Giardia duodenalis: Base excision repair pathway enzyme FEN1 carries out catalytic activities pertaining to NER pathway

Giardia duodenalis is a binuclear protozoan that causes intestinal infection in humans and animals. The life cycle of G. duodenalis is comprised by 2 stages: trophozoite (vegetative, ploidy: 4 N) and cyst (infective, ploidy: 8-16 N) and the transition from one to another requires a precise coordination as well as the support of the DNA repair machinery. While DNA homologous recombination DNA repair has recently been characterized, NER and BER are pathways that had not been explored. Most of the structure specific enzymes (SSE) participate in a variety of processes like DNA replication stress, DNA adduct repair and Holliday junction processing. In an effort to explore these kinds of enzymes in G. duodenalis, a minimalist parasite, we aimed at characterizing the Fen1 enzyme by cloning its gene to study its catalytic properties (binding and nuclease) using flap and bubble DNA substrates. Unexpectedly, we found that GdFen-1 is able to cleave bubble DNA, then to shed light on which domains of this enzyme are responsible for this activity, giardial acid block and a portion of a cap region were substituted by their human counterparts, and while acid block substitution did not affect this activity, the modification in the cap region did. The possible implications of these findings are addressed.