 7-9) and in vivo (8,(10)(11)(12)) is 8,9-dihydro-8-(N7-guanyl)-9-hydroxyaflatoxin B1 (AFBl-N7-Gua). The positively charged imidazole ring of this adduct promotes depurination, resulting in apurinic (AP) site formation. Alternatively, under slightly basic conditions the imidazole ring of AFB1-N7-Gua opens to form the chemically and biologically stable AFB1 formamidopyrimidine (AFBI-FAPY) (1). The initial AFB,-N7-Gua adduct, the AFB1-FAPY, and the AP site, individually or collectively, represent the likely chemical precursors to the genetic effects of AFB1.The nature of mutations induced by electrophilic derivatives of AFB1 has been studied in several forward mutation assays.Upon examining AFB1 mutagenesis in the endogenous Escherichia coli lacI gene, Foster et al. (13) (20,21). The same mutations are observed in human hepatocytes grown in culture (22).The genetic studies cited above indicate that several mutations occur in DNA globally modified by AFB1, but the predominant mutation induced or selected in vivo appears to be the GC -> TA transversion. At least three DNA lesio...

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Section: Resultsmentioning

confidence: 99%

Mutational properties of the primary aflatoxin B1-DNA adduct.

Bailey

Iyer

Stone

et al. 1996

Proc. Natl. Acad. Sci. U.S.A.

159

143

View full text Add to dashboard Cite

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“…Collections of binding sites to construct both PSWM were obtained from previous publications. For LexA1, 23 experimentally verified binding sites from E. coli (15,23,29,31) were used to construct the PSWM, while seven LexA2 binding sites from several Pseudomonas and Xanthomonas species were used to construct the LexA2 PSWM (1,41). Genomic sequences were all downloaded from the NCBI GenBank repository through its FTP server (ftp.ncbi.nlm.nih.gov).…”

Section: Methodsmentioning

confidence: 99%

Cohabitation of Two DifferentlexARegulons inPseudomonas putida

et al. 2007

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In contrast to the vast majority of the members of the domain Bacteria, several Pseudomonas and Xanthomonas species have two lexA genes, whose products have been shown to recognize different LexA binding motifs, making them an interesting target for studying the interplay between cohabiting LexA regulons in a single species. Here we report an analysis of the genetic composition of the two LexA regulons of Pseudomonas putida KT2440 performed with a genomic microarray. The data obtained indicate that one of the two LexA proteins (LexA1) seems to be in control of the conventional Escherichia coli-like SOS response, while the other LexA protein (LexA2) regulates only its own transcriptional unit, which includes the imuA, imuB, and dnaE2 genes, and a gene (PP_3901) from a resident P. putida prophage. Furthermore, PP_3901 is also regulated by LexA1 and is required for DNA damage-mediated induction of several P. putida resident prophage genes. In silico searches suggested that this marked asymmetry in regulon contents also occurs in other Pseudomonas species with two lexA genes, and the implications of this asymmetry in the evolution of the SOS network are discussed.

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“…mucAB also restores inducible mutagenesis to normally nonmutable recA430 and lexA4(Ind-) strains of E. coli (6,7,29,31,57). In this report, we demonstrate that mucAB can also restore mutability to recA433 and recA435 strains.…”

mentioning

confidence: 99%

The enhanced mutagenic potential of the MucAB proteins correlates with the highly efficient processing of the MucA protein

et al. 1992

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Inducible mutagenesis in Escherichia coli requires the direct action of the chromosomally encoded UmuDC proteins or functional homologs found on certain naturally occurring plasmids. Although structurally similar, the five umu-like operons that have been characterized at the molecular level vary in their ability to enhance cellular and phage mutagenesis; of these operons, the mucAB genes from the N-group plasmid pKM101 are the most efficient at promoting mutagenesis. During the mutagenic process, UmuD is posttranslationally processed to an active form, UmuD'. To explain the more potent mutagenic efficiency of mucAB compared with that of umuDC it has been suggested that unlike UmuD, intact MucA is functional for mutagenesis. To examine this possibility, we have overproduced and purified the MucA protein. Although functionally similar to UmuD, MucA was cleaved much more rapidly both in vitro and in vivo than UmuD. In vivo, restoration of mutagenesis functions to normally nonmutable recA430, recA433, recA435, or recA730 A(umuDC)595::cat strains by either MucA+ or mutant MucA protein correlated with the appearance of the cleavage product, MucA'. These results suggest that most of the differences in mutagenic phenotype exhibited by MucAB and UmuDC correlate with the efficiency of posttranslational processing of MucA and UmuD rather than an inherent activity of the unprocessed proteins.

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LexA-independent expression of a mutant mucAB operon

Cited by 20 publications

References 49 publications

Mutational properties of the primary aflatoxin B1-DNA adduct.

Mutational properties of the primary aflatoxin B1-DNA adduct.

Cohabitation of Two DifferentlexARegulons inPseudomonas putida

The enhanced mutagenic potential of the MucAB proteins correlates with the highly efficient processing of the MucA protein

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