First Betalain-Producing Bacteria Break the Exclusive Presence of the Pigments in the Plant Kingdom

The biosynthesis of antioxidant pigments, namely, betalains, was believed to be restricted to Caryophyllales plants. This paper changes this paradigm, and enzyme mining from bacterial hosts promoted the discovery of bacterial cultures producing betalains. The spectrum of possible sources of betalain pigments in nature is broadened by our description of the first betalain-forming bacterium, Gluconacetobacter diazotrophicus. The enzyme-specific step is the extradiol cleavage of the precursor amino acid l-dihydroxyphenylalanine (l-DOPA) to form the structural unit betalamic acid. Molecular and functional work conducted led to the characterization of a novel dioxygenase, a polypeptide of 17.8 kDa with a K(m) of 1.36 mM, with higher activity and affinity than those of its plant counterparts. Its superior activity allowed the first experimental characterization of the early steps in the biosynthesis of betalains by fully characterizing the presence and time evolution of 2,3- and 4,5-seco-DOPA intermediates. Furthermore, spontaneous chemical reactions are characterized and incorporated into a comprehensive enzymatic-chemical mechanism that yields the final pigments.