This domain is found at the C-terminal end of the small subunit (PriS) of the core primase from the archaeon Sulfolobus solfataricus, an RNA polymerase that catalyzes the synthesis of the RNA primers required by the DNA polymerases. PriS is the catalytic subunit, with the active site located within an exposed groove in the concave side of two divergent beta-sheets, surrounded on the outside by alpha-helices This C-terminal domain, which is not found in other homologues, shows a mixed alpha-beta fold. It seems to support and position the elongated hairpin structure leading into the zinc-binding motif (PriS-Zn) [1,2].

1: Structure of the heterodimeric core primase. Lao-Sirieix SH, Nookala RK, Roversi P, Bell SD, Pellegrini L; Nat Struct Mol Biol. 2005;12:1137-1144. PMID:16273105

2: Primer synthesis by a eukaryotic-like archaeal primase is independent of its Fe-S cluster. Holzer S, Yan J, Kilkenny ML, Bell SD, Pellegrini L; Nat Commun. 2017;8:1718. PMID:29167441