l-isoleucine dioxygenase (IDO) specifically transforms l-isoleucine (Ile) to 4-hydroxyisoleucine (4-HIL), and 4-HIL is a promising drug for diabetes. To enhance the activity and catalytic efficiency of IDO, we used directed evolution and site-specific mutagenesis. The IDO gene (ido) derived from Bacillus weihenstephanensis was cloned and expressed in Escherichia coli. Directed evolution using error prone (EP)-PCR and site-specific mutagenesis were conducted. Two improved mutants were obtained after one round of EP-PCR, with Ido(N126H) exhibiting a 2.8-fold increase in activity. Two improved mutants were obtained through site-specific mutagenesis, with Ido(T130K) showing a 170% increase in activity. Although the activity of the combined mutant Ido(N126H/T130K) (0.95 +/- 0.08 U/mg) was slightly higher than that of the wild-type Ido, its catalytic efficiency was 2.4-fold and 3.0-fold higher than Ido with Ile and alpha-ketoglutaric acid as substrates. After biotransformation of Ile by E. coli BL21(DE3) expressing Ido(N126H/T130K) and Ido, 66.50 +/- 0.99 mM and 26.09 +/- 1.85 mM 4-HIL was synthesized, respectively, in 24 h. Ido(N126H/T130K) had a higher enzyme activity and catalytic efficiency and can therefore be used as a more suitable candidate for 4-HIL production.