Identification and isolation of a regulator protein for 3,17β-HSD expressional regulation in Comamonas testosteroni

•Comamonas testosteroni harbors several genes for steroid degrading enzymes.•The genes code for hydroxysteroid dehydrogenases and are inducible by steroids.•We have identified a new regulatory gene (brp) for 3,17β-HSD expression.•BRP works in concert with other cis- and trans-acting regulators.

Comamonas testosteroni (C. testosteroni) is able to catabolize a variety of steroids and polycyclic aromatic hydrocarbons. 3,17β-Hydroxysteroid dehydrogenase (3,17β-HSD) from C. testosteroni is a member of the short-chain dehydrogenase/reductase (SDR) superfamily. It is an inducible and key enzyme in steroid degradation. Elucidating the mechanism of 3,17β-HSD gene (βhsd) regulation may help us to generate prospective C. testosteroni mutants for bioremediation. The genome of C. testosteroni ATCC11996 was sequenced in our previous work. Upon examining the genome with bioinformatics tools, a gene (brp) coding for a regulator protein (BRP) for 3,17β-HSD expression was found upstream of the βhsd gene. A Blast search revealed high identities to a nucleotide binding protein with unknown function in other bacteria. Two potential promoters and two repeat sequences (RS, 16 bp), spaced to each other by 1661 bp, were also found upstream of the βhsd gene C. testosteroni. The brp gene was cloned into plasmid pK18 and pET-15b, expressed in Escherichia coli, and the recombinant BRP protein was purified on a Ni-column. In addition, a brp gene knock-out mutant of C. testosteroni was prepared. These knock-out mutants showed an enhanced expression of both the βhsd gene and the hsdA gene (the latter coding for 3α-HSD/CR) in the presence of testosterone. To characterize the BRP functional DNA domain, different fragments of the βhsd upstream regulatory region were tested in a cotransformation system. Our data reveal that the βhsd gene undergoes complex regulation involving the two promoters, a loop structure via the two repeat sequences, and the steroid testosterone. Furthermore, a proximal repressor gene for βhsd expression, phaR, had been identified in our previous investigations. The exact interplay between all these factors will be determined in future experiments.