## 2 Materials and Methods
### Constructing the Engineered Bacterial Strains  
To construct the artificial operon _P165-g10-merT-merP-merB1-merB2-ppk-rpsT_, a series of primers were designed and synthesized (Table 1). Based on the sequences of plasmids pMR26 (GenBank accession number: D83080.2) and pMR28 (GenBank accession number: AB013925.1), the artificial operon _merT-merP-merB1-merB2_ was designed and synthesized by TAKARA (Dalian, China). The synthesized DNA fragment was inserted into the region between the _Nde_ I and _Not_ I restriction sites of plasmid pET28a using an In-Fusion HD Cloning Kit (Clontech, code number: 639633). The resulting plasmid was named p1.  
The _ppk_ gene was amplified from _Enterobacter aerogenes_ genomic DNA using 2\(\times\) Pfu MasterMix (ComWin Biotech, code number: CW0686A). The primers used were PPPKF and PPPKR. The amplified fragment was inserted into the area between the _Not_ I and _Xho_ I restriction sites of plasmid p1. The resulting plasmid was named p2 (Table 1). According to published DNA sequences (GenBank accession number: Z00044.2, reference) [11], P1F and P1R were synthesized and 1 mmol/L P1F and 1 mmol/L P1R were mixed with 25 uL of 2\(\times\) Pfu MasterMix. The volume of reaction was 500 uL. The mixture was annealed in a PCR amplifier. The annealed product, the strong constitutive promoter _P16S_[12; 13; 14] and the translation enhancer _g10_[11], was inserted into plasmid pET28a. The resulting plasmid was named p3 (Table 1). Primers P2F and P2R were synthesized and DNA fragment _merT-merP-merB1-merB2-ppk_ was amplified using p2 as a template. The amplified fragment was inserted into the region between the _Bal_ I and _Eco_ R I sites of p3. The resulting plasmid was named p4 (Table 1).  
Primers P3F and P3R were synthesized according to a previously published sequence (GenBank accession number: Z00044.2). Using P3F and P3R as primers, _rp5T_, a strong terminator that can enhance transcript stability by forming a secondary structure at the 3\({}^{\prime}\) end of the mRNA [15], was amplified from tobacco chloroplast DNA. The amplified _rp5T_ was inserted into the _Eco_ R I site of p4. The resulting plasmid, in which the transcription direction of _rp5T_ was consistent with that of _merT-merP-merB1-merB2-ppk_, was named p5 (Table 1). The _E. coli_ BL21 strain harboring p5 was named BL21-7. The sequence of the artificial operon _P16S-g10-merT-merP-merB1-merB2-ppk-rpsT_ is shown in Supplementary Materials S1. To characterize the roles of the genes in the artificial operon _P16S-g10-merT-merP-merB1-merB2-ppk-rpsT_ and the transcription of the operon, a series of artificial operons were constructed (Table 2). The detailed procedures can be found in Supplementary Materials S2.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Primer** & **Sequence** & **Plasmid** \\ \hline PPPKF & 5\({}^{\prime}\)-ATAAGAATGCGCCGCCGCATTACCACCCTGATT-3\({}^{\prime}\); & \\  & _N_of I restriction site is underlined. & p2 \\ \hline PPPKR & 5\({}^{\prime}\)-CCGATCCCCGACGTTTAATCGGCTTCCG-3\({}^{\prime}\); & \\  & _X_h0 I restriction site is underlined. & p2 \\ \hline P1F & 5\({}^{\prime}\)-CGACATATGGCTCCGCCGTTCAAATGGAA