We introduced the Escherichia coli glycolate catabolic pathway into Arabidopsis thaliana chloroplasts to reduce the loss of fixed carbon and nitrogen that occurs in C(3) plants when phosphoglycolate, an inevitable by-product of photosynthesis, is recycled by photorespiration. Using step-wise nuclear transformation with five chloroplast-targeted bacterial genes encoding glycolate dehydrogenase, glyoxylate carboligase and tartronic semialdehyde reductase, we generated plants in which chloroplastic glycolate is converted directly to glycerate. This reduces, but does not eliminate, flux of photorespiratory metabolites through peroxisomes and mitochondria. Transgenic plants grew faster, produced more shoot and root biomass, and contained more soluble sugars, reflecting reduced photorespiration and enhanced photosynthesis that correlated with an increased chloroplastic CO(2) concentration in the vicinity of ribulose-1,5-bisphosphate carboxylase/oxygenase. These effects are evident after overexpression of the three subunits of glycolate dehydrogenase, but enhanced by introducing the complete bacterial glycolate catabolic pathway. Diverting chloroplastic glycolate from photorespiration may improve the productivity of crops with C(3) photosynthesis.

We present a novel method for visualizing intracellular metabolite concentrations within single cells of Escherichia coli and Corynebacterium glutamicum that expedites the screening process of producers. It is based on transcription factors and we used it to isolate new L-lysine producing mutants of C. glutamicum from a large library of mutagenized cells using fluorescence-activated cell sorting (FACS). This high-throughput method fills the gap between existing high-throughput methods for mutant generation and genome analysis. The technology has diverse applications in the analysis of producer populations and screening of mutant libraries that carry mutations in plasmids or genomes.

A number of D-amino acids occur in nature, and there is growing interest in their function and metabolism, as well as in their production and use. Here we use the well-established L-amino-acid-producing bacterium Corynebacterium glutamicum to study whether D-amino acid synthesis is possible and whether mechanisms for the export of these amino acids exist. In contrast to Escherichia coli, C. glutamicum tolerates D-amino acids added extracellularly. Expression of argR (encoding the broad-substrate-specific racemase of Pseudomonas taetrolens) with its signal sequence deleted results in cytosolic localization of ArgR in C. glutamicum. The isolated enzyme has the highest activity with lysine (100%) but also exhibits activity with serine (2%). Upon overexpression of argR in an L-arginine, L-ornithine, or L-lysine producer, equimolar mixtures of the D-and L-enantiomers accumulated extracellularly. Unexpectedly, argR overexpression in an L-serine producer resulted in extracellular accumulation of a surplus of D-serine (81 mM D-serine and 37 mM L-serine) at intracellular concentrations of 125 mM D-serine plus 125 mM L-serine. This points to a nonlimiting ArgR activity for intracellular serine racemization and to the existence of a specific export carrier for D-serine. Export of D-lysine relies fully on the presence of lysE, encoding the exporter for L-lysine, which is apparently promiscuous with respect to the chirality of lysine. These data show that D-amino acids can also be produced with C. glutamicum and that in special cases, due to specific carriers, even a preferential extracellular accumulation of this enantiomer is possible.Corynebacterium glutamicum is well known for its extraordinary L-amino acid production properties. Its most prominent feature is probably its capacity to produce L-glutamate, 1.8 million tons of which are currently produced per year and used as a sodium salt to be added to food (24). Another amino acid made with C. glutamicum is L-lysine, which is required for animal nutrition. Indeed, over the years, C. glutamicum strains have been developed for the production of almost all the L-amino acids with commercial potential. For instance, our own group has contributed to the development of C. glutamicum strains producing L-isoleucine (7), L-valine (30), L-threonine (5), and L-serine (37).Obviously, given the success of amino acid production by C. glutamicum and other bacteria, and the fact that sugar is a renewable substrate, bacterial production of further compounds has great appeal. Processes to enable the production of succinate (28), lactate (29), cadaverine (14), putrescine (32), 2-oxoisovalerate (19), and isobutanol (35) by C. glutamicum have been described. D-Amino acids constitute another interesting group of products, although their direct biotechnological production by bacteria has not yet been investigated. A significant reason is probably that it is unclear whether the cell can tolerate the coexistence of D-and L-amino acids. In fact, the growth of Currently, D-amino acids are produced ...

The pyridoxal-5'-phosphate (PLP)-dependent amino acid racemases occur in almost every bacterium but may differ considerably with respect to substrate specificity. We here isolated the cloned broad substrate specificity racemase ArgR of Pseudomonas taetrolens from Escherichia coli by classical procedures. The racemase was biochemically characterized and amongst other aspects it was confirmed that it is mostly active with lysine, arginine and ornithine, but merely weakly active with alanine, whereas the alanine racemase of the same organism studied in comparison acts on alanine only. Unexpectedly, sequencing the amino-terminal end of ArgR revealed processing of the protein, with a signal peptide cleaved off. Subsequent localization studies demonstrated that in both P. taetrolens and E. coli ArgR activity was almost exclusively present in the periplasm, a feature so far unknown for any amino acid racemase. An ArgR-derivative carrying a carboxy-terminal His-tag was made and this was demonstrated to localize even in an E. coli mutant devoid of the twin-arginine translocation (Tat) pathway in the periplasm. These data indicate that ArgR is synthesized as a prepeptide and translocated in a Tat-independent manner. We therefore propose that ArgR translocation depends on the Sec system and a post-translocational insertion of PLP occurs. As further experiments showed, ArgR is necessary for the catabolism of D: -arginine and D: -lysine by P. taetrolens.