Isolation and characterisation ofCandida sp. mutants enriched in S-adenosylmethionine (SAM)

The ability to accumulate S-adenosylmethionine (SAM) of 572 yeast strains isolated from environmental sources were surveyed. An S-adenosylmethionine enriching strain S42-12, identified asCandida sp., was chose to develop a SAM-accumulating mutant successfully. The final SAM-accumulating mutant strain YQ-5 was isolated by UV radiation or by NTG treatment using ethionine selection and nystatin selection method. The mutant strain YQ-5 accumulated 112.1 mg per gram biomass, was 3.14-fold higher than the original strain S42-12. When cultivated in the optimal medium with a favourable fermentation conditions, SAM content of the mutant strain reached at 1740 mg L^?1. Trend of SAM and ergosterol contents and methionine adenosyltransferase activity of SAM-accumulating mutants during fermentation were analysed. The results suggested that one of the reasons why the mutants accumulated SAM in significantly high amounts may be the lower consumption of SAM for ergosterol biosynthesis, other than improvement of methionine adenosyltransferase activity.