Background Corrinoids are an essential cofactor of reductive dehalogenases, the key enzymes of the environmental-friendly process of organohalide respiration (OHR). Dehalobacter restrictus strain PER-K23 is an obligate OHR bacterium (OHRB) able to conserve energy with tetrachloroethene, but is unable to de novo synthesize corrinoids (1). Genome analysis of D. restrictus however revealed the presence of the complete corrinoid biosynthesis pathway (2,3). Objectives The aim of the present study is to understand the corrinoid metabolism of D. restrictus at the level of biosynthesis, regulation and transport and to compare it to contrasting situations in other OHR bacteria. Methods Genome analysis was performed with standard bioinformatic tools. Both transcriptomic and proteomic approaches were applied on D. restrictus cells cultivated in media with alternative corrinoid conditions. Gene expression was further addressed using targeted reverse transcription and quantitative PCR. Conclusions 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 spp., indicates that cbiH represents a possible checkpoint behind corrinoid auxotrophy. The expression of most corrinoid biosynthetic genes is likely to be controlled by cobalamin riboswitches. Experimental evidence of this latter mechanism is under scrutiny. 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 has recently lost its capacity of de novo corrinoid synthesis. References (1) Holliger et al. (1998), Arch Microbiol 169, 313. (2) Kruse et al. (2013), Stand Genomic Sci, submitted. (3) Rupakula et al. (2013), R Soc Phil Trans B, in press.