Catalytically inactive T7 DNA polymerase imposes a lethal replication roadblock

Bacteriophage T7 encodes its own DNA polymerase, the product of gene 5 (gp5). In isolation, gp5 is a DNA polymerase of low processivity. However, gp5 becomes highly processive upon formation of a complex withEscherichia colithioredoxin, the product of thetrxAgene. Expression of a gp5 variant in which aspartate residues in the metal-binding site of the polymerase domain were replaced by alanine is highly toxic toE. colicells. This toxicity depends on the presence of a functionalE. coli trxAallele and T7 RNA polymerase-driven expression but is independent of the exonuclease activity of gp5.In vitro, the purified gp5 variant is devoid of any detectable polymerase activity and inhibited DNA synthesis by the replisomes ofE. coliand T7 in the presence of thioredoxin by forming a stable complex with DNA that prevents replication. On the other hand, the highly homologous Klenow fragment of DNA polymerase I containing an engineered gp5 thioredoxin-binding domain did not exhibit toxicity. We conclude that gp5 alleles encoding inactive polymerases, in combination with thioredoxin, could be useful as a shutoff mechanism in the design of a bacterial cell-growth system.