The Cys(4) 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. T he primase contains a single Cys(4) zinc-binding motif that is essential fo r recognition. Biochemical and mutagenic analyses suggest that the Cys(4) m otif contacts cytosine of 5'-GTC-3' and may also contribute to thymine reco gnition. 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 ss DNA by Cys(4) motifs bears little resemblance to the recognition of trinucl eotides of double-stranded DNA by Cys(2)His(2) zinc fingers.