ИСТИНА

Guanine quadruplexes in DNA (hereafter G4) are four-stranded non-canonical forms of DNA. Present in genomes of living organisms, G4 were shown to perform important regulatory functions. However, they contribute to genome instability alongside. A number of proteins are endowed with the ability of interaction with G4. As was discovered previously, mismatch repair (MMR) protein MutS from E. coli and H. sapiens is capable of specific and efficient tetramolecular G4 binding. MMR system corrects the mistakes of DNA polymerase and therefore serves to genome stability. MutS is the protein responsible for misincorporated nucleotides recognition and recruitment of MutL protein which introduces single-strand cut alone or by the engagement of MutH protein in some γ-proteobacteria including E. coli. There is no available data on MutL and MutH interaction with G4 as well as additional information is required to comprehend the authentic role and mechanism of interaction between MutS and G4. Therefore, in order to investigate the G4 influence on the initial steps of MMR we suggested DNA model system that included the motif of parallel intramolecular G4 flanked with duplex regions carrying MutH recognition site and GT-mismatch in some cases. DNA with (GT)n-loop of exact the same size as G4 motif flanked with corresponding duplex sequences was used as a control. The presence of G4 structure in conditions employed in further experiments was proved by dimethylsulphate-footprinting. Applying gel mobility shift assay we investigated the binding of MutS from E. coli and Rhodobacter sphaeroides to fluorophore-labeled G4 DNA constructions in conditions providing different MutS conformations. The determined affinity of both MutS proteins hereby changed in the row G4 DNA > DNA with GT-mismatch, DNA with (GT)n-loop > canonical DNA duplex if ADP was present as binding cofactor, yet the difference in affinity between G4 and other DNA molecules greatly increased upon ATP or ATPγS addition. The proposed DNA constructions were thereafter subjected to hydrolysis induced by R. sphaeroides MutL (rsMutL) possessing the endonuclease function or E. coli MutH. The hydrolysis efficiency was demonstrated to be not dependent of G4 presence in DNA duplex in case of MutH. However in case of rsMutL the shift between yields of non-specific hydrolysis products was observed for G4 DNA compared to canonical DNA duplex. Hence, in this work for the first time the potential influence of intramolecular G4 DNA on MMR functioning was demonstrated.