Thioredoxin is a small protein participating in a number of oxidation-reduction and regulatory functions in various cells. In addition, thioredoxin is required for replication or assembly of some bacteriophages. Thioredoxin is built up of two domains. To investigate the structure-function relationship of thioredoxin, a C-terminal mutant of E. coli thioredoxin was constructed by the use of a single ClaI site on the trxA gene encoding thioredoxin. Plasmid pCK1, which resulted from plasmid pCJF4 carrying E. coli wild-type thioredexin gene by ClaI digestion, filling-up, and ligation, produced a C-terminal mutant thioredoxin containing very different C-terminal domain. The C-terminal mutant thioredoxin was able to weakly support the growth of M13 bacteriophage. This mutant protein was in vivo characterized in other aspects and its apparent production was confirmed.