The Gly74→Ser and Ser3→Ala Mutations in Rhodobacter sphaeroides Y Thioredoxin

In this study, we report the effects of two different substitutions in Rhodobacter sphaeroides thioredoxin on two regions of the protein: the N‐terminus end and the hydrophobic area implicated in protein/protein interactions. We have produced by site‐directed mutagenesis R. sphaeroides thioredoxin single and double mutants in which the glycine residue at position 74 is changed to a serine and the serine: it position 3 is changed to an alanine; the three mutant proteins have been purified. The two substitutions are not equivalent. Substitution of serine by alanine increased the pI from 5.2 to 6.1; this pI value was the same in the double‐mutated protein, which demonstrates the presence of a local conformational change. In vivo studies showed that the Gly74→Ser substitution completely prevented phage T3/7 growth whereas the Ser3→Ala substitution had no effect. This finding was corroborated by the large decrease (100‐fold) of polymerase activity for the double mutant in the in vitro measurement of phage T7 DNA polymerase activity with the corresponding pure proteins. Although marginal(within a factor of two), the effects of the two substitutions on the catalytic activities of the thioredoxin reductase reaction confirmed their difference. Substitution of serine by alanine had no effect on the K m and resulted in an improvement in the catalytic efficiency. In contrast, the second substitution increased the K m value, without improving the catalytic efficiency. The following can be concluded (a) glyciiie74 of R. sphaeroides thioredoxin has a direct role in the binding of T7 gene 5 protein and the hydrophobic area of thioredoxin: (b) the N‐terminus plays a role in maintaining the conformational integrity of the active site; (c) the flexibility of Gly74 in the hydrophobic region involved in protein/protein interaction is the operative factor in the case of the activity of thioredoxin in the T7 DNA polymerase.