Microbial catabolism of steroids, from enzymes to ecosystems

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Microbial steroid catabolism plays important roles in biomass decomposition, decontamination of wastewaters, synthesis of drugs and pathogenesis. My Discovery Grant research program has made very significant advances in our understanding of steroid catabolism. The next phase of the program has the goals of (1) elucidating the poorly-understood initial steps of steroid degradation and (2) exploring the diversity, evolution and ecology of steroid catabolism. I hypothesize that the initial catabolic steps vary according to ecological niches and potentially play roles in microbe-host interactions. I will investigate the roles of cholesterol oxidases and hydroxysteroid dehydrogenases, including their relative importance for the use of membrane-associated versus free steroids by Rhodococcus jostii. This investigation will involve analysis of gene expression, production and characterization of mutant strains as well as enzyme expression and activity assays. More broadly, microbial genomic and metagenomic approaches offer exciting new potential to explore the diversity of microbial metabolism and ecology, but these approaches are severely hampered by the lack of functionally-characterized proteins, which leads to mis-annotation of genes. I will use a unique combination of bioinformatic and biochemical analyses in an iterative process to develop and expand a reference set of steroid catabolism genes/proteins. With this knowledge base, I will map the occurrence of steroid catabolism pathways across microbial taxa and different environments. Thusly, in microbial strains and in environmental DNA, I will identify novel genes and pathways that warrant biochemical investigation. Overall, this research program will make important advances in our mechanistic understanding of microbial steroid catabolism and in our understanding of the evolution and ecology of the pathways. This understanding will maximize our ability to exploit microbial steroid transformations for industrial and environmental processes and may open the door to a new paradigm for microbe-host interactions.