Expression of an L-alanine dehydrogenase gene in Zymomonas mobilis and excretion of L-alanine.

An approach to broaden the product range of the ethanologenic, gram-negative bacterium Zymomonas mobilis by means of genetic engineering is presented. Gene alaD for L-alanine dehydrogenase (EC 1.4.1.1.) from Bacillus sphaericus was cloned and introduced into Z. mobilis. Under the control of the strong promoter of the pyruvate decarboxylase (pdc) gene, the enzyme was expressed up to a specific activity of nearly 1 mu mol . min -1 . mg of protein -1 in recombinant cells. As a results of this high L-alanine dehydrogenase activity, growing cells excreted up to 10 mmol of alanine per 280 mmol of glucose utilized into a mineral salts medium. By the addition of 85 mM NH4+ to the medium, growth of the recombinant cells stopped, and up to 41 mmol alanine was secreted. As alanine dehydrogenase competed with pyruvate decarboxylase (PDC) (EC 4.1.1.1.) for the same substrate (pyruvate), PDC activity was reduced by starvation for the essential PDC cofactor thiamine PPi. A thiamine auxotrophy mutant of Z. mobilis which carried the alaD gene was starved for 40 h in glucose-supplemented mineral salts medium and then shifted to mineral salts medium with 85 mM NH4+ and 280 mmol of glucose. The recombinants excreted up to 84 mmol of alanine (7.5 g/liter) over 25 h. Alanine excretion proceeded at an initial velocity of 238 nmol . min-1 . mg [dry weight]-1. Despite this high activity, the excretion rate seemed to be a limiting factor, as the intracellular concentration of alanine was as high as 260 mM at the beginning of the excretion phase and decreased to 80 to 90 mM over 24 h.