THE ROLE OF THE ZINC MOTIF IN SEQUENCE RECOGNITION BY DNA PRIMASES

The DNA primase of bacteriophage T7 has a zinc-binding motif that is e ssential for the recognition of the sequence 3'-CTG-5'. The T7 primase also catalyzes helicase activity, a reaction coupled to nucleotide hy drolysis. We have replaced the zinc motif of the T7 primase with those found in the gene 61 primase of phage T4 and the DnaG primase of Esch erichia coli. The T4 and E. coil primases recognize the sequences 3'-T (C/T)G-5' and 3'-GTC-5', respectively. Both chimeric proteins can part ially replace T7 primase in vivo. The two chimeric primases catalyze t he synthesis of oligoribonucleotides albeit at a reduced rate and DNA dependent dTTPase activity is reduced by 3-10-fold. Both chimeric prot eins recognize 3'-(A/G)CG-5' sites on single-stranded DNA, sites that differ from those recognized by the T7, T4, or E. coil primases, indic ating that the zinc motif is only one determinant in site-specific rec ognition.