Microbial sterol side chain degradation in Actinobacteria

PhD ceremony: Mr. M.H. Wilbrink, 13.15 uur, Academiegebouw, Broerstraat 5, Groningen

Title: Microbial sterol side chain degradation in Actinobacteria

Promotor(s): prof. L. Dijkhuizen

Faculty: Mathematics and Natural Sciences

 

The investigations of Maarten Wilbrink for the first time have led to identification of genes involved in the degradation of sterol side chains.

Bacterial species belonging to the genus Rhodococcus are renown for their capability to degrade a wide variety of biomolecules, including sterols and steroids. The microbial degradation of sterols involves oxidation of the steroid nucleus and stepwise removal of the side chain via β -oxidation. The genes and enzymes involved in the latter process were hitherto unknown. Detailed information on the genetics and enzymes involved in sterol side chain degradation is of fundamental importance and industrially relevant, e.g. for the engineering of microbial strains to produce (bioactive) steroids from cheap sterol precursors.

Transcriptome analysis identified a cholesterol catabolic gene cluster in the soil bacterium Rhodococcus jostii RHA1. The gene cluster is also present in the related pathogenic bacterium Mycobacterium tuberculosis, which causes tuberculosis in humans. The cholesterol degradation genes were previously found to be important for survival of the pathogen in macrophages, but their roles had remained unclear. Wilbrinks results thus provided novel insights in the ability of M. tuberculosis to survive in macrophages, namely via degradation of cholesterol.

His thesis describes the molecular characterization of several genes (cyp125, fadD19, ltp3 and ltp4), involved in sterol side chain degradation. Their roles were determined using mutant strain RG32 of Rhodococcus rhodochrous. Strain RG32, constructed in Wilbrinks lab, is blocked in steroid nucleus degradation and accumulates steroids from sterols. His studies, combined with biochemical analysis of the encoded enzymes (in the case of CYP125, FadD19) gave detailed insight into the microbial degradation of sterols in rhodococci that extends to related (pathogenic) bacteria including M. tuberculosis.