Characterization of tetracycline modifying enzymes using a sensitive in vivo reporter system

Metabolic Engineering of Corynebacterium glutamicum for the High-Level Production of l-Valine under Aerobic Conditions

l-Valine, an essential amino acid, serves as a valuable compound in various industries. However, engineering strains with both high yield and purity are yet to be delivered for microbial l-valine production. We engineered a Corynebacterium glutamicum strain capable of highly efficient production of l-valine. We initially introduced an acetohydroxy acid synthase mutant from an industrial l-valine producer and optimized a cofactor-balanced pathway, followed by the activation of the nonphosphoenolpyruvate-dependent carbohydrate phosphotransferase system and the introduction of an exogenous Entner-Doudoroff pathway. Subsequently, we weakened anaplerotic pathways, and attenuated the tricarboxylic acid cycle via start codon substitution in icd, encoding isocitrate dehydrogenase. Finally, to balance bacterial growth and l-valine production, an l-valine biosensor-dependent genetic circuit was established to dynamically repress citrate synthase expression. The engineered strain Val19 produced 103 g/L of l-valine with a high yield of 0.35 g/g glucose and a productivity of 2.67 g/L/h. This represents the highest reported l-valine production in C. glutamicum via direct fermentation and exhibits potential for its industrial-scale production, leveraging the advantages of C. glutamicum over other microbes.

TETX: a novel nuclear selection marker for Chlamydomonas reinhardtii transformation

BACKGROUND

Transformation of microalgae to obtain recombinant proteins, lipids or metabolites of economic value is of growing interest due to low costs associated with culture growth and scaling up. At present there are only three stable nuclear selection markers for the transformation of Chlamydomonas reinhardtii, which is the most commonly transformed microalgae, specifically: the aminoglycoside phosphotransferaseses aph7and aphVIII and the phleomycin resistance ble gene. As several microalgae are resistant to some of the antibiotics associated with the mentioned resistance genes, we have developed another alternative, tetX, a NADP-requiring Oxidoreductase that hydroxylates tetracycline substrates. We provide evidence that tetX can be used to obtain nuclear transformants of Chlamydomonas reinhardtii.

RESULTS

We obtained nuclear transformants harbouring the tetX gene under the control of beta 2 tubulin or HSP70ARBCS2 promoters at an efficiency of transformation of 3.28 and 6.18 colony forming units/μg DNA respectively. This is the first report of a eukaryotic cell transformed using tetracycline as a selectable marker.

CONCLUSIONS

We developed a protocol for the nuclear transformation of Chlamydomonas reinhardtii using tetX as a selectable marker that confers stable resistance to tetracycline up to 100 μg/mL. We believe tetX can be used to transform Chlamydomonas reinhardtii chloroplasts, related microalgae and other aerobic organisms sensitive to any tetracycline antibiotic.