ИСТИНА

G-quadruplexes (G4) are present in living cells of many organisms and can perform regulatory functions. However, the formation of these noncanonical structures is also associated with genome instability. It is known that G4s have a variety of effects on the functioning of base (BER) and nucleotide (NER) excision repair, as well as mismatch repair (MMR) systems, manifested both in attenuation and activation of repair along with no effect in different cases. A fragment of human telomerase reverse transcriptase (hTERT) gene promoter is capable of forming a noncanonical structure consisting of three G-quadruplexes (hTERT-G4). The study of hTERT-G4 in relation to repair systems is of great interest, since certain positions in it are often subjected to point mutations in some types of malignant tumors, including 60-80% cases of urothelial carcinomas, melanomas and glioblastomas. We investigated the effect of most wide-spread G228A, G250A, and double G242A/G243A mutations in single-stranded hTERT-G4 on recognition by key proteins of E. coli MMR — MutS and MutL, which were previously shown to preferentially bind perfect G4. As demonstrated by DMS footprinting, these mutations do not significantly impede the formation of G4 structure as a whole compared to the native sequence and only caused a destabilization of mutated G-tracts indicating an imperfect G4 arrangement. Accordingly, the mutations do not interfere with the affinity of MMR proteins towards the hTERT-G4 variants which remains almost as high as for wild-type sequence. The analysis of the activity of E. coli uracil-DNA glycosylase (UDG) — a BER component, towards hTERT-G4 substrates with different positions of deoxyuridine relative to three tandem G4s is in progress.