The Cys4 zinc finger of bacteriophage T7 primase in sequence-specific single-stranded DNA recognition

Bacteriophage T7 DNA primase recognizes 5′-GTC-3′ in single-stranded DNA. The primase contains a single Cys 4 zinc-binding motif that is essential for recognition. Biochemical and mutagenic analyses suggest that the Cys 4 motif contacts cytosine of 5′-GTC-3′ and may also contribute to thymine recognition. Residues His 33 and Asp 31 are critical for these interactions. Biochemical analysis also reveals that T7 primase selectively binds CTP in the absence of DNA. We propose that bound CTP selects the remaining base G, of 5′-GTC-3′, by base pairing. Our deduced mechanism for recognition of ssDNA by Cys 4 motifs bears little resemblance to the recognition of trinucleotides of double-stranded DNA by Cys 2 His 2 zinc fingers.