To genetically engineer cyanobacteria, we chose Synechococcus elongatus PCC 7942 as our engineering host. Our main strategy is to embark on gene double-crossover homologous recombination in S. elongatus PCC 7942 genome, which is the first cyanobacterial strain to be transformed by exogenous DNAs and is reliably transformable through natural uptake of extracellular DNAs.

　　First, we constructed a vector which is able to finish double-crossover homologous gene recombination in S. elongatus PCC 7942. The vector (pPIGBACK) contains 5’- and 3’-ends of the neutral site II (NSII) and an ampicillin resistance gene (AmpR) for antibiotic selection. Then we fused AmpR with double terminator, BBa_B0015, which is proved to be functional in cyanobacteria. Additionally, in order to easily manipulate DNAs for gene cloning and plasmid preparation in E. coli DH5α, the replication origin (ORI) of pBR322 was also introduced to make the plasmid vector replicable in E. coli. Then, in order to overexpress foreign genes in the cyanobacteria, the intrinsic promoter of Rubisco large subunit (PrbcL) was chosen as the target for vector construction. PrbcL regulates the expression of the most abundant proteins in photosynthetic species and has been proven to have a high activity to express foreign genes, so we chose PrbcL as the promoter of our pigment gene.¹

　　The strategy we chose to construct the vector is to fuse B0015 and AmpR together first. Secondly, we fused 5’- and 3’-ends of the neutral site II (NSII) with PBR322 replication origin (ORI) together. At last, we ligated two parts together. The vector (pPIGBACK) is used to transform into PCC7942 with the inserted pigment gene in our experiments. After mass reproduction in E. coli DH5α, PCC7942 were transformed through the uptake of plasmid DNAs extracted from E. coli DH5α. The transformed strains (transformants) were usually successfully obtained after 2 to 3 weeks and survived the ampicillin treatment.

　　Indigoidine is a bacterial natural product with antioxidant and antimicrobial activities. Its bright blue color resembles the industrial dye indigo, thus representing a new natural blue dye that may find uses in industry. In nowadays studies, an Indigoidine synthetase Sc-IndC and an associated helper protein Sc-IndB were identified from Streptomyces chromofuscus ATCC 49982 and successfully expressed in Escherichia coli BAP1 to produce the blue pigment². The IndB gene codes for a putative phosphatase and the IndC gene codes for Indigoidine synthase. Together, these enzymes convert L-glutamine into Indigoidine. Recently, it has been shown that IndC alone can produce Indogoidine, and the inclusion of IndB expression in the system will increase yields significantly³.

　　As we know, L-Glutamine is the direct biosynthetic precursor of Indigoidine, and it is a key amino acid in primary metabolism and thus naturally exists in S. elongatus PCC7942. Because glutamine related products are already existed in S. elongatus PCC7942, we only need to activate the expression of Sc-IndC in S. elongatus PCC7942 which leads to the production of Indigoidine. However, due to the access difficulties of Streptomyces chromofuscus ATCC 49982, we decided to use the previous part for IndC, which has been submitted to the iGEM Parts Registry (BBa_K1152008)⁴. According to the part design, our Indigoidine gene comes from Photorhabdus luminescens laumondii TT01 (DSM15139).

　　Zeaxanthin belongs to carotenoid family and is widely found in the nature. It is also a natural color making corns, carrots and marigolds yellow. Moreover, zeaxanthin is an essential nutrient substance to human’s eyes, and some healthy supplements are made of it. Most of green plants produce zeaxanthin as an intermediate in carotenoid pathway. However, some photosynthetic bacteria such as cyanobacteria lack of zeaxanthin. Therefore, we try to transform zeaxanthin-related genes to cyanobacteria to make them yellow.

　　After paper study, we find nobody has done it before. Under our instructor’s help, we develop a way to make cyanobacteria yellow. We compare the complete carotenoid pathway with Synechococcus elongatus PCC 7942 whole genomic DNA on KEGG and we find every zeaxanthin-related gene is included in PCC 7942 genomic DNA except beta-carotene hydroxylase (crtZ) . And we find crtZ coding sequence with ribosome-binding site is an igem released part (BBa_I742158), which is from a plant pathogen, Pantoea ananatis.

　　We had successfully transformed CrtZ to Escherichia coli to reproduce massively, and then transformed CrtZ with pPIGBACK to Synechococcus elongatus PCC 7942. After a week, the transformed Synechococcus elongatus PCC 7942 expressed more yellow than the control group. To test whether the photosynthetic efficiency is better, we used iodine to measure starch concentration and compare it with wild type.

　　Melanin, a biological pigment widely found in terrestrial flora and fauna, is a complex oxidation product of amino acid tyrosine. Melanin plays diverse roles in a myriad of organisms. As far as the human is concerned, melanin is the primary determinant of human skin color and pupils or irises of the eyes. It is also an important signal molecule in the human neural system. For microorganisms, melanin would protect them by against ultraviolet radiation effect from sunlight, which is detrimental to most of the organisms.

　　Due to the fact that the dark pigment derived from MelA gene has extensive wavelength absorbance, we decided to transform MelA gene combined with particular constitute promoter into Synechococcus elongates PCC 7942 to measure the growth curve and photosynthetic efficiency of it. Based on the well-elaborated procedure provide by IGEM Tokyo Tech 2009, we had intended to replicate their experiment to produce melanin massively first in E. coli, but failed to succeed due to inconsistence of DNA sequence. Therefore, we had no alternative but to turn to DNA synthesis and directly transform MelA gene ligated with our backbone into microalgae.

　　Astaxanthin is a high value and natural pink pigment which can be found in microalgae, yeast and some sea creatures. It’s special due to its antioxidant activity and has been suggested to be beneficial in cardiovascular, immune, inflammatory and neurodegenerative diseases and skin health. Although it has lots of benefits, astaxanthin is still a product result in a minority amount in the carotenoid synthesis pathway compare with other carotenoid families and yet, the artificial chemical synthesis cost high and result in the least production.

　　Astaxanthin synthesis does not naturally exist in the S. elongatus PCC7942. But fortunately, after paper research, we found out that S. elongatus PCC7942 has a similar pathway with other microalgae which can synthesize astaxanthin, and the only different is, S. elongatus PCC7942 lack of two necessary gene: beta-carotene ketolase (crtW) and beta-carotene hydroxylase (crtZ) to undergo this pathway. Thus, we use IDT to synthesis these two genes and construct it on pPIGBACK, a vector which can express the carrying genes in S. elongatus PCC7942.

　　We expected the transformed PCC7942 with CrtZ gene could produce zeaxanthin and become more yellow. Just like our prediction, the result showed that the transformed PCC7942 was more yellow than wild type obviously.

　　Moreover, we expected the transformed PCC7942 which produce zeaxanthin would change wavelength absorbance and have better photosynthetic efficiency.

Wavelength Absorbance

　　To test this hypothesis, we used spectrophotometer to measure the absorbance of CrtZ and wild type at 400 to 700 nm. The outcome was that the OD value of CrtZ at 400 to 500 nm was hihger than wild type. The result verified our first prediction, a change of wavelength absorbance. Not only this, the change of wavelength absorbance was at 400 to 500 nm, which was blue light. This indicated CrtZ absorbed blue light and reflected yellow light, so CrtZ was more yellow than wild type. The measurement matched what we saw intuitively.

Photosynthetic Efficiency

　　To test whether the photosynthetic efficiency of CrtZ is better than wild type, we used iodine to measure starch concentration. First, the initial concentration of microalgae of CrtZ and wild type should be same, so that the measurement could be fair. We measured the OD value at 730 nm, which represented the concentration of microalgae. Then we calculated how much microalgae and BG11 (the medium) we should add to make same amount of microalgae in each plate. Second, we started to measure starch concentration. We measured the OD value of each plate at 730 nm, which represented the cell number. Then we added 50 μl iodine into each cuvette, waited for five minutes, and measured the OD value at 620 nm, which represented the starch content. We repeated this step for seven days. Here are our results.

　　Then we divided the starch content by cell number, and we knew there was how much starch in every unit cell.

　　Then we calculated the variation of starch content per cell per day.

　　The first day, wild type (WT) had more starch than CrtZ. However, the increase of starch per day of CrtZ was more than wild type. The result implied that CrtZ could produce more starch per day than the wild type. Moreover, the result corresponds to our hypothesis -- CrtZ actually had better photosynthetic efficiency!

Survival Ability

　　We also considered about the survival ability of CrtZ. Therefore, we conducted a competitive experiment of CrtZ and wild type. We co-cultured CrtZ and wild type together, and then observed the color after seven days. Similarly, the initial concentration of microalgae of CrtZ and wild type should be same, and we used the same method we used in photosynthetic efficiency experiments.

　　After seven days, CrtZ and wild type both survived. The result indicated that CrtZ was strong and had great survival ability!

Conclusion

Reference