Search

The SOS chromotest is reviewed through over 100 publications corresponding to the testing of 751 chemicals. 404 (54%) of these chemicals present a genotoxic activity detectable in the SOS chromotest. Their SOS inducing potencies span more than 8 orders of magnitude. For 452 compounds, the results obtained in the SOS chromotest could be compared to those obtained in the Ames test. It was found that 373 (82%) of these compounds give similar responses in both tests (236 positive and 137 negative responses). Thus the discrepancies between both tests concern 79 compounds (18%). A case by case analysis shows that many of these compounds are at the same time very weak SOS inducers and very weak mutagens. Thus we think that, most of the time, the discrepancies between the two tests may be accounted for by differences in the interpretation of the results rather than by the experimental results themselves. However, there are some compounds which are clearly SOS inducers but devoid of mutagenic activity in the Ames test (such as quinoline-1-oxide) and to a larger extent, clearly mutagenic compounds which do not induce the SOS response in the SOS chromotest (such as benzidine, cyclophosphamide, acridines, ethidium bromide). We also analyzed the correlation between SOS induction, mutagenesis and carcinogenesis according to the classification of Lewis. For 65 confirmed carcinogens (class 1), the sensitivity, i.e., the capacity to identify carcinogens, was 62% with the SOS chromotest and 77% with the Ames test. For 44 suspected carcinogens (class 2), the sensitivity was 66% with the SOS chromotest and 68% with the Ames test. Thus, we confirmed previous observations made on 83 compounds that there is a close correlation between the results given by both bacterial tests. The capacity of the Ames test to identify carcinogens is higher than that of the SOS chromotest. However, because the number of false positive compounds was lower in the SOS chromotest, the specificity, i.e., the capacity to discriminate between carcinogens and non-carcinogens of the SOS chromotest, appeared higher than that of the Ames test. Thus, the results of the SOS chromotest and of the Ames test can complement each other. The SOS chromotest is one of the most rapid and simple short-term test for genotoxins and is easily adaptable to various conditions, so that it could be used as an early--perhaps the earliest--test in a battery.

The characterization of target nucleotides involved in the binding to DNA of 7-methoxy-2-nitro-naphtho[2,1-b]furan (R7000), a very potent genotoxic nitrofuran derivative, was investigated. Since R7000 undergoes metabolic activation prior to interacting with DNA, plasmids containing AT-rich and GC-rich sequences were devised and treated by R7000 in bacterial cells presenting nitroreductase activity. The nucleotide modifications to these homogeneous fragments that resulted from R7000 treatment were analyzed using the 'postlabeling' method. A preferential binding to the GC segment was demonstrated. Using a modification of the Maxam-Gilbert sequencing technique, it was demonstrated that activated R7000 creates alkali-labile phosphodiester bonds at the positions of guanines. In addition, the analysis of DNA replication-blocking properties of R7000 lesions was performed using avian myeloblastosis virus (AMV) reverse transcriptase as DNA polymerase. The termination of DNA replication occurred preferentially at the sites of guanine residues in the template strand, indicating that one nucleotide was inserted opposite a lesion. All these results indicate that guanine residues are the preferential sites of formation of R7000-DNA adducts.

In order to investigate the mutational events induced by the very potent genotoxic agent 7-methoxy-2-nitronaphtho[2,1-b]furan (R7000), we evaluated its mutagenic potency in a battery of his- mutants of Salmonella typhimurium designed to study mutagenic specificity (Levin and Ames, 1986). Using this system we could show that R7000 is able to induce base-pair substitutions, mainly GC----TA transversions and, perhaps to a lower extent, AT----TA transversions. In addition, our results suggest that this chemical may be a very efficient inducer of base-pair deletions/insertions resulting in frameshifts. Whereas the induction of base-pair substitutions by R7000 is dependent on the presence of plasmid pKM101 carrying the mucA,B operon, the induction of base-pair deletions/insertions is independent of the presence of the plasmid.

Bacterial tests can be used for several related purposes: the screening for genotoxic agents, genetic analysis of the mode of action of genotoxins and attempts to predict their effects in mammals. We examine various aspects of the assembly of tests into batteries with emphasis on the genetic properties of target bacterial cells. We discuss the problems of carcinogenicity prediction, the identification of particular types of DNA lesions, the study of mutagenic specificity and the elucidation of metabolic steps towards the ultimate genotoxin.

The genotoxic activities of refined smoke flavor (RSF) produced in Poland and used in food processing were investigated in 2 bacterial short-term tests. Its mutagenic activity was examined in the Salmonella/histidine plate assay and its SOS-inducing capacity in the SOS Chromotest both without and with 'activation' by a rat liver homogenate. No genotoxic activity was detected using these 2 bacterial tests.

The SOS Chromotest is a quantitative bacterial colorimetric assay for genotoxins. Substantial validation is now available. We describe the tester strain as well as results of validation studies. Comparison of the results with those obtained in the Mutatest (the Ames test) showed that most (90 percent) of the mutagenic compounds were also SOS inducers. For these compounds, a quantitative correlation was observed between the mutagenic potency and the SOS-inducing potency (SOSIP). The present data indicate that the SOS Chromotest has many practical advantages and may be used as a primary screening tool or as part of a battery of short-term tests for carcinogens. Theoretical considerations on the relation between the SOSIP and the mutagenic potency are briefly discussed.

The existence of damaged-site independent mutagenesis is confirmed here by scoring the appearance of clear-plaque (c-) or virulent (vir) forward mutations on intact (non-irradiated) phage lambda grown on UV-irradiated E. coli K12 hosts. The mutation frequency was measured as a function of the incubation time between the occurrence of host DNA lesions and phage infection. The time course of mutagenesis of intact phage followed the induction pattern observed upon UV-reactivation of UV-damaged phage by Defais et al. (1976). Intact phage did not mutate in UV-irradiated hosts carrying the uvm-25 mutation known to prevent the occurrence of UV-reactivation. These findings suggest that damaged-site independent mutagenesis results from inducible error-prone repair. Clear-plaque mutations arising on intact phage were mostly found in phage bursts consisting of clear and turbid plaque formers whereas UV-damaged phage gave rise to mostly clear-plaque formers. Contrarily to damaged-site dependent mutagenesis, damaged-site independent mutagenesis can arise even at late times during the phage replication cycle. Our data indicate that about half of the phage mutations that arise upon UV-reactivation are damaged-site independent mutations. Replication of intact phage DNA in a host during induction of SOS functions provides a sensitive assay for the detection of damaged-site independent mutagenesis.

We used a gene fusion, placing the lacZ gene encoding beta-galactosidase under the control of the sfiA promoter, to construct a new tester strain for genotoxic agents. The assay is performed in a few hours and involves simple enzymatic assays. The dose response curves contain a linear portion which enables to define the SOS Inducing Potency (SOSIP) of compounds. For the compounds tested SOSIPs extend over 7 decades and correlate generally well with the mutagenic potency assayed in the Salmonella/microsome assay (Mutatest) and in a phage induction assay (Inductest). Sensitivities (lowest amount detected) are comparable in the SOS Chromotest and Mutatest but lower in the Inductest. Our results suggest that at least part of the response in the Mutatest depends on the induction of an SOS function, and that most of the genotoxins are inducer of the SOS system -i.e. can lead to activation of the RecA protease.