Interaction of mutant thioredoxins of Escherichia coli with the gene 5 protein of phage T7. The redox capacity of thioredoxin is not required for stimulation of DNA polymerase activity.
DNA polymerase activity in Escherichia coli cells infected with bacteriophage T7 resides in a protein complex consisting of the T7 gene 5 protein and E. coli thioredoxin in a 1 to 1 stoichiometry. We have analyzed nine mutant thioredoxins, both in vivo and in vitro, for their ability to interact with the T7 gene 5 protein and stimulate the DNA polymerase and exonuclease activities inherent in gene 5 protein. The efficiency of plating of T7 on E. coli thioredoxin mutants depends strongly on the copy number of the respective mutant thioredoxin allele. Plating efficiencies at a constant copy number correlate well with the affinity of the purified mutant proteins for T7 gene 5 protein. The observed dissociation constant, Kobs, is increased between 5 and several hundredfold at 42 degrees C compared to wild-type thioredoxin. The maximum polymerase activity of the reconstituted gene 5 protein-thioredoxin complex at saturating concentrations of mutant thioredoxins, however, is reduced by less than 20%. Consequently, none of the mutant thioredoxins acts as a competitive inhibitor of wild-type thioredoxin. The active-site disulfide of thioredoxin is not essential for the activities of the gene 5 protein-thioredoxin complex. Both cysteines can be replaced without significantly affecting the maximum polymerase or exonuclease activities. Substitution or alkylation of either cysteine, however, reduces the affinity for gene 5 protein drastically, indicating that the active site is part of the thioredoxin surface involved in the protein-protein interaction.