Thioredoxins are ubiquitous proteins which catalyze the reduction of disulfide bridges on target proteins and are involved in many cellular reactions. In a previous work, a thioredoxin from the thermophilic organism Aliciclobacillus acidocaldarius (Alitrx) was purified, characterized, and its gene expressed in Escherichia coh. In order to produce larger quantities of Alitrx, the protein has been expressed in the methylotrophic yeast Pichia pastoris and in the gram positive bacteria Bacillus subtilis. The growth conditions of strains showing high-level expression of Alitrx were optimized for both systems in shake-flask cultures. Active proteins were secreted in the culture media at a level of approximately 0.9 and 0.5 g/L, respectively, for P. pastoris and B. subtilis. The proteins were purified almost to homogeneity by a thermal precipitation procedure, with a 90-fold and 50-fold higher total yield with respect to that obtained with the same protein expressed in E coli. The results indicate that either of these two systems could be utilized as a host for large-scale production of recombinant Alitrx. (C) 2003 Elsevier Science (USA). All rights reserved.