Hi Roel,

Well, cerevisiae produces cobalamin and, indeed, dried yeasts is a source of cobalamin in alimentary supplements for humans.

Moreover, the yeast protein AVT4p is homologous to bacterial CbiMs, the cobalamin transporters. So yeast is probably able to take out cobalamin from culture meidum.

Finally, you might be able to boost production of cobalamin in yeast by supplementing the culture medium with CoCl2 (2.8 mg per liter), as is done in the Kapelli synthetic medium.

Best regards

Philippe Urban

My understanding, supported by some sentences on the internet (I know, not very strong), was that S. cerevisiae does not contain B-12, and certainly does not produce it (from the dietary mineral cobalt in inorganic form?).  The sources include:

·         “Yeast cannot produce B12…“ - https://en.wikipedia.org/wiki/Nutritional_yeast

·         “Yeast does not contain B12 unless it is added.” - http://www.naturalnews.com/028516_vitamin_B_supplements.html

·         “…saccharomyces cerevisiae does not contain vitamin B12.” - http://www.sharecare.com/health/dietary-supplements/saccharomyces-cerevisiae-contain-all-vitamins

Are these sources mistaken?  If so, do you have an online reference so I can modify the wiki entry.

Hi Roel

vitamin B12 is mostly used for two processes: Biosynthesis of methionine and degradation of odd-chain fatty and branched-chain amino acids (by conversion of propionyl CoA into succinyl-CoA ). 

S. cerevisiae has (like most plants) a B12 independent methionine synthase (MET6). It also lacks the gene for methylmalonyl-CoA mutase. So from a physiological point of view there is no need for B12. 

It is true that there are some genes annotated as lying on the pathway of B12 synthesis. However, B12 is produced as a late branch-off from the heme biosynthetic pathway and those genes are relevant for heme synthesis only. 

The surprising situation is that many yeast minimal media have additions of vitamin B12. This I think just reflects the fact that the inventors of these media wanted to be on the safe side:  Rather that omitting a nutrient that might be required they added everything that was known to be required in other organisms.

In summary,  I do not think that S. cerevisiae will take up notable amounts of B12 or enrich it in the cytoplasm. According to yeastgenome.org Avt4p is a vacuolar protein, thus it is unlikely involved in the uptake of B12.

Best wishes

Jürgen