Cross-Species Investigation into the Requirement of XPA  for Nucleotide Excision Repair
After reconstitution of nucleotide excision repair (excision repair) with XPA, RPA, XPC, TFIIH, XPF-ERCCI, and XPG in vitro, it was concluded that these six factors are the minimal essential components of the excision repair machinery. All six factors are highly conserved across diverse organisms spanning yeast, worms, fruit flies, and humans. Yet, no identifiable homolog of the XPA gene exists in many eukaryotes including green plants. Nevertheless, excision repair is reported to be robust in at least one XPA-lacking organism, Arabidopsis thaliana, which raises a fundamental question of whether excision repair could occur without XPA in other organisms. Here we performed a phylogenetic analysis of XPA across all species with annotated genomes and then quantitatively measured excision repair in the absence of XPA using the sensitive whole-genome qXR-Seq method in human cell lines and two model organisms, Drosophila melanogaster and Caenorhabditis elegans. We found that although the absence of XPA results in inefficient excision repair and extreme UV-sensitivity in humans, flies, and worms, excision repair of UV-induced DNA damage is detectable over the background. These studies have yielded a significant discovery regarding the evolution of XPA protein and its mechanistic role in nucleotide excision repair.