Methionine synthesis and glycine betaine demethylation are intricately intertwined in cosmopolitan marine bacteria

Across all domains of life, cobalamin-dependent methyltransferases have diversified to perform a range of crucial functions, such as methionine synthesis and the demethylation of various reduced nitrogen and sulfur compounds. These large modular enzymes typically possess three substrate-binding domains, two binding either the methyl donor or methyl acceptor, as well as a cobalamin-binding domain. Here, by challenging the current paradigm of glycine betaine (GBT) catabolism, we have identified a unique methyltransferase in aerobic environmental bacteria that has a dual function both as a methionine synthase and a GBT methyltransferase. Using the marine bacterium <i>Ruegeria pomeroyi</i> DSS-3 as a model, we demonstrate that a core cobalamin-binding domain (MtgC) and a bidirectional methyltransferase (MtgD) are essential for both methionine synthesis and GBT demethylation. MtgC is phylogenetically distinct from the cobalamin-binding domains of either the classical methionine synthase (MetH) or the GBT methyltransferases found in anaerobic bacteria and archaea. Across the global ocean, <i>mtgC</i> expression is frequently greater than previously known GBT catabolic pathways due to its occurrence in abundant cosmopolitan marine bacteria. Thus, we uncover a unique relationship between GBT catabolism and methionine synthesis in nature and identify a major route for N-osmolyte demethylation in the global ocean.