Methyl phosphotriesters in alkylated DNA are repaired by the Ada regulatory protein of E. coli

Mutant Escherichia coli Ada proteins simultaneously defective in the repair of O6-methylguanine and in gene activation

The activated Ada protein triggers expression of DNA repair genes in Escherichia coli in response to alkylation damage. Ada also possesses two distinct suicide alkyltransferase activities, for O6-alkylguanines and for alkyl phosphotriesters in DNA. The mutant Ada3 and Ada5 transferases repair O6-methylguanine in DNA 20 and 3000 times more slowly, respectively, than the wild-type Ada protein, but both exhibit normal DNA phosphotriester repair. These same proteins also exhibit delayed and sluggish induction of the ada and alkA genes. Since the C-terminal O6-methylguanine methyltransferase domain of Ada is not implicated in the direct binding of specific DNA sequences, this part of the Ada protein is likely to play an alternative mechanistic role in gene activation, either by promoting Ada dimerization, or via direct contacts with RNA polymerase.

Alkylation mutagenesis in Saccharomyces cerevisiae: lack of evidence for an adaptive response

We have found no evidence for an adaptive response for either lethality or mutagenesis following treatment of Saccharomyces cerevisiae with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). The rad6 and rad52 mutants of S. cerevisiae are highly defective in MNNG and ethyl methanesulfonate induced mutagenesis of both stationary and exponential phase cells. These and other observations indicate that the mechanisms of repair of alkylation damage and mutagenesis differ markedly between S. cerevisiae and Escherichia coli.

The intracellular signal for induction of resistance to alkylating agents in E. coli

The E. coli ada gene positively controls its own expression and that of other genes (alkA, alkB, aidB) involved in repair of DNA alkylation damage. The cloned ada and alkA genes and purified Ada protein have been used in cell-free systems to identify the inducing signal. Self-methylation of the Ada protein by transfer of a methyl group from a phosphotriester in alkylated DNA to a cysteine residue in the protein converts it to an activator of transcription. The covalently modified Ada protein binds specifically to promoter regions containing the sequence d(AAANNAAAGCGCA) immediately upstream of the RNA polymerase binding sites. This is apparently the first example of conversion of a regulatory gene product to a transcriptional activator by a posttranslational modification event.

Exposure of HeLa cells to 0(6)-alkylguanines increases sensitivity to the cytotoxic effects of alkylating agents

Exposure of HeLa S3 cells to 0.4mM 0(6)-methylguanine or 0(6)-n-butylguanine for 24 h led to a substantial decrease in the activity of 0(6)-alkylguanine-DNA alkyltransferase. Such pretreatment caused a marked increase in the sensitivity of the cells to the cytotoxic effects of the cross-linking alkylating agent 1-(2-chloroethyl)-1-nitroso-3-cyclohexylurea and a smaller increase in the sensitivity to N-methyl-N'-nitro-N-nitrosoguanidine. These results indicate that the repair of DNA by the alkyltransferase plays an important role in the protection of cells from the cytotoxic effects of certain alkylating agents particularly those such as 1-(2-chloroethyl)-1-nitroso-3-cyclohexylurea which ultimately lead to the formation of lethal interstrand cross-links.

Stereospecific removal of methyl phosphotriesters from DNA by an Escherichia coli ada+ extract

The ada+ gene product, a DNA methyltransferase present in extracts from an Escherichia coli strain constitutive for the adaptive response, removes only half of the methyl phosphotriesters from alkylated DNA. Since DNA phosphotriesters occur in two isomeric configurations (denoted Rp and Sp), we examined whether this reflects a stereospecific mode of repair by the methyltransferase. Analysis by reverse-phase HPLC, phosphorus NMR and circular dichroism established that only triesters in the Sp configuration are acted upon by the E. coli extract.

E. coli Ada regulatory protein repairs the SP diastereoisomer of alkylated DNA

Using HPLC and 31P NMR spectroscopy on a chemically synthesized asymmetric mixture of the diastereoisomers of thymidyl(3'----5')thymidyl-O-methyl phosphate absolute configuration has been correlated with chromatographic mobility. The methyl phosphotriester system in alkylated DNA which is repaired by the Ada regulatory protein of E. coli has consequently been established to possess the Sp configuration.

Repair of DNA alkylation adducts in mammalian cells

Carcinogenic alkylating agents, including nitrosamines, are able to alkylate DNA at various sites. This review presents evidence of the high degree of specificity in the type of DNA damage induced by various N-nitroso compounds and in the DNA repair processes among tissues or cells of different species. The O6-alkylguanine DNA alkyltransferase activity in various human and rodent tissues is discussed as well as the detection of O6-methylguanine in human DNA, using monoclonal antibodies and radioimmunoassay. The relevance of these findings to the mechanisms of cancer induction by nitrosamines is discussed.