Salvage of the essential corrinoid cofactor in Dehalobacter restrictus

The corrinoid cofactor is essential for organohalide respiration (OHR), a bacterial metabolism assisting towards environmental depollution. Two different strategies for obtaining corrinoids, are used by OHR bacteria, either de novo biosynthesis or salvaging from the environment. Although Dehalobacter restrictus depends strictly on OHR metabolism for growth, it cannot produce the essential corrinoid de novo. Genome analysis of D. restrictus revealed a complete set of corrinoid biosynthesis genes with the exception of one non-functional gene (cbiH) involved in the contraction of the corrin ring, which could explain its corrinoid auxotrophy. However comparative genome analyses showed that other Dehalobacter spp. have an intact cbiH gene. Proteomic analysis of D. restrictus revealed a strong up-regulation of two operons (#1, 2) involved in cobalt/corrinoid transport and salvaging as a response to corrinoid limitation. They contain multiple genes encoding transporters and CbiZ proteins. These two operons are found only in D. restrictus and generally not present in the genome of other Dehalobacter spp. Targeted transcriptional analysis further highlighted the importance of these two operons in D. restrictus. Another important feature of corrinoid in OHR metabolism is the nature of the nucleotide loop, as illustrated by the unusual corrinoid identified in the organohalide-respiring Sulfurospirillum multivorans. In order to investigate the corrinoid structure in D. restrictus, cultures supplemented with cobalamin or cobinamide (a corrinoid intermediate lacking the nucleotide loop) are currently performed. Analysis of the corrinoids extracted from cells of these different cultures will show whether they are used as provided or D. restrictus modifies the nucleotide loop.