Base J, found in nuclear DNA of Trypanosoma brucei, is not a target for DNA glycosylases

Abstract

Base excision repair (BER) is an evolutionarily conserved system which removes altered bases from DNA. The initial step in BER is carried out by DNA glycosylases which recognize altered bases and cut the N-glycosylic bond between the base and the DNA backbone. In kinetoplastid flagellates, such as Trypanosoma brucei, the modified base beta- d-glucosyl-hydroxymethyluracil (J) replaces a small percentage of thymine residues, predominantly in repetitive telomeric sequences. Base J is synthesized at the DNA level via the precursor 5-hydroxymethyluracil (5-HmU). We have investigated whether J in DNA can be recognized by DNA glycosylases from non-kinetoplastid origin, and whether the presence of J and 5-HmU in DNA has required modifications of the trypanosome BER system. We tested the ability of 15 different DNA glycosylases from various origins to excise J or 5-HmU paired to A from duplex oligonucleotides. No excision of J was found, but 5-HmU was excised by AlkA and Mug from Escherichia coli and by human SMUG1 and TDG, confirming previous reports. In a combination of database searches and biochemical assays we identified several DNA glycosylases in T. brucei, but in trypanosome extracts we detected no excision activity towards 5-HmU or ethenocytosine, a product of oxidative DNA damage and a substrate for Mug, TDG and SMUG1. Our results indicate that trypanosomes have a BER system similar to that of other organisms, but might be unable to excise certain forms of oxidatively damaged bases. The presence of J in DNA does not require a specific modification of the BER system, as this base is not recognized by any known DNA glycosylase.