Abstract RNA polymerase (RPase) from E. coll contains two tightly incorporated Zn(II) ions, while the monomeric RPase from bactenophage T7 does not contain zinc and does not require Zn(II) in the assay. One of the two Zn(II) ions can be differentially removed from E coli RPase with p-hydroxymer-curiphenylsulfonate (PMPS) combined with EDTA and thiol. The resultant Zn 1 or Zn A RPase shows no alteration in transcription initiation and elongation rate from σ-specific promoters. Biosynthesis of a Co 2 RPase and formation of Co A RPase by similar treatment shows the tetrahedral-type Co(II) d-d absorption bands to be associated only with the Co(II) at the A site with maxima at 760 (ϵ = 800), 710 (ϵ = 900), 602 (ϵ = 1500), and 484 (ϵ = 4000) nm. Sulfur to Co(II) charge transfer bands are present at 350 (ϵ = 9600) and 370 (ϵ = 9500) nm. The absorption characteristics strongly suggest that the A site is a tetrathiolate site. While DNA polymerases do not in general appear to contain zinc, gene 32 protein (g32P) from bactenophage T4, an accessory protein essential for DNA replication and recombination and translational control in the T4 life cycle, is a Zn(II) metalloprotein and contains 1 gram atom of tightly incorporated Zn(II) PMPS displaces the zinc by reacting with three SH groups. Apo-g32P shows markedly altered DNA binding properties. Co(II) substitution gives a protein with intense d-d transitions typical of a tetrahedral Co(II) complex with absorption maxima at 680 (ϵ = 480), 645 (ϵ = 660), 605 (ϵ = 430), 355 (ϵ = 2250), and 320 (ϵ = 3175) nm. The data support a 3 Cys, 1 His coordination site located in the middle of the DNA binding domain of g32P. Data thus far suggest that the Zn(II) binding sites in multisubunit RNA polymerases and in accessory proteins involved in polynucleotide biosynthesis are more likely to play structural or allosteric (regulatory) roles rather than directly participating in catalysis.