The corrinoid metabolism in desulfitobacterium hafniense - genetic and regulatory aspects

Corrinoids (e.g. vitamin B12, cobalamin) represent a family of complex organometallic cofactors essential to the metabolism of microorganisms, animal and humans, and are involved in several enzymes such as isomerases, methyltransferases and reductive dehalogenases. Microorganisms - and not all of them - however, are the only natural source of corrinoids. Desulfitobacterium hafniense is a strictly anaerobic bacterium harboring a versatile energy metabolism, including a process called organohalide respiration (OHR) during which chlorinated organic pollutants are used as terminal electron acceptors. The first genome sequences of D. hafniense isolates revealed the presence of a complete set of corrinoid biosynthesis genes together with many corrinoid-dependent pathways, such as OHR, and enzymes. We successfully cultivated D. hafniense strain TCE1 in absence of corrinoid in the medium, and preliminary HPLC analysis of extracted corrinoids suggested that strain TCE1 produces several types of corrinoids. A thorough analysis of corrinoid-related genes in the genomes of D. hafniense strains Y51 and DCB-2 revealed the presence of sixteen and eleven cobalamin riboswitches (Cbl-RS), respectively. Cbl-RS structures are known to regulate the transcription or translation of downstream located genes by corrinoid-dependent conformational changes occurring in the 5’-untranslated region of the corresponding RNA transcripts. In vitro in-line probing analysis of three significantly different riboswitches of D. hafniense highlighted their role as regulatory elements in the corrinoid metabolism. In vivo analysis of the transcription of the directly downstream genes upon addition of various corrinoids clearly confirmed it. Our study extends the knowledge on the functional diversity of cobalamin riboswitches.