[Mechanisms of targeted frameshift mutations--insertion formation under error-prone or SOS synthesis of DNA...

Up to now the mechanism of formation of frameshift mutations caused by cyclobutane pyrimidine dimers has not been yet explained satisfactorily. Mechanisms of different mutations are usually considered in polymerase model. Here, the alternative polymerase-tautomer model of ultraviolet mutagenesis is developed. The mechanism of targeted insertion formation caused by cis-syn cyclobutane thymine dimers is proposed. Insertions are mutations when one or several DNA bases are inserted.Targeted insertions are mutations of a frameshift type--when one or severalnucleotides are inserted opposite damageswhich may stop synthesis of DNA. Targeted insertions are induced bycyclobutane pyrimidine dimmers. Ultraviolet irradiation may result in a change of tautomer state of DNA bases. A thymine base may form 5 rare tautomer forms that are stable if the base is a part of cyclobutane dimer. As it was shown by structural analysis, one rare tautomeric form of thymine forms hydrogen bonds with no one canonical DNA base. Therefore, under SOS or error-prone synthesis of DNA containing cis-syn cyclobutane thymine dimers with such rare tautomeric_form a specialize or modified DNA polymerase leaves a single nucleotide gap opposite the cis-syn cyclobutane thymine dimer. According to Streisinger model, if the DNA composition within this region is homogeneous, the end of the growing DNA strand can slip and form complementary pairs with a template nucleotide neighboring to the dimer of such type a loop is formed. Further elongation of the daughter strand leads to the appearance of targeted insertion in the daughter strand. Here, it is first shown that cis-syn cyclobutane thymine dimers with one or both bases in the specific tautomer conformation--opposite which it is impossible to insert a canonical base with a hydrogen bond formation--results in targeted insertions. Moreover, the model of forming targeted single--and several-base insertions is developed. The polymerase-tautomer model of ultraviolet mutagenesis can explain a nature and mechanism of appearing not only hot and cold spots, targeted and untargeted base substitution mutations but also targeted frameshift mutations.