1. Introduction
Terpenes are a large family of chemicals with the formula (C
5H
8)
n that are often produced by plants where they act in their photoautotrophic metabolism, protection against pathogens and/or attraction of pollinators. Many high-value terpenes are exploited by cosmetic, food, pharmaceutical and/or biofuel industries, including bisabolene, farnesene, limonene and pinene, the focus of the present study [
1,
2].
All terpenes are derived from the five-carbons (C5) building blocks, isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP) that are condensed to form the geranyl pyrophosphate (GPP, C10) precursor of monoterpenes (C
10H
16) such as limonene and pinene. The subsequent addition of one IPP unit on GPP forms the farnesyl pyrophosphate (FPP, C15) precursor of sesquiterpenes (C
15H
24), such as bisabolene and farnesene (
Figure 1).
Instead of plants that must be preserved for food production, cyanobacteria, the robust photosynthetic prokaryotes that colonize our planet, can be used for the sustainable production of terpenes from the plentiful natural resources: solar energy, water, CO
2 and minerals [
1]. Cyanobacteria possess the methylerythritol-4-phosphate (MEP) pathway that produces GPP and FPP (
Figure 1), which can be transformed into terpenes. For this purpose, synthetic genes encoding terpene synthases need to be adapted to the cyanobacterial codon usage, and subsequently introduced and expressed in cyanobacteria [
1,
2].
The model (unicellular) cyanobacteria
Synechococcus PCC 7002 (hereafter S.7002) and
Synechococcus PCC 7942 (S.7942) are attractive for this purpose, in having good genetic tools and interesting physiological difference (for review see [
3]) which make their comparative analysis very interesting. The salt-tolerant marine model S.7002 (genome size 3.41 Mb) is able to grow on nitrate (the classical nitrogen source for cyanobacteria) or urea (cheaper than nitrate) [
4], but it requires vitamin B12 for growing [
5]. In contrast, the salt-sensitive freshwater model S.7942 (genome size 2.74 Mb) does not require vitamin B12, but is unable to grow on urea [
6]. S.7002 and S.7942 have other key differences in their DNA repair [
7] and glutathione-dependent systems that play a prominent role in tolerance to stresses [
3]).
Two proof-of-concept studies showed that S.7002 can be engineered for the photosynthetic production of bisabolene and limonene [
8] or pinene [
9], but not farnesene as we presently report. Concerning S.7942, several laboratories reported that S.7942 can be engineered for the production of farnesene [
10,
11,
12] or limonene [
13,
14], but neither bisabolene nor pinene as we report here. The limited number of previous studies and their multiple experimental differences (nature of the terpene synthase genes and their promoters, and of the chromosome/plasmid vectors propagating them) make it is difficult to know whether S.7002 and/or S.7942 are suitable chassis for the photoproduction of chemically-diverse terpenes or only a few of them.
In this study, we have employed the same genetic strategy to engineer S.7002 and S.7942 for the photoproduction of four chemically-different high-value terpenes: two monoterpenes, limonene (cyclic molecule) and pinene (bicyclic), and two sesquiterpenes, bisabolene (cyclic) and farnesene (linear), in order to compare the potential of these two model cyanobacteria for the production of various terpenes. Each terpene synthase gene was expressed from the same strong promoter propagated by the same autonomously replicating plasmid vector in both S.7002 and S.7942. We showed for the first time that S.7002 and S.7942 can produce farnesene and bisabolene, respectively. Both S.7002 and S.7942 produced farnesene and pinene with the best and lowest efficiencies, respectively. Furthermore, S.7002 produced limonene better than bisabolene, whereas S.7942 produced bisabolene better than limonene. We also showed for the first time, that S.7002 cultivated on urea (a frequent pollutant) produce terpenes more efficiently than cultures growing on either nitrate or ammonium (standard, but more expensive, nitrogen sources for cyanobacteria). Finally, we showed in both S.7002 and S.7942 that increasing the copy number of the terpene synthase-encoding gene increases the level of terpene production, suggesting that terpene production is more limited by the activity of terpene synthases rather than the abondance of terpene precursors.
4. Conclusions
We have performed the first comparative analysis of the ability of two physiologically-diverse model unicellular cyanobacteria
Synechococcus PCC 7002 (marine host) and
Synechococcus PCC 7942 (fresh-water host), to photosynthetically produce chemically-diverse terpenes of high-value (bisabolene, farnesene, limonene and pinene). For this purpose, each terpene synthase gene expressed from the same strong promoter was propagated by the same RSF1010-derived replicative plasmid vector. We showed, for the first time, that S.7002 and S.7942 can be engineered for the photoproduction of farnesene and bisabolene, respectively. S.7942 and S.7002 (in this order) produced farnesene more efficiently than the other tested terpenes (especially pinene the weakest produced terpene) similarly to what we previously observed in the other model cyanobacterium
Synechocystis PCC 6803 [
20]. Furthermore, S.7002 produced limonene better than bisabolene, whereas S.7942 produced bisabolene better than limonene. We showed for the first time that S.7002 can produce terpenes when growing on urea (a frequent pollutant) as the sole nitrogen source, similarly to what we observed for
Synechocystis PCC 6803 [
20]. In the case of S.7002, higher levels of terpenes were produced by cultures growing on urea, as compared to nitrate or ammonium, the standard or frequent nitrogen source for cyanobacteria, respectively. Also interestingly, higher levels of terpenes were produced by both S.7002 and S.7942 expressing the terpene synthase gene from both a RSF1010-derived replicating pC plasmid and a neutral chromosomal site, as we observed in
Synechocystis PCC 6803 [
20]. These results suggest that the production of terpene
s in cyanobacteria is more limited by the activity of terpene synthases than the abondance of terpene precursors, similarly to what we discussed in the case of
Synechocystis PCC 6803 [
20]. Finally, we compared the levels of terpene production by S.7002 and S.7942 with what we previously observed for
Synechocystis PCC 6803 that was engineered the same way than S.7002 and S.7942 [
20]. We found that S.6803 produces higher levels of the sesquiterpenes farnesene and bisabolene than S7942 and S.7002 (in this order), suggesting that S.6803 could be a better cyanobacterial chassis for future industrial photoproduction of these chemicals.
Author Contributions
Conceptualization, F.C. and C.C.-C.; methodology, F.C., C.C.-C. and A.G.; validation, C.C., V-B.G., M.V., E.D-S. and A.G.; formal analysis, C.C., V-B.G., M.V. and E.D-S.; investigation, C.C., V-B.G., M.V., E.D-S., C.C.-C. and F.C.; resources, F.C. and C.C.-C.; writing—original draft preparation, F.C., C.C.-C., C.C., V-B.G. and M.V.; writing—review and editing, F.C. and C.C.-C.; supervision, F.C. and C.C.-C.; project administration, F.C. and C.C.-C.; funding acquisition, F.C. and C.C.-C. All authors have read and agreed to the published version of the manuscript.