Dietary derived Vitamin B12 protects the nematode Caenorhabditis elegans from thiol reducing agents.

We describe a novel SAM methyl transferase in the nematode Caenorhabditis elegans that is upregulated by thiol reducing agents and hydrogen sulfide with expression controlled by the hypoxia inducible factor pathway. This methyl transferase, RIPS-1, is expressed in the gut and hypodermis of this nematode with homologues found in a small subset of eukaryotes and bacteria, many of which can adapt to fluctuations in environmental oxygen levels. We identified RIPS-1 through forward genetic screens as the only gene that when mutated allowed worms to survive normally lethal concentrations of thiol reducing agents such as dithiothreitol (DTT) and {beta}-mercaptoethanol. The RIPS-1 methyl transferase is an important player in the methionine cycle and its activation consumes methionine in a methionine synthetase and vitamin B12-dependant manner. This requirement limits the availability of vitamin B12 in the mitochondrion. Mitochondrial involvement was also established through a targeted enhancer screen that identified methylmalonyl-CoA epimerase as a strong genetic enhancer of RIPS-1 mutant resistance to DTT. Toxicity associated with thiol reducing agent exposure can be overcome in C. elegans by adding methionine, loss of RIPS-1, or by addition of excess vitamin B12. This work highlights the central importance of dietary vitamin B12 in normal metabolic processes in C. elegans and defines a new role in countering reductive stress