Spontaneous Mutation Rates

The presence of mutator plasmids. Excision-deficient strains containing pKMIOl have a higher spontaneous mutation rate at both base substitution and frameshift loci than excision-proficient strains. 

The importance of these checks together with careful experiment-to-experiment controls of spontaneous mutation rates, response to reference mutation rates and... 

As an alternative to random mutagenesis by error-prone PCR, expression vectors containing the gene of interest may be propagated in mutator E. coli strains like XL 1-Red (Stratagene). This strains contains mutations in three DNA repair pathways and exhibits a more than 5000-fold increased spontaneous mutation rate (3.5 x 10-6), compared to wildtype E. coli (7 x 10-10) [22]. Provided that all mu-... 

The concern over integration of pDNA into the host chromosomal pDNA is a significant safety concern since random integration could result in the activation of a proto-oncogene. To date, in preclinical studies, there is no direct evidence that pDNA integrates into chromosomal DNA following i.m. administration (Nichols et al., 1995 Martin et al., 1999 Ledwith et al., 2000 Manam et al., 2000). It has been determined that if such an event were to occur, it would be well below the level of the natural spontaneous mutation rate and would not pose a significant safety concern (Martin et al., 1999). 

The test for mutagenic activity of the detergent (Tween) extract did not induce any revertants all plate counts were in the range of spontaneous mutation rate, with and without S-9 mix. From the ethanol distillate we got a positive response demonstrated in Table II. In the case of milk-crumb the numbers of induced revertants were about a hundredfold those of the spontaneously reverted colonies with the most sensitive strains (TA 98, 100) only without metabolic activation. In the same experiment the test strain TA 100 responded to the soya-crumb sample, also only without metabolic activation. 

The health consequence of deleterious mutation in populations is poorly understood, despite the large effort that has been expended in the study of mutation. Spontaneous-mutation rates are known only roughly, the forces that maintain or alter gene frequencies in populations are not well understood, and the relative impacts of different types of mutations are obscure. 

The sex-linked recessive-lethal test is the most widely applied test for mutation in Drosophila. It has proved to be sufficiently sensitive to mutagenic activity of chemicals, many of which exert only weak activity at the concentrations used. The test can routinely demonstrate a doubling of the spontaneous-mutation rate. The X-linked recessive-... 

For chemical mutagens, there is no easily measured background exposure for some newly manufactured compounds, the natural background in the past was obviously zero. < it may some day be possible to determine the extent to which the spontaneous-mutation rate is caused by chemicals in the body, but it is not likely that there will be any information nearly as reliable as that on radiation in the next few years. 

Although the subject is in its infancy, it is already clear that a substantial fraction, possibly a majority, of spontaneous mutations in Drosophila have such a cause. If that turns out to be true and generalizable to mammals (there is reason to believe that such elements are present in mice and humans), it will require a revision in our thinking about mutation and mutagens, and it would mean that the kinds of mutagenic mechanisms discussed in this report account for only a part of the spontaneous-mutation rate. On the one hand, we can conclude optimistically that classical mutagens are relatively less important. On the other hand, we cam assume pessimistically that chemicals not detectable with existing test systems increase the rate of transposon-induced mutations. 

That is true of spontaneous, as well as induced, mutation. The current spontaneous-mutation rate may or may pot be too high from the standpoint of evolution (thousands or more generations), but there is no doubt that for several generations in the future we would be better off if our spontaneous-mutation rate (and induced-mutation rate) were lower. 

Perhaps it will be possible to counter the effects of an increase in environmental mutagens by lowering the spontaneous-mutation rate (which must be due in part to chemical influences) with exogenous chemicals. It is not likely that a pill that reduces the mutation rate and possibly reduces carcinogenesis as a byproduct and that has no adverse side effects will suddenly appear. But new research has led to much greater knowledge of what controls the rates of mutation. 

In exponentially growing liquid cultures, the spontaneous mutation rate to lactose utilization (lac+) is high (Table II), which may explain the early appearance of Lac+ papillae. However, neither L-gulonic acid-utilizing nor L-arabitol-utilizing mutants were ever obtained by plating liquid cultures directly onto selective media. 

PMSl). The DNA located between the nick and the mismatch is removed by 3 to 5 or 5 to 3 exonucleases depending on whether the nick is 3 or 5 to the mismatch. The gap is then filled by DNA Pol III in E. coli (Fig. 10) by DNA Pol 5 in humans and is ligated. Mismatch repair increases the spontaneous mutation rate in E. coli and yeast. Humans defective in mismatch repair exhibit a cancer-prone syndrome called hereditary nonpolyposis colon cancer (HNPCC), and sporadic mutations of mismatch repair genes are also associated with up to 50% of all human cancers (42 4). 

We now turn from DNA replication to DNA mutations and repair. Several types of mutations are known (1) the substitution of one base pair for another, (2) the deletion of one or more base pairs, and (3) the insertion of one or more base pairs. The spontaneous mutation rate of T4 phage is about 10 per base per replication. E. coli aaA Drosophila melanogaster have much lower mutation rates, of the order of lO" . 

In bacteria, mutation is an important mechanism by which resistance to antibiotics and other antimicrobial chemicals is achieved, although the receipt of entirely new genes directly from other bacteria is also clinically very important. Spontaneous mutation rates (rates not influenced by mutagenic chemicals or ionizing radiation) vary substantially depending on the gene and the organism in question, but rates of 10-5-10-7 are typical. These values... 

Wild-type Escherichia coli strains exhibit a spontaneous mutation frequency of about 2.5 x 10 4 mutations per 1000 nucleotides of DNA propagated on a pBlue-script-like plasmid after 30 generations of growth. Strains which contain mutations in various DNA repair pathways show a considerable increase in the spontaneous mutation rate which is 5- to 100-fold higher than that of a wild-type... 

---