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J Biol Chem, Vol. 273, Issue 33, 21246-21252, August 14, 1998

A Carboxyl-terminal Cys₂/His₂-type Zinc-finger Motif in DNA Primase Influences DNA Content in Synechococcus PCC 7942

From the Department of Biochemistry and Genetics, The Medical School, University of Newcastle, Newcastle NE2 4HH, United Kingdom

The DNA primase gene, dnaG, has been isolated from the cyanobacterium Synechococcus PCC 7942. It is not part of a macromolecular synthesis operon but is co-transcribed with pheT and located adjacent to the metallothionein divergon, smt. At the carboxyl terminus of this DnaG is a Cys₂/His₂ zinc-finger motif. The carboxyl-terminal 91 residues bound ⁶⁵Zn and 0.95 g atom of Zn²⁺ mol¹ were detected with 4-(2-pyridylazo)resorcinol. Following exposure to Cd²⁺, 0.95 g atom of Cd²⁺ was displaced by 2 equivalents of p-(hydroxymercuri) phenylsulfonate mol¹, while only 0.03 g atom of Cd²⁺ was displaced mol¹ polypeptide missing the carboxyl-terminal (residue 592 onward) zinc-finger motif. Zn²⁺ caused an increase in intensity, and a reduction in wavelength, of Trp fluorescence at the tip of the predicted zinc-finger, while EDTA caused the converse. Cells containing a single chromosomal codon substitution (C597S), altering the zinc-finger, were generated by exploiting Zn²⁺-sensitive smt mutants and the proximity of dnaG to smt. Cells in which smt and dnaG(C597S) had integrated into the chromosome were selected via restored Zn²⁺ tolerance. Synechococcus PCC 7942 and its dnaG(C597S) mutant grew at equivalent rates, but the latter had a reduced number of chromosomes.

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