Sequencing of the DNA region located upstream of the alpha-acetolactate synthase and decarboxylase (alsS-alsD) cluster of Oenococcus oeni allowed identification of an ORF, named trxA. This encodes a protein of 104 amino acids very similar to known thioredoxins. The protein encoded by the cloned fragment was able to complement Escherichia coli strains lacking a functional thioredoxin. Considering the results of protein sequence comparisons and complementation experiments, it was concluded that the trxA gene encodes a functional thioredoxin. Studies of trxA expression showed that the abundance of trxA mRNA was similar during all growth stages. A significant increase in trxA mRNA levels was observed in the presence of hydrogen peroxide in the medium or after heat shock. A single transcriptional start site was determined with total RNA isolated from cells subjected or not subjected to oxidative stress or heat shock. In each case the same promoter region was identified and shown to have a high similarity to the consensus promoter sequence of Gram-positive bacteria, as well as to that of E. coli and the previously mapped promoters from O. oeni.