A conserved 5′ to 3′ exonuclease activity in the yeast and human nucleotide excision repair proteins RAD2 and XPG

Saccharomyces cerevisiae RAD2 protein and its human homolog xeroderma pigmentosum group G (XPG) protein function in the incision step of nucleotide excision repair of DNA damaged by ultraviolet light. Both RAD2 and XPG proteins have been shown previously to possess an endonuclease activity. Using DNA substrates labeled at either the 5′ end or 3′ end, we now demonstrate that RAD2 protein also digests both single-stranded and double-stranded DNAs exonucleolytically with a 5′ to 3′ directionality. A 5′ to 3′ exonuclease activity is also present in the XPG protein, indicating evolutionary conservation of this activity. The possible role of RAD2 and XPG 5′ to 3′ exonuclease activity in nucleotide excision repair is discussed.