Fig. 3.

The primase and helicase activities of the T4 primosome are optimal at an approximately 6∶1 gp41∶gp61 subunit ratio. RNA primer synthesis activity (filled circles) measures the amount of [α³²P]rCTP incorporated into RNA primer pentamers after a reaction time of 20 min (see Materials and Methods). Increasing concentrations of gp61 subunits were titrated into a reaction mix containing a fixed concentration of gp41 helicase hexamers. The priming activity reached a maximum at a gp61 concentration ratio somewhat in excess of one primase subunit per gp41 helicase hexamer and then decreased progressively as more gp61 was added (see text). The helicase data were taken from Fig. 4 of Richardson and Nossal (5), and show that the unwinding activity of either 50 pmols (open squares) or 100 pmols (open circles) of T4 gp41 hexamer on a helicase substrate consisting of a 3-tailed ssDNA M13 template complexed with a 51 bp duplex construct reached a plateau value at a gp61 subunit concentration ratio of approximately one gp61 subunit per gp41 hexamer.