Background Corrinoids are an essential cofactor of reductive dehalogenases, the key enzymes of organohalide respiration (OHR). Dehalobacter restrictus is an obligate OHR bacterium (OHRB) able to conserve energy with tetrachloroethene, but is unable to de novo synthesize corrinoids. Genome analysis of D. restrictus strain PER-K23 however revealed the presence of the complete corrinoid biosynthesis pathway. Objectives The aim of the present study is to understand the corrinoid metabolism of D. restrictus strain PER-K23 at the level of biosynthesis, regulation and transport and to compare it to contrasting situations in other Dehalobacter strains. Methods Genome analysis was performed with standard bioinformatic tools. Both transcriptomic and proteomic approaches were applied on D. restrictus strain PER-K23 cells cultivated in media with alternative corrinoid conditions. Gene expression was further addressed using targeted reverse transcription and quantitative PCR. Growth and dechlorination of D. restrictus strain PER-K23 was followed by measuring cell numbers and chloride release, respectively. Results and Conclusion Annotation and analysis of genes involved in corrinoid metabolism revealed a 101-bp deletion in the cbiH gene resulting in a shift of the reading frame and leading to a non-functional enzyme. This mutation, which is not present in the genome of other Dehalobacter strains, indicates that cbiH represents a possible checkpoint behind corrinoid auxotrophy. The expression of most corrinoid biosynthetic genes is controlled by cobalamin riboswitches. Experimental evidence of this latter mechanism was obtained. Comparative ’omics’ analyses of corrinoid-starved cells revealed an increased production of corrinoid transporters and proteins involved in corrinoid salvaging. Taken together, these data suggest that D. restrictus strain PER-K23 has recently lost its capacity of de novo corrinoid synthesis. References : Holliger et al., 1998, Kruse et al., 2013; Rupakula et al., 2013, Tang et al., 2012