B12-BIOSYNTHESIS IN THE TETRACHLOROETHENE-RESPIRING OBLIGATE ANAEROBE DESULFITOBACTERIUM
Tetrachloroethene (PCE) reductive dehalogenases are the key catalysts in the respiratory chain of dehalorespiring microorganisms, where they act as the terminal electron-accepting enzymes in the process. On protein level, reductive dehalogenases have been shown to belong to the family of corrinoid-dependent enzymes. However reductive dehalogenases are not involved in alkyl-transfer as other corrinoid-containing enzymes but in electron transfer and investigation on the nature of the cofactor present in tetrachloroethene reductive dehalogenase of Sulfurospirillum multivorans has revealed a novel type of corrinoid. Preliminary investigation with our model strain Desulfitobacterium hafniense strain TCE1 showed that strain TCE1 was able to de novo synthesize its own corrinoid. The bacterium was grown for a long-term period without B12 amendment in the medium before a pulse of vitamin B12 was operated. The expression level of key genes in B12 biosynthesis was then followed by RT-PCR. A down-regulation was clearly observed on transcription level for some of the key genes (cbiX, cbiC, cbi(ET), cbiJ, cobA), while no or a late regulation was reported for others (btuF, cobS). Analysis of the genetic structure around B12-biosynthetic genes in the genome sequence of strain Y51, a closely related strain, revealed that several riboswitches are present directly upstreams of most transcriptional units, indicating that the B12 biosynthesis pathway is essentially regulated by RNA elements. The question of which B12-derivates can interact with these riboswitches remains to be elucidated, as well as the regulation mode for each B12-riboswitch predicted in the genome.