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Background  Aerobic side chain degradation of phytosterols by actinobacteria is the basis for the industrial production of androstane steroids which are the starting materials for the synthesis of steroid hormones. A native strain of  Mycobacterium  sp. VKM Ac-1817D effectively produces 9α-hydroxyandrost-4-ene-3,17-dione (9-OH-AD) from phytosterol, but also is capable of slow steroid core degradation. However, the set of the genes with products that are involved in phytosterol oxidation, their organisation and regulation remain poorly understood.    Results  High-throughput sequencing of the global transcriptomes of the  Mycobacterium  sp. VKM Ac-1817D cultures grown with or without phytosterol was carried out. In the presence of phytosterol, the expression of 260 genes including those related to steroid catabolism pathways significantly increased. Two of the five genes encoding the oxygenase unit of 3-ketosteroid-9α-hydroxylase ( kshA ) were highly up-regulated in response to phytosterol (55- and 25-fold, respectively) as well as one of the two genes encoding its reductase subunit ( kshB ) (40-fold). Only one of the five putative genes encoding 3-ketosteroid-∆ 1 -dehydrogenase (KstD_1) was up-regulated in the presence of phytosterol (61-fold), but several substitutions in the conservative positions of its product were revealed.  Among the genes over-expressed in the presence of phytosterol, several dozen genes did not possess binding sites for the known regulatory factors of steroid catabolism. In the promoter regions of these genes, a regularly occurring palindromic motif was revealed. The orthologue of TetR-family transcription regulator gene  Rv0767c  of  M. tuberculosis  was identified in  Mycobacterium  sp. VKM Ac-1817D as  G155_05115 .    Conclusions  High expression levels of the genes related to the sterol side chain degradation and steroid 9α-hydroxylation in combination with possible defects in KstD_1 may contribute to effective 9α-hydroxyandrost-4-ene-3,17-dione accumulation from phytosterol provided by this biotechnologically relevant strain. The TetR-family transcription regulator gene  G155_05115  presumably associated with the regulation of steroid catabolism. The results are of significance for the improvement of biocatalytic features of the microbial strains for the steroid industry.    Electronic supplementary material  The online version of this article (10.1186/s12896-019-0533-7) contains supplementary material, which is available to authorized users.

Genome-wide response on phytosterol in 9-hydroxyandrostenedione-producing strain of Mycobacterium sp. VKM Ac-1817D