J. Bacteriol., 12 1996, 6701-6705, Vol 178, No. 23
Copyright © 1996, American Society for Microbiology

Interaction of Escherichia coli primase with a phage G4ori(c)-E. coli SSB complex

W Sun and GN Godson
Biochemistry Department, New York University Medical Center, New York, New York 10016, USA.

We earlier reported that Escherichia coli single-stranded DNA-binding protein (SSB) bound in a fixed position to the stem-loop structure of the origin of complementary DNA strand synthesis in phage G4 (G4ori(c)), leaving stem-loop I and the adjacent 5' CTG 3', the primer RNA initiation site, as an SSB-free region (W. Sun and G. N. Godson, J. Biol. Chem. 268:8026-8039, 1993). Using a small 278-nucleotide (nt) G4ori(c) single-stranded DNA fragment that supported primer RNA synthesis, we now demonstrate by gel shift that E. coli primase can stably interact with the SSB-G4ori(c) complex. This stable interaction requires Mg2+ for specificity. At 8 mM Mg2+, primase binds to an SSB- coated 278-nt G4ori(c) fragment but not to an SSB-coated control 285-nt LacZ ss-DNA fragment. In the absence of Mg2+, primase binds to both SSB- coated fragments and gives a gel shift. T4 gene 32 protein cannot substitute for E. coli SSB in this reaction. Stable interaction of primase with naked G4ori(c). single-stranded DNA was not observed. DNase I and micrococcal nuclease footprinting, of both 5' and 3' 32P- labeled DNA, demonstrated that primase interacts with two regions of G4ori(c): one covering stem-loop I and the 3' sequence flanking stem- loop I which contains the pRNA initiation site and another located on the 5' sequence flanking stem-loop III.

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