Bacteriophage T7 encodes its own DNA polymerase, the product of gene 5. In isolation, gene product 5 (gp5) is a DNA polymerase of low processivity, however, it becomes highly processive upon formation of a complex with E. coli thioredoxin. In the course of investigating the transfer of primers from T7 DNA primase to T7 DNA polymerase, produced a gp5 variant lacking polymerase activity. Expression of a gp5 variant where the metal binding site in the polymerase domain was inactivated was found to be highly toxic to E. coli. This toxicity was strictly dependent on the presence of a functional E. coli trxA allele and T7 RNA polymerase‐driven expression. The closely related Klenow fragment of DNA polymerase I containing an engineered gp5 thioredoxin‐binding domain did not show any toxicity. Purified polymerase‐inactive gp5 drastically inhibited the E. coli replisome in vitro, but only in the presence of thioredoxin. Mutation of two critical metal binding site residues promote the formation of long‐lived protein‐DNA complexes that may block the displacement of the E. coli replication machinery. On the other hand, the inactive gp5 mutant only partially inhibits the T7 replisome, T7 DNA polymerase, or Klenow fragment. We envision that this T7 gp5 allele, in combination with a functional copy of TrxA, can be used a genetic building block in the design of synthetic biology approaches as a cell‐growth shutoff mechanism or a timed protein antibiotic.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.