Figure 6: Trimming of DNA ends by Artemis nuclease. (a) Substrate specificity for trimming of 3′-overhangs. Long overhangs are trimmed 4-5 bases from the single-strand/double-strand junction. Shorter overhangs can be trimmed to as little as 2 bases, but 2 unpaired bases are required on each side of the cleavage site. A 3′-PG can substitute for the 3′-terminal base. (b) Trimming of blunt ends by Artemis. Several bases are first removed endonucleolytically from the 5′-terminal strand (which can be either phosphate- or hydroxyl terminated), generating a 3′-overhang that is then also trimmed, whether it is terminated in a 3′-PG ( ) or a 3′-hydroxyl. All endonucleolytic trimming by Artemis requires autophosphorylated DNA-PK.