# Advances in the Metabolic Engineering of _Escherichia coli_ for the Manufacture of Monoterpenes
## 1 Introduction
Similar problems also exist in the subsequently developed chemical synthesis strategy with the complex reaction process and the environment pollution risks, even though the productivity could be increased. Moreover, such modes of production could hardly synthesize compounds with complex molecular structure and specific affinity and specificity [24]. To address these problems and achieve broad commercial applications, scholars have developed a series of biological manufacturing methods with the advantages of self-assembly, proliferation, mild reaction condition requirements, and environment-friendly features to produce valuable monoterepenes, especially in this rapid development era of synthetic biology and bioengineering [25; 26]. Several microorganisms, such as _Escherichia coli_ and _Saccharomyces cerevisiae_, are considered as perfect chassis and have been designed as microbial cell factories for the industrialized production of significant monotereponids [27; 28]. Both MEV and MEP pathways and their downstream enzyme systems are available to be incorporated and engineered in these chassis with different strategies to produce various valuable monoprenoids with high yield. This review summarizes the advances of different strategies for the establishment and optimization of heterologous monoterepene synthesis in the prokaryotic cell factory (Table 1). \begin{table}
\begin{tabular}{p{42.7 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Pathway** & \begin{tabular}{c} **Strains of** \\ **the E. coli** \\ **Chassis** \\ \end{tabular}  & \begin{tabular}{c} **Origin of the Integrated** \\ **Enzymes for the** \\ **Monotepene Production** \\ \end{tabular}  & **Engineering Design** & \begin{tabular}{c} **Monotepene** \\ **Product** \\ **Monotepene Yield,** \\ **Conductance and** \\ **Recovery Methods** \\ \end{tabular}  & \begin{tabular}{c} **Maximal** \\ **Monotepene Yield,** \\ **Reference** \\ **Reference** \\ \end{tabular}  \\ \hline MEP & \begin{tabular}{c} BLR (DE3) \\ Codon \\ Plus-RIL cells \\ \end{tabular}  & 1. GPIPS from _Ahies grandis_ & 1. Ableestone of enhanced MEP or 2. uTLS from _Methus spiata_ & Limonene & \begin{tabular}{c} -5 mg/L, \\ Steam distillation; \\ \end{tabular}  & [29] \\ MEP & \begin{tabular}{c} 1. GPIPS from _Ahies grandis_ \\ 3. DNS and IDI from _E. coli_ \\ \end{tabular}  & 2. Plasmid vector and enzyme & \begin{tabular}{c} 1. Codon optimization \\ 2. Plasmid vector and enzyme \\ arrangement selection \\ 3. Integration of _spress_ and _ls_ in \\ one plasmid; integration of _dsx_ \\ and _li in another plasmid_ \\ \end{tabular}  & \begin{tabular}{c} 35.8 mg/L, \\ two-phase culture of \\ _n_-hexadecane \\ organic layer \\ \end{tabular}  & [30] \\ MEP & \begin{tabular}{c} 1. AACT and IDI from _E. coli_ \\ 2. HMGS and HMGR from _E. coli_ \\ _Saccharomyces cerevisiae_ \\ 4. GPIPS from _Ahies grandis_ \\ 5. tLS from _Methus spiata_ \\ 6. efflux pump from _Achomica_ \\ \end{tabular}  & \begin{tabular}{c} 1. Codon optimization \\ 2. Exogenous pathway \\ introduction \\ 3. Replication origin and \\ promoter strength selection \\ 4. Integration of seven MEV \\ pathway genes in one plasmid; \\ integration of spgps and _ls_ in \\ another plasmid; efflux pump \\ genes integrated in the last \\ plasmid alone. \\ \end{tabular}  & \begin{tabular}{c} 1. Codon optimization \\ 2. Exogenous pathway \\ introduction \\ 3. Strong promoter \\ \end{tabular}  & \ \begin{tabular}{c} -435 mg/L, \\ two-phase culture of \\ dodecane organic \\ \end{tabular}  & [31] \\ MEP &
\begin{tabular}{c} 31. \\ 32. \\ 33. \\ 3.