Rhodococcus erythropolis IGTS8 that possesses dibenzothiophene sulfone monooxygenase mutated at residue 345 (Q345A), can degrade octyl sulfide on which the wild strain cannot grow. Residue 345 and the neighbouring residues were changed by site-directed mutagenesis. Only DszA changed at residue 345 gave an altered C-S bond cleavage pattern of 3-methyl DBT sulfone. This residue is therefore involved in C-S bond cleavage specifically for alkylated DBT sulfone.