3a-Hydroxysteroid dehydrogenase/carbonyl reductase from Comamonas testosteroni is a key enzyme in the degradation of steroids in the environment. In this work the hsdA gene was replaced by the green fluorescent protein (GFP) gene, which was homologous integrated into the chromosomal DNA, the C. testosteroni mutant strain CT-GFP5-1 was generated. In the mutant strain GFP gene could be expressed when some steroid inducers were added in the culture medium. The Fluorescence intensity is correlated to th...