To investigate the biological role of thioredoxin in the facultative photosynthetic bacterium , attempts were made to construct a thioredoxin-deficient mutant by site-specific mutagenesis, using the Tn kanamycin resistance gene for selection. and Southern hybridization analyses have demonstrated that the TrxAmutation is lethal for growth under anaerobic conditions with DMSO as terminal electron acceptor and under aerobic conditions. In addition, the DNA region upstream of the initiation codon is essential for aerobic growth of An ORF of unknown function was identified in this region and is suggested to encode a product essential for aerobic metabolism of The mechanism of thioredoxin action was also analysed by using the procedure for gene replacement to introduce a Cys33 to Ser mutation into the chromosomal copy. The strain carrying this mutation produced a thioredoxin impaired in its protein-disulfide reductase activity and was also not viable. These data suggest that the physiological function of thioredoxin is redox-dependent. Thioredoxin purified from was shown to have a glutathione-disulfide oxidoreductase activity typical of glutaredoxins. This unexpected finding suggests that thioredoxin, in contrast to thioredoxin, has the potential to act in GSH-dependent processes. Thus, the fundamental role of thioredoxin in cell growth probably originates from the multiple functions it can serve .


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