Weihua Wang

Photosynthesis driven conversion of carbon dioxide to fatty alcohols and hydrocarbons in cyanobacteria

The production of high value biochemicals and high energy biofuels from sustainable resources through the use of microbial based, green conversion technologies could reduce the dependence on petrochemical resources. However, a sustainable source of carbon and a clean, cost effective method for its conversion to high quality biofuel products are obstacles that must be overcome. Here we describe the biosynthesis of fatty alcohols in a genetically engineered cyanobacterial system through heterologously expressing fatty acyl-CoA reductase and the effect of environmental stresses on the production of fatty alcohols in the mutant strains. Hydrocarbon production in three representative types of native cyanobacterial model strains and the mutant strain overexpressing acetyl-CoA carboxylase was evaluated. The results of this investigation demonstrate the potential for direct production of high value chemicals and high energy fuels in a single biological system that utilizes solar energy as the energy source and carbon dioxide as the carbon source.

Engineering cyanobacteria to improve photosynthetic production of alka(e)nes

BackgroundCyanobacteria can utilize solar energy and convert carbon dioxide into biofuel molecules in one single biological system. Synechocystis sp. PCC 6803 is a model cyanobacterium for basic and applied research. Alkanes are the major constituents of gasoline, diesel and jet fuels. A two-step alkane biosynthetic pathway was identified in cyanobacteria recently. It opens a door to achieve photosynthetic production of alka(e)nes with high efficiency by genetically engineering cyanobacteria.ResultsA series of Synechocystis sp. PCC6803 mutant strains have been constructed and confirmed. Overexpression of both acyl-acyl carrier protein reductase and aldehyde-deformylating oxygenase from several cyanobacteria strains led to a doubled alka(e)ne production. Redirecting the carbon flux to acyl- ACP can provide larger precursor pool for further conversion to alka(e)nes. In combination with the overexpression of alkane biosynthetic genes, alka(e)ne production was significantly improved in these engineered strains. Alka(e)ne content in a Synechocystis mutant harboring alkane biosynthetic genes over-expressed in both slr0168 and slr1556 gene loci (LX56) was 1.3% of cell dry weight, which was enhanced by 8.3 times compared with wildtype strain (0.14% of cell dry weight) cultivated in shake flasks. Both LX56 mutant and the wildtype strain were cultivated in column photo-bioreactors, and the alka(e)ne production in LX56 mutant was 26 mg/L (1.1% of cell dry weight), which was enhanced by 8 times compared with wildtype strain (0.13% of cell dry weight).ConclusionsThe extent of alka(e)ne production could correlate positively with the expression level of alkane biosynthetic genes. Redirecting the carbon flux to acyl-ACP and overexpressing alkane biosynthetic genes simultaneously can enhance alka(e)ne production in cyanobacteria effectively.

afsQ1-Q2-sigQ is a pleiotropic but conditionally required signal transduction system for both secondary metabolism and morphological development in Streptomyces coelicolor

Two-component system AfsQ1-Q2 of Streptomyces coelicolor was identified previously for its ability to stimulate actinorhodin (ACT) and undecylprodigiosin (RED) production in Streptomyces lividans. However, disruption of either afsQ1 or afsQ2 in S. coelicolor led to no detectable changes in secondary metabolite formation or morphogenesis. In this study, we reported that, when cultivated on defined minimal medium (MM) with glutamate as the sole nitrogen source, the afsQ mutant exhibited significantly decreased ACT, RED, and calcium-dependent antibiotic (CDA) production and rapid growth of aerial mycelium. In addition, we also found that deletion of sigQ, which is located upstream of afsQ1-Q2 and encodes a putative sigma factor, led to the precocious hyperproduction of these antibiotics and delayed formation of sporulating aerial mycelium in the same glutamate-based defined MM. Reverse-transcription polymerase chain reaction and egfp fusion analyses showed that the expression of sigQ was under control by afsQ. In addition, deletion of both afsQ-sigQ resulted in the phenotype identical to that of afsQ mutant. The results suggested that afsQ1-Q2 and sigQ worked together in the regulation of both antibiotic biosynthesis and morphological development, and sigQ might be responsible for antagonizing the function of AfsQ1-Q2 in S. coelicolor, however, in a medium-dependent manner. Moreover, the study showed that the medium-dependent regulation of antibiotic biosynthesis by AfsQ1-Q2-SigQ was through pathway-specific activator genes actII-ORF4, redD, and cdaR. The study provides new insights on regulation of antibiotic biosynthesis and morphological development in S. coelicolor.

Characterization of a novel two-component regulatory system involved in the regulation of both actinorhodin and a type I polyketide in Streptomyces coelicolor

To seek more information on function of two-component regulatory systems (TCSs) in Streptomycescoelicolor, a dozen TCS-knockout mutants were generated, and phenotype changes were determined. One TCS (SCO5403/5404)-deleted mutant with phenotype change was obtained. Here, we report the characterization of this novel TCS, designated as RapA1/A2 (regulation of both actinorhodin and a type I polyketide), using genetic and proteomic approaches. Although growth and morphological analyses showed no difference between the knockout mutant and wild-type strain M145, a visible decrease of the production of actinorhodin (Act) was observed in rapA1/A2 mutant. The decrease can be restored by introducing rapA1/A2 genes on an integrative vector. A 2D-gel based proteomic analysis showed that knockout of rapA1/A2 resulted in reduced expression of a putative 3-oxoacyl-[acyl-carrier protein] reductase that is part of a biosynthetic cluster for a cryptic type I polyketide. Further reverse-transcriptase-polymerase chain reaction (RT-PCR) analyses confirmed that expression levels of several biosynthetic genes and the respective pathway-specific regulatory genes actII-ORF4 and kasO for these two clusters were all down-regulated in the rapA1/A2 mutant, compared to M145. Taken together, the results demonstrated that RapA1/A2 may serve as a positive regulator for biosynthesis of both Act and the uncharacterized polyketide in S. coelicolor, and the effects exerted by RapA1/A2 were dependent on the pathway-specific regulatory genes.

Effects of fatty acid activation on photosynthetic production of fatty acid-based biofuels in Synechocystis sp. PCC6803

BackgroundDirect conversion of solar energy and carbon dioxide to drop in fuel molecules in a single biological system can be achieved from fatty acid-based biofuels such as fatty alcohols and alkanes. These molecules have similar properties to fossil fuels but can be produced by photosynthetic cyanobacteria.ResultsSynechocystis sp. PCC6803 mutant strains containing either overexpression or deletion of the slr1609 gene, which encodes an acyl-ACP synthetase (AAS), have been constructed. The complete segregation and deletion in all mutant strains was confirmed by PCR analysis. Blocking fatty acid activation by deleting slr1609 gene in wild-type Synechocystis sp. PCC6803 led to a doubling of the amount of free fatty acids and a decrease of alkane production by up to 90 percent. Overexpression of slr1609 gene in the wild-type Synechocystis sp. PCC6803 had no effect on the production of either free fatty acids or alkanes. Overexpression or deletion of slr1609 gene in the Synechocystis sp. PCC6803 mutant strain with the capability of making fatty alcohols by genetically introducing fatty acyl-CoA reductase respectively enhanced or reduced fatty alcohol production by 60 percent.ConclusionsFatty acid activation functionalized by the slr1609 gene is metabolically crucial for biosynthesis of fatty acid derivatives in Synechocystis sp. PCC6803. It is necessary but not sufficient for efficient production of alkanes. Fatty alcohol production can be significantly improved by the overexpression of slr1609 gene.

Cross-talk between an orphan response regulator and a noncognate histidine kinase in Streptomyces coelicolor

Two-component systems (TCSs), typically consisting of a histidine kinase (HK) and a cognate response regulator (RR), are the most common signaling systems in bacteria. Besides paired genes encoding TCSs, there also exists unpaired HKs and orphan RRs. In Streptomyces coelicolor, 13 orphan RRs have been annotated. Because of lack of cognate HKs, little is known as yet about the regulation of orphan RRs. Bioinformatic analysis revealed that several orphan RRs had high amino acid sequence identities with RRs from typical TCSs in S. coelicolor. Among them, the orphan RR SCO3818 and RR SCO0204, which paired with HK SCO0203, showed the highest identity (65%), suggesting that the two RRs might both be under the regulation of SCO0203. Following studies showed that SCO0203 could phosphorylate not only SCO0204 but also SCO3818. Deletion of either sco0203 or sco3818 led to enhanced production of blue-pigmented antibiotic actinorhodin, which indicated a functional correlation between SCO0203 and SCO3818. These results suggested that SCO3818 might be regulated by SCO0203. This is the first report describing the regulation of an orphan RR by an HK. Moreover, this is also the first identification of cross-talk between different TCS components in S. coelicolor.

Microbial Synthesis of Alka(e)nes.

Alka(e)nes are the predominant constituents of gasoline, diesel, and jet fuels. They can be produced naturally by a wide range of microorganisms. Bio-alka(e)nes can be used as drop-in biofuels. To date, five microbial pathways that convert free fatty acids or fatty acid derivatives into alka(e)nes have been identified or reconstituted. The discoveries open a door to achieve microbial production of alka(e)nes with high efficiency. The modules derived from these alka(e)ne biosynthetic pathways can be assembled as biological parts and synthetic biology strategies can be employed to optimize the metabolic pathways and improve alka(e)ne production.

Microbial recycling of glycerol to biodiesel

The sustainable supply of lipids is the bottleneck for current biodiesel production. Here microbial recycling of glycerol, byproduct of biodiesel production to biodiesel in engineered Escherichia coli strains was reported. The KC3 strain with capability of producing fatty acid ethyl esters (FAEEs) from glucose was used as a starting strain to optimize fermentation conditions when using glycerol as sole carbon source. The YL15 strain overexpressing double copies of atfA gene displayed 1.7-fold increase of FAEE productivity compared to the KC3 strain. The titer of FAEE in YL15 strain reached to 813 mg L(-1) in minimum medium using glycerol as sole carbon source under optimized fermentation conditions. The titer of glycerol-based FAEE production can be significantly increased by both genetic modifications and fermentation optimization. Microbial recycling of glycerol to biodiesel expands carbon sources for biodiesel production.

Versatility of hydrocarbon production in cyanobacteria

Cyanobacteria are photosynthetic microorganisms using solar energy, H2O, and CO2 as the primary inputs. Compared to plants and eukaryotic microalgae, cyanobacteria are easier to be genetically engineered and possess higher growth rate. Extensive genomic information and well-established genetic platform make cyanobacteria good candidates to build efficient biosynthetic pathways for biofuels and chemicals by genetic engineering. Hydrocarbons are a family of compounds consisting entirely of hydrogen and carbon. Structural diversity of the hydrocarbon family is enabled by variation in chain length, degree of saturation, and rearrangements of the carbon skeleton. The diversified hydrocarbons can be used as valuable chemicals in the field of food, fuels, pharmaceuticals, nutrition, and cosmetics. Hydrocarbon biosynthesis is ubiquitous in bacteria, yeasts, fungi, plants, and insects. A wide variety of pathways for the hydrocarbon biosynthesis have been identified in recent years. Cyanobacteria may be superior chassis for hydrocabon production in a photosynthetic manner. A diversity of hydrocarbons including ethylene, alkanes, alkenes, and terpenes can be produced by cyanobacteria. Metabolic engineering and synthetic biology strategies can be employed to improve hydrocarbon production in cyanobacteria. This review mainly summarizes versatility and perspectives of hydrocarbon production in cyanobacteria.

Production of Photosynthetic Biofuels by Genetically Engineering Cyanobacteria

Cyanobacteria, photosynthetic bacteria with conversion capability to utilize solar energy and carbon dioxide and genetic engineering capacity to be easily modified to build non-native and improve native biosynthetic pathways, have displayed huge potential for biotechnology applications for direct production of biofuels by using solar energy as energy source and carbon dioxide as carbon source. Here research progress on microbial production of photosynthetic biofuels including hydrogen, ethanol, higher alcohols, isoprene and fatty acid-based biofuels in genetically engineered cyanobacteria is reviewed and the engineering challenges for using cyanobacteria as model hosts to make biofuels with high efficiency are discussed.