Ethyl methanesulfonate

Ethyl methanesulfonate (EMS) is a mutagenic, teratogenic, and carcinogenic organic compound with formula C3H8SO3. It produces random mutations in genetic material by nucleotide substitution; particularly through G:C to A:T transitions induced by guanine alkylation. EMS typically produces only point mutations. Due to its potency and well understood mutational spectrum, EMS is the most commonly used chemical mutagen in experimental genetics. Mutations induced by EMS exposure can then be studied in genetic screens or other assays. EMS can induce mutations at a rate of 5x10−4 to 5x10−2 per gene without substantial killing. A 5x10−4 per gene mutation rate observed in a typical EMS mutagenesis experiment of the model organism C. elegans, corresponds to a raw mutation rate of ~7x10−6 mutations per G/C base pair, or about 250 mutations within an originally mutagenized gamete (containing a ~100 Mbp, 36% GC haploid genome). Such a mutagenized gamete would have about 9 different loss-of-function mutations in genes, with 1 to 2 of these mutations being within essential genes and therefore lethal. However, since it is unlikely the same essential gene is mutated in independent gametes, and if loss of the essential gene did not kill the gamete itself, downstream gamete fusion often allows for survival of the resulting zygote and organism, as the now heterozygous non-functional mutated allele may be rescued by the still wildtype allele provided by the other gamete. The ethyl group of EMS reacts with guanine in DNA, forming the abnormal base O6-ethylguanine. During DNA replication, DNA polymerases that catalyze the process frequently place thymine, instead of cytosine, opposite O6-ethylguanine. Following subsequent rounds of replication, the original G:C base pair can become an A:T pair (a transition mutation). This changes the genetic information, is often harmful to cells, and can result in disease. RNA polymerase can also place uridine (RNA analog of thymine) opposite an O6-ethylguanine lesion.