The present invention provides an isolated polynucleotide comprising the kstD promoter from Rhodococcus erythropolis. The polynucleotide can very advantageously be used as a controllable transcription activator. Said controlling function can be provided by providing said isolated polynucleotide with i nucleotide sequence encoding a transcription regulator of said promoter, [n the oresent invention, such a transcrimion regulator may be externally indueea. s^cr. as by introduction of steroidal compounds. In an alternative embodiment :f :he present invention the isolated polynucleotide may comprise the kstR gene ;r i homologue or a functional part thereof as the transcription regulator of the kstD promoter.