Control of misincorporation of de novo synthesized norleucine into recombinant interleukin-2 in E. coli

Interleukin-2 produced from a recombinant E. coli was found to contain as much as 19% norleucine in place of methionine in a minimal medium fermentation. Medium supplementation experiments and use of a leucinerequiring mutant host strain indicated the origin of norleucine to be de novo biosynthesis by reactions involving the enzymes of the leucine biosynthetic pathway. The misincorporation was highly suppressed by addition of either Lleucine or L-methionine to the fermentation and completely suppressed by adding both amino acids.