Fig. 6.

Model of primer length determination by Sulfolobus primase. The initiating nucleotide, which is bound by PriX and retains its 5′ triphosphate, is shown as a red circle; subsequent nucleotides, added via the PriS polymerization active site, are shown as black circles. We propose that PriX remains engaged with the 5′ end of the growing RNA chain during primer synthesis. The primer length is thus defined by the maximum spatial distance that can be accommodated between the PriX and PriS nucleotide-binding sites.