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One of the largest natural product molecules, vitamin B12, is a molecular marvel and an enzymatic enigma. The biosynthetic enzymes responsible for the formation of one of its components, dubbed DMB, have remained elusive — until now. On page 449, biologist Graham Walker at the Massachusetts Institute of Technology and his colleagues identify a role for the bacterial enzyme BluB in DMB formation and show that this enzyme catalyses complex and novel chemistry — including the cannibalization of flavin.

How did you finally identify the enzymes in the DMB biosynthetic pathway?

Our success has its roots in an undergraduate lab experiment with the soil microbe, Sinorhizobium meliloti, which led years later to identification of the bluB mutant, which cannot form a symbiotic relationship with its plant host. It was some time before we recognized the clues implicating bluB in vitamin B12 biosynthesis. Other workers unearthed a bluB homologue in a cluster of vitamin B12 genes in another bacterium, and a colleague discovered that bluB was regulated in response to vitamin B12. Determining the function of the BluB protein then required expertise in basic biology, enzyme mechanisms and crystallography.

What was most surprising about the enzyme?

We knew that three bonds were cut and one new bond formed during DMB synthesis, but it was not obvious that one enzyme could do this. My postdoc Michiko Taga conducted clever genetic screens for other genes involved in DMB biosynthesis, but found nothing. Then she discovered that purified BluB protein could make DMB all by itself.

What makes the reaction so unusual?

BluB has a structure similar to an oxidoreductase but has a different function. Many enzymes use flavin as a coenzyme to help a reaction proceed, but the flavin is not consumed or changed. This is the first example we know of in which flavin is destroyed by the enzyme and transformed into part of another coenzyme. That's what we're calling cannibalization.

How will this help us understand soil microorganisms and symbiosis better?

We're investigating how making vitamin B12 could enable the bacterium to carry out symbiosis. If you can grow fine without vitamin B12, why carry around 30 genes to make it? We're testing whether B12-containing forms of these enzymes are more resistant to the oxidative stress that microbes undergo when trying to get into a plant.