SUMMARY: Two species of were isolated from soil by their ability to grow aerobically on L-threonine as sole source of energy and cellular carbon and nitrogen. Threonine-grown fungi contained a highly active inducible L-threonine: NAD dehydrogenase and a significantly less active constitutive ‘biosynthetic' threonine dehydratase, but possessed no L-threonine: acetaldehyde lyase activity. The 2-amino-3-oxobutyrate formed by initial dehydrogenation of threonine was subsequently cleaved in both fungi to acetyl-CoA and glycine in a coenzyme-A-dependent cleavage catalysed by a 2-amino-3-oxobutyrate: CoA ligase which was inducibly synthesized during growth on threonine and not during growth on acetate plus glycine.

During growth of both fungi on threonine, C from L-[U-C]threonine was rapidly incorporated into glycine and malate, and thereafter into citrate, aspartate, glutamate, succinate and various other metabolites. The time-dependent distribution of C among metabolites in these short-term incubations with L-[U-C] threonine showed that acetyl-CoA produced by the NAD plus coenzyme-A-dependent cleavage of threonine was metabolized via the tricarboxylic acid cycle plus glyoxylate cycle.

Comparative enzyme induction patterns after growth of both fungi on a wide range of carbon sources showed that glycine produced by the NAD plus coenzyme-A-dependent cleavage of threonine was metabolized via the glycerate pathway.

There was no evidence from either comparative enzyme induction patterns or incorporation of C from L-[U-C]threonine of aminoacetone production and further metabolism by both fungi, even though a small amount of this amino-ketone appeared in culture media during growth on threonine.


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