Vitamin B12 Synthesis and Salvage Pathways Were Acquired by Horizontal Gene Transfer to the Thermotogales

The availability of genome sequences of Thermotogales species from across the order allows an examination of the evolutionary origins of phenotypic characteristics in this lineage. Several studies have shown that the Thermotogales have acquired large numbers of genes from distantly related lineages, particularly Firmicutes and Archaea. Here, we report the finding that some Thermotogales acquired the ability to synthesize vitamin B-12 by acquiring the requisite genes from these distant lineages. Thermosipho species, uniquely among the Thermotogales, contain genes that encode themeans to synthesize vitamin B-12 de novo from glutamate. These genes are split into two gene clusters: the corrinoid synthesis gene cluster, that is unique to the Thermosipho and the cobinamide salvage gene cluster. The corrinoid synthesis cluster was acquired from the Firmicutes lineage, whereas the salvage pathway is an amalgam of bacteria- and archaea-derived proteins. The cobinamide salvage gene cluster has a patchy distribution among Thermotogales species, and ancestral state reconstruction suggests that this pathway was present in the common Thermotogales ancestor. We show that Thermosipho africanus can grow in the absence of vitamin B-12, so its de novo pathway is functional. We detected vitamin B-12 in the extracts of T. africanus cells to verify the synthetic pathway. Genes in T. africanus with apparent B-12 riboswitches were found to be down-regulated in the presence of vitamin B-12 consistent with their roles in B-12 synthesis and cobinamide salvage.