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Appl. Environ. Microbiol. doi:10.1128/AEM.00888-08
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Characterization of a second 3-ketosteroid 9-hydroxylase activity in Rhodococcus erythropolis SQ1 comprised of a terminal oxygenase homologue KshA2 active with oxygenase-reductase component KshB

Department of Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute (GBB), University of Groningen, P.O. Box 14, 9750 AA Haren, The Netherlands

^* To whom correspondence should be addressed. Email: L.Dijkhuizen{at}rug.nl.

	Abstract

Previously we have characterized 3-ketosteroid 9-hydroxylase (KSH), a key enzyme in microbial steroid degradation in Rhodococcus erythropolis strain SQ1, as a two-component iron-sulphur monooxygenase, comprised of the terminal oxygenase component KshA1 and the oxygenase-reductase component KshB. Deletion of the kshA1 gene resulted in loss of ability of mutant strain RG2 to grow on the steroid substrate 4-androstene-3,17-dione (AD). Here we report characteristics of a close KshA1 homologue, KshA2 of strain SQ1, sharing 60% identity at the amino acid level. Expression of the kshA2 gene in mutant strain RG2 restored growth on AD and ADD, indicating that kshA2 also encodes 3-ketosteroid 9-hydroxylase activity. The functional complementation was shown to be dependent on the presence of kshB. Transcriptional analysis showed that expression of kshA2 is induced in parent strain R. erythropolis SQ1 in the presence of AD. However, promoter activity studies, using -lactamase of E. coli as a convenient transcription reporter protein for Rhodococcus, revealed that the kshA2 promoter in fact is highly induced in the presence of 9-hydroxy-4-androstene-3,17-dione (9OHAD), or a metabolite thereof. Inactivation of kshA2 in parent strain SQ1 by unmarked gene deletion did not affect growth on 9OHAD, cholesterol or cholic acid. We speculate that KshA2 plays a role in preventing accumulation of toxic intracellular concentrations of ADD during steroid catabolism. A third kshA homologue was additionally identified in a kshA1 kshA2 double gene deletion mutant strain of R. erythropolis SQ1. The developed degenerate PCR primers for kshA may be useful for isolation of kshA homologues from other (actino)bacteria.