Extending the biosynthetic repertoires of cyanobacteria and chloroplasts

Abstract

SUMMARYChloroplasts in plants and algae and photosynthetic microorganisms such as cyanobacteria are emerging hosts for sustainable production of valuable biochemicals, using only inorganic nutrients, water, CO 2 and light as inputs. In the past decade, many bioengineering efforts have focused on metabolic engineering and synthetic biology in the chloroplast or in cyanobacteria for the production of fuels, chemicals and complex, high-value bioactive molecules. Biosynthesis of all these compounds can be performed in photosynthetic organelles/organisms by heterologous expression of the appropriate pathways, but this requires optimization of carbon flux and reducing power, and a thorough understanding of regulatory pathways. Secretion or storage of the compounds produced can be exploited for the isolation or confinement of the desired compounds. In this review, we explore the use of chloroplasts and cyanobacteria as biosynthetic compartments and hosts, and we estimate the levels of production to be expected from photosynthetic hosts in light of the fraction of electrons and carbon that can potentially be diverted from photosynthesis. The supply of reducing power, in the form of electrons derived from the photosynthetic light reactions, appears to be non-limiting, but redirection of the fixed carbon via precursor molecules presents a challenge. We also discuss the available synthetic biology tools and the need to expand the molecular toolbox to facilitate cellular reprogramming for increased production yields in both cyanobacteria and chloroplasts.

CyanoGate: A Golden Gate modular cloning suite for engineering cyanobacteria based on the plant MoClo syntax

“…The increasing availability of genome-scale metabolic models for different cyanobacterial species and their utilisation for guiding engineering strategies for producing heterologous high-value biochemicals has helped to re-invigorate interest in the industrial potential of cyanobacteria (95)(96)(97)(98). Future efforts should focus on combining genome-scale metabolic models with synthetic biology approaches, which may help to overcome the production yield limitations observed for cyanobacterial cell factories (8), and will accelerate the development of more complex and precise gene control circuit systems that can better integrate with host metabolism and generate more robust strains (99)(100)(101). The future development of truly 'programmable' photosynthetic cells could provide significant advancements in addressing fundamental biological questions and tackling global challenges, including health and food security (102)(103)(104).…”

“…They are metabolically diverse and encode many components (e.g. P450 cytochromes) necessary for generating high-value pharmaceutical products that can be challenging to produce in other systems (8)(9)(10)(11). Furthermore, cyanobacteria show significant promise in biophotovoltaic devices for generating electrical energy (12,13).…”

Harnessing transcription for bioproduction in cyanobacteria

“…Usually, the simple expression and optimization of a biosynthetic pathway is not enough, and the complete redesign of the metabolism is required to overcome metabolic bottlenecks, avoid feedback regulation, might require insertion of alternate pathways with optimal metabolite usage, and in general design a specialized strain for optimal production. These metabolic considerations and the coupling of metabolic engineering with systems biology have been extensively discussed elsewhere (Gudmundsson and Nogales , Nielsen et al , Oliver et al ).…”

Cyano-assassins: Widespread cyanogenic production from cyanobacteria

“…many terpenoids, quinolizidine alkaloids, piperidine alkaloid coniine) are produced completely or partly in chloroplasts and/or mitochondria (Wink, 2003, 2008). Moreover, there is an indication that many enzymes and pathways are common in plant and cyanobacteria secondary metabolite production (Chen et al, 2016; Nielsen et al, 2016).…”