Abstract: The invention relates to a process for the preparation of L-3,4-dihydroxyphenylalanine wherein L-3,4,-dihydroxyphenylalanine is produced in a fermentation medium by aerobic fermentation of a recombinant microorganism having L-tyro-sine-3-hydroxy-mono-oxygenase activity and at least the metabolic pathways: glycolysis, pentose phosphate pathway, aromatic amino acid pathway, or derivative pathways thereof, which process comprises (i) a growth phase and a production phase, wherein L-3,4-dihydroxy-phenylalanine is produced in the fermentation medium, and 10 (ii) a downstream processing phase, and in which process L-3,4-dihydroxy phenylalanine is produced from a carbon source and the pH is in the range of from 1 to 7 during at least part of the production phase and/or downstream processing phase.

Abstract: The invention makes available, by means of an increased provision of intracellular metabolic intermediates, in particular of erythrose 4-phosphate, alternative processes for the microbial preparation of substances, in particular of aromatic amino acids such as L-phenylalanine, in which processes the activity of a transaldolase is increased in a microorganism producing these substances. In preferred embodiments of the invention, the activity of a transketolase or the activity of a transport protein for the PEP-independent uptake of a sugar and/or the activity of a kinase which phosporylates the relevant sugar are/is additionally increased. The invention also relates to gene structures, and to transformed cells carrying these gene structures, which make it possible to implement these processes in a particularly successful manner.