The biosynthesis of threonine by mammalian cells: expression of a complete bacterial biosynthetic pathway in an animal cell.

The coding regions for the Escherichia coli gene for aspartokinase I/homoserine dehydrogenase I (thrA) and the Corynebacterium glutamicum gene for aspartic semialdehyde dehydrogenase (asd) have been subcloned into a Simian Virus 40 (SV40)-based mammalian expression vector. Both enzyme activities are expressed in mouse 3T3 cells after transfer of the corresponding chimaeric gene. The kinetic parameters are similar to those of the native bacterial enzymes, and aspartokinase I/homoserine dehydrogenase I retains its allosteric regulation by threonine. An extract of the cells expressing aspartokinase I/homoserine dehydrogenase I, mixed with one from cells expressing aspartic semialdehyde dehydrogenase, produced homoserine when the mixture was incubated with aspartic acid, ATP and NADPH. The thrA and asd expression cassettes were combined into a single plasmid which, when transfected into 3T3 cells, enabled them to produce homoserine from aspartic acid. Homoserine-producing 3T3 cells were transfected with the plasmid pSVthrB/C (homoserine kinase and threonine synthase) and selected for growth on homoserine. Cell lines isolated from these cells expressed the complete bacterial threonine pathway, were independent of threonine for growth and could be maintained in medium which contained no free threonine. The threonine in the proteins of these cells became enriched in 15N when the culture medium contained [15N]aspartic acid. The production of homoserine and the growth of cells was at a maximum when there was more than 2.5 mM aspartate in the medium. Below this concentration the high Km of aspartokinase limited the flux through the pathway. In the presence of additional aspartic acid the new pathway could sustain a cell cycle time close to that of the same cells cultured in threonine-containing medium.