The relationship of cobalt requirement to vitamin B12-dependent methionine synthesis in Rhizobium meliloti.

As a growth factor, Rhizobium meliloti required cobalt ion, or vitamin B12 which was found to be incorporated into the cells without decomposition to cobalt ion. Trial of replacement for cobalt ion by the addition of various compounds to the cobalt-deficient medium revealed that methionine could substitute for cobalt ion and promote the growth in response to its concentration. Furthermore, B12-dependent methionine synthetase was demonstrated in the cell-free extracts of this microorganism. The morphological change of R. meliloti by the additions to the medium was observed microscopically.