The applicant will investigate the biochemical role of the cbiB protein in de novo coenzyme B12 synthesis. This work is needed to fill an important gap of knowledge of this major biosynthetic pathway. This work will have important implications on our understanding of how precursors available in the cells environment are salvaged and converted into coenzyme B12. A combination of in vivo genetics, molecular biological biochemical, and crystallographic approaches will be employed to dissect how this important enzyme works.