## 4 Common Strategies for Optimizing Sesquietpenol Biosynthesis
### The Strategies for the Regulation of the Synthesis Pathway  
#### 4.2.1 The Engineering of Host Metabolism  
The production of sesquiterpenols via microbial endogenous metabolism alone remains insufficient for industrial applications. A common strategy to enhance their production involves engineering rate-limiting pathways to improve precursor supply (Figure 5a, Table 2). The HMG1 protein, a reductase that catalyzes the conversion of HMG-CoA to mevalonate, is the rate-limiting enzyme in the MVA pathway and represents a critical bottleneck in terpenoid biosynthesis. Overexpression of HMG1 significantly enhances metabolic flux through the mevalonate pathway. During farnesol (FOH) biosynthesis, ten genes in the MVA pathway were overexpressed to assess their contributions to FOH production. Among these, overexpression of HMG-CoA reductase (HMG1) demonstrated the most pronounced enhancement [61]. In _S. cerevisiae_, HMG1 activity is regulated by feedback and cross-regulation in the MVA pathway, primarily through protein degradation via its SSD domain [107]. Truncating the SSD domain prevents post-transcriptional degradation of HMG1, thereby enhancing its catalytic stability [108]. Consequently, the N-terminal truncated tHMG1 (500 amino acids) exhibits stable cytoplasmic expression. By overexpressing this truncated HMG-CoA reductase (tHMGR1), [PERSON] et al. increased farnesol production 50.8-fold to 5.08 mg/L [55]. Given the multi-enzyme nature of the MVA pathway, combinatorial overexpression strategies generally outperform single-enzyme optimization. [PERSON] et al. overexpressed tHMG1, and the resulting strain showed a 2.5-fold increase of the (-)-\(\alpha\)-bisabolol yield. When both tHMG1 and ERG20 were overexpressed, the final production of (-)-\(\alpha\)-bisabolol was increased 3.4-fold compared with the initial engineered strain. Thus, it can be concluded that overexpression of tHMG1 and ERG20 can effectively increase the FPP supply, thereby significantly enhancing the production of (-)-\(\alpha\)-bisabolol [70]. Recent advances include the Global Metabolic Engineering Strategy (GMES), which employs cocktail \(\delta\)-integration in _S. cerevisiae_ to enable multi-copy integration of pathway genes at \(\delta\)-sites for complex pathway optimization [109]. [PERSON] et al. applied this strategy and achieved patchoul production at 42.1 mg/L through pathway gene copy number optimization [75]. Introducing heterologous pathways is a commonly used strategy to enhance precursor availability (Figure 5a, Table 2). Recent advances in heterologous terpenoid biosynthesis have gradually been increasingly reported and applied. [PERSON] et al. successfully integrated the heterologous MVA pathway into _S. marcescens_ and used inducible and constitutive promoters to adjust key enzyme expression levels, thereby increasing (-)-\(\alpha\)-bisabolol yield to 2.1 g/L [5]. [PERSON] et al. successfully expressed the mevalonate pathway in _E. coli_, achieving a farnesol concentration of 526.1 mg/L [110]. The isopentenol utilization pathway (IUP), a two-step enzymatic cascade, enables precursor supplementation via exogenous isopentenol administration [111]. This system utilizes _Arabidopsis thaliana_ choline kinase (CK) and isopentenyl phosphate kinase (AtIPK) to sequentially phosphate isopentenol, thereby generating isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP)--universal precursors for terpenoid biosynthesis. [PERSON] et al. introduced the IUP pathway in _S. cerevisiae_, achieving a remarkable 147-fold increase in the IPP/DMAPP precursor pool compared to native MVA pathway [112]. Furthermore, [PERSON] et al. developed the isoprenoid alcohol (IPA) pathway [113], which demonstrates superior energetic efficiency through reduced ATP and cofactor demands, thereby establishing a versatile platform for isoprenoid production. Overexpression of IPP isomerase (IDI) is also a critical strategy for pathway optimization due to its role in flux expansion.