Abstract
Using a minimal medium containing a methionine analog together with a small amount of S-adenosylmethionine (SAM), many SAM requiring mutants which responded only to SAM and not to methionine, S-adenosylhomocysteine, or homocysteine were efficiently isolated from Corynebacterium glutamicum TLD-140 after mutagenesis. Among them, SAM-14 and SAM-19 selected from selenomethionine resistant mutants were subjected to further investigation. Both mutants were unable to grow in a minimal medium and had no detectable activity of SAM synthetase. Both mutants acquired higher resistance to methionine hydroxamate and ethionine as well as to selenomethionine than TLD-140 and produced L-methionine in a medium.
Homoserine-O-transacetylase in SAM-19 was subject to full repression by the addition of excess SAM to the growth medium and was not repressed under SAM limitation, whereas addition of excess L-methionine under SAM limitation caused a partial repression of the enzyme. SAM synthetase as well as L-methionine biosynthetic enzymes in a methionine auxotroph of C. glutamicum was repressed by the addition of L-methionine to the growth medium.
These results suggest that SAM is implicated in the repression of L-methionine synthesizing enzymes in C. glutamicum.
