Fig. 6.

The arrangement of gp4, gp5/trx, and the primer/template in the priming complex. (A) The RPD of one subunit within the hexamer and the ZBD of an adjacent subunit position ATP and CTP at the primase recognition site, and the catalytic site within the RPD catalyzes the formation of the dinucleotide pppAC. (B) The RPD synthesizes a tetraribonucleotide, and gp5/trx is recruited to the primer/template. (C) The contacts of the ZBD and RPD with gp5/trx stabilize the tetraribonucleotide in the DNA binding crevice. Both RPD and ZBD make contacts with the cryptic cytosine. Gp5/trx extends the tetraribonucleotide.