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CrtE, CrtB, CrtI, and CrtY are key enzymes involved in carotenoid biosynthesis. CrtE participates in geranylgeranyl pyrophosphate synthesis, CrtB catalyzes phytoene formation, CrtI mediates phytoene desaturation, and CrtY converts lycopene into beta-carotene. Although these enzymes are essential for carotenoid production, recent metabolic engineering studies suggest that carotenoid-specific enzymes do not solely limit lycopene yield. Instead, precursor availability, metabolic flux through the MEP pathway, cofactor balance, and overall cellular metabolism strongly influence production efficiency. (Huang et al., 2025; Jing et al., 2021; Sandmann & Misawa, 1992)
Although phytoene desaturation mediated by CrtI has historically been considered an important control point in carotenoid biosynthesis, recent studies suggest that carotenoid production is often limited by precursor supply and metabolic flux rather than by a single carotenoid-specific enzyme. In engineered E. coli systems, optimization of the MEP pathway, carbon metabolism, and cofactor availability can significantly influence lycopene and β-carotene yields (Wang et al., 2019; Liu et al., 2026).
I would choose Escherichia coli as the production organism for this construct because the goal of this lab is to design a microbial system for carotenoid bioproduction, especially lycopene or β-carotene. E. coli is a convenient chassis because it grows quickly, is inexpensive to culture, and has well-established plasmid-based expression systems. In addition, several metabolic engineering strategies for carotenoid production in E. coli focus on improving precursor supply through the MEP pathway, optimizing central carbon metabolism, and balancing expression of heterologous carotenoid genes. For this reason, E. coli would be a practical host for a first plasmid-based design. (Liu et al., 2026; Wu et al., 2020)
For the expression construct, I would choose CrtY, the lycopene β-cyclase enzyme. This enzyme converts lycopene into β-carotene, so it is directly relevant if the desired final product is β-carotene. In this design, lycopene can be considered the precursor, and CrtY would redirect the pathway toward β-carotene accumulation. This choice also makes the construct easy to explain because the presence or absence of CrtY activity helps determine whether the engineered system accumulates lycopene or produces β-carotene. (Wang et al., 2020; Liu et al., 2026)
Based on Liu et al, 2026
A promoter is a regulatory DNA sequence that controls the initiation of transcription by recruiting RNA polymerase and transcriptional machinery. In engineered microbial systems, promoters are essential for regulating the expression level of heterologous genes involved in biosynthetic pathways.
Promoters can be classified as constitutive, inducible, or repressible promoters. Constitutive promoters continuously drive gene expression, whereas inducible promoters activate transcription in response to specific molecules such as IPTG or arabinose. Repressible promoters decrease transcription in response to regulatory signals or metabolites. In metabolic engineering, inducible promoters are commonly used to balance cell growth and product synthesis while minimizing metabolic burden.
If the goal is to suppress transcription in response to a metabolite, repressible promoters would be useful because they decrease gene expression after sensing a specific molecule. Conversely, inducible promoters are useful when gene expression should increase in the presence of a metabolite or inducer. Dynamic promoter regulation is important in metabolic engineering because excessive expression of biosynthetic genes can impose a metabolic burden and reduce cell growth.