AN N-TERMINAL FRAGMENT OF THE GENE-4 HELICASE PRIMASE OF BACTERIOPHAGE-T7 RETAINS PRIMASE ACTIVITY IN THE ABSENCE OF HELICASE ACTIVITY/

Primase and helicase activities of bacteriophage T7 are present in a s ingle polypeptide coded by gene 4, Because the amino terminal region o f the gene 4 protein contributes to primase activity, we constructed a truncated gene 4 encoding the N-terminal 271-aa residues. The truncat ed protein, purified from cells overexpressing the protein, is a dimer in solution; the full-length protein is a hexamer, Although the fragm ent is devoid of dTTPase and helicase activities, it catalyzes templat e-directed synthesis of di-, tri-, and tetranucleotides. The rates for tetraribonucleotide synthesis and for dinucleotide extension on a 20- nucleotide template are similar for the full-length and truncated prot eins. However, the activity of the primase fragment is unaffected by d TTP whereas the primase activity of the full-length protein is stimula ted >14-fold. The primase fragment is defective in the interaction wit h T7 DNA polymerase in that primer synthesis cannot be coupled to DNA synthesis.