The invention relates to an isolated polynucleotide sequence comprising a nucleic acid sequence encoding the amino acid sequence of KshA protein or of KshB protein, encoded by nucleotides 499 1695 of SEQ ID NO: 1 or by nucleotides 387 1427 of SEQ ID NO:2, respectively, and functional homologues thereof. The polynucleotides of the invention can be used to construct genetically modified microorganisms blocked in 3-ketosteroid 9.alpha.-hydroxylase activity, which are useful in the microbial degradation of steroids to accumulate certain steroid products.