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Background3-Hydroxypropionic acid (3-HP) is an important platform chemical that boasts a variety of industrial applications. Gene expression systems inducible by 3-HP, if available, are of great utility for optimization of the pathways of 3-HP production and excretion.ResultsHere we report the presence of unique inducible gene expression systems in Pseudomonas denitrificans and other microorganisms. In P. denitrificans, transcription of three genes (hpdH, mmsA and hbdH-4) involved in 3-HP degradation was upregulated by 3-HP by the action of a transcriptional regulator protein, LysR, and a cis-acting regulatory site for LysR binding. Similar inducible systems having an LysR transcriptional regulator were identified in other microorganisms that also could degrade 3-HP. A docking study showed that the 3-HP binding pocket is located between the N-terminal helix-turn-helix motif and the C-terminal cofactor-binding domain.ConclusionsThis LysR-regulated 3-HP-inducible system should prove useful for control of the level of gene expression in response to 3-HP.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0353-5) contains supplementary material, which is available to authorized users.

Coenzyme B12 (Vitamin B12 ) is one of the most complex biomolecules and an essential cofactor required for the catalytic activity of many enzymes. Pseudomonas denitrificans synthesizes coenzyme B12 in an oxygen-dependent manner using a pathway encoded by more than 25 genes that are located in six different operons. Escherichia coli, a robust and suitable host for metabolic engineering was used to produce coenzyme B12 . These genes were cloned into three compatible plasmids and expressed heterologously in E. coli BL21 (DE3). Real-time PCR, SDS-PAGE analysis and bioassay showed that the recombinant E. coli expressed the coenzyme B12 synthetic genes and successfully produced coenzyme B12 . However, according to the quantitative determination by inductively coupled plasma-mass spectrometry, the amount of coenzyme B12 produced by the recombinant E. coli (0.21 ± 0.02 μg/g cdw) was approximately 13-fold lower than that by P. denitrificans (2.75 ± 0.22 μg/g cdw). Optimization of the culture conditions to improve the production of coenzyme B12 by the recombinant E. coli was successful, and the highest titer (0.65 ± 0.03 μg/g cdw) of coenzyme B12 was obtained. Interestingly, although the synthesis of coenzyme B12 in P. denitrificans is strictly oxygen-dependent, the recombinant E. coli could produce coenzyme B12 under anaerobic conditions.

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3-Hydroxypropionic acid (3-HP) is an important platform chemical that can be used to synthesize a range of chemical compounds. A previous study demonstrated that recombinant Escherichia coli stains can produce 3-HP from glycerol in the presence of vitamin B₁₂ (coenzyme B₁₂), when overexpressed with a coenzyme B₁₂-dependent glycerol dehydratase (DhaB) and an aldehyde dehydrogenase. The present study examined the production of 3-HP in recombinant Klebsiella pneumoniae strains, which naturally synthesizes vitamin B₁₂ and does not require supplementation of the expensive vitamin. The NAD⁺-dependent gamma-glutamyl-gamma-aminobutyraldehyde dehydrogenase (PuuC) of K. pneumoniae alone or with its DhaB was overexpressed homologously, and two major oxidoreductases, DhaT and YqhD, were disrupted. Without vitamin B₁₂ addition, the recombinant K. pneumoniae ΔdhaTΔyqhD overexpressing PuuC could produce ∼3.8 g/L 3-HP in 12 h of flask culture. However, this was possible only under the appropriate aeration conditions; 1,3-propanediol (1,3-PDO) (instead of 3-HP) was mainly produced when aeration was insufficient, whereas a very small amount of both 3-HP and 1,3-PDO were produced when aeration was too high. The production of a small amount of 3-HP under improper aeration conditions was attributed to either slow NAD⁺ regeneration (under low aeration) or reduced vitamin B₁₂ synthesis (under high aeration). In a glycerol fed-batch bioreactor experiment under a constant DO of 5%, the strain, K. pneumoniae ΔdhaTΔyqhD, overexpressing both PuuC and DhaB could produce >28 g/L 3-HP in 48 h with a yield of >40% on glycerol. Only small amount of 3-HP was produced when cultivation was carried out at a constant aeration of 1 vvm or constant 10% DO. These results show that K. pneumoniae is potentially useful for the production of 3-HP in an economical culture medium that does not require vitamin B₁₂. The results also suggest that the aeration conditions should be optimized carefully for the efficient production of 3-HP while using this strain.