The secondary metabolites of higher plants include diverse chemicals, such as alkaloids, isoprenoids and phenolic compounds (phenylpropanoids and flavonoids). Although these compounds are widely used in human health and nutrition, at present they are mainly obtained by extraction from plants and extraction yields are low because most of these metabolites accumulate at low levels in plant cells. Recent advances in synthetic biology and metabolic engineering have enabled tailored production of plant secondary metabolites in microorganisms, but these methods often require the addition of expensive substrates. Here we develop an Escherichia coli fermentation system that yields plant alkaloids from simple carbon sources, using selected enzymes to construct a tailor-made biosynthetic pathway. In this system, engineered cells cultured in growth medium without additional substrates produce the plant benzylisoquinoline alkaloid, (S)-reticuline (yield, 46.0 mg l−1 culture medium). The fermentation platform described here offers opportunities for low-cost production of many diverse alkaloids.

A novel bacterial putrescine utilization pathway was discovered. Seven genes, the functions of whose products were not known, are involved in this novel pathway. Five of them encode enzymes that catabolize putrescine; one encodes a putrescine importer, and the other encodes a transcriptional regulator. This novel pathway involves six sequential steps as follows: 1) import of putrescine; 2) ATP-dependent ␥-glutamylation of putrescine; 3) oxidization of ␥-glutamylputrescine; 4) dehydrogenation of ␥-glutamyl-␥-aminobutyraldehyde; 5) hydrolysis of the ␥-glutamyl linkage of ␥-glutamyl-␥-aminobutyrate; and 6) transamination of ␥-aminobutyrate to form the final product of this pathway, succinate semialdehyde, which is the precursor of succinate.

Opiates such as morphine and codeine are mainly obtained by extraction from opium poppies. Fermentative opiate production in microbes has also been investigated, and complete biosynthesis of opiates from a simple carbon source has recently been accomplished in yeast. Here we demonstrate that Escherichia coli serves as an efficient, robust and flexible platform for total opiate synthesis. Thebaine, the most important raw material in opioid preparations, is produced by stepwise culture of four engineered strains at yields of 2.1 mg l−1 from glycerol, corresponding to a 300-fold increase from recently developed yeast systems. This improvement is presumably due to strong activity of enzymes related to thebaine synthesis from (R)-reticuline in E. coli. Furthermore, by adding two genes to the thebaine production system, we demonstrate the biosynthesis of hydrocodone, a clinically important opioid. Improvements in opiate production in this E. coli system represent a major step towards the development of alternative opiate production systems.

β-Glucosidase from Kluyveromyces marxianus (KmBglI) belongs to the GH3 (glycoside hydrolase family 3). The enzyme is particularly unusual in that a PA14 domain (pf07691), for which a carbohydrate-binding role has been claimed, is inserted into the catalytic core sequence. In the present study, we determined the enzymatic properties and crystal structure of KmBglI in complex with glucose at a 2.55 A (1 A=0.1 nm) resolution. A striking characteristic of KmBglI was that the enzyme activity is essentially limited to disaccharides, and when trisaccharides were used as the substrates the activity was drastically decreased. This chain-length specificity is in sharp contrast with the preferred action on oligosaccharides of barley β-D-glucan glucohydrolase (ExoI), which does not have a PA14 domain insertion. The structure of subsite (-1) of KmBglI is almost identical with that of Thermotoga neapolitana β-glucosidase and is also similar to that of ExoI, however, the structures of subsite (+1) significantly differ among them. In KmBglI, the loops extending from the PA14 domain cover the catalytic pocket to form subsite (+1), and hence simultaneously become a steric hindrance that could limit the chain length of the substrates to be accommodated. Mutational studies demonstrated the critical role of the loop regions in determining the substrate specificity. The active-site formation mediated by the PA14 domain of KmBglI invokes α-complementation of β-galactosidase exerted by its N-terminal domain, to which the PA14 domain shows structural resemblance. The present study is the first which reveals the structural basis of the interaction between the PA14 domain and a carbohydrate.

Glutathione protects cells and organisms from oxygen species and peroxides and is indispensable for aerobically living organisms. Moreover, it acts against xenobiotics and drugs by the formation and excretion of glutathione S conjugates. In this study, we show that the yliA, -B, -C, and -D genes of Escherichia coli K-12 encode a glutathione transporter with the ATP-binding cassette. The transporter imports extracellular glutathione into the cytoplasm in an ATP-dependent manner. This transporter, along with ␥-glutamyltranspeptidase, has an important role in E. coli growth with glutathione as a sole sulfur source.

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Tetrahydropapaveroline (THP), a benzylisoquinoline alkaloid (BIA) found in diverse pharmaceutical compounds, is used as a starting material for the production of BIA. THP also has various neurobiological properties but is difficult to synthesize. Therefore, a simple method for THP production is desired. Recent studies have shown that microbes, especially bacteria, can serve as platforms for synthesizing these complex compounds; however, because bacteria lack organelles, the designed synthetic pathway cannot be compartmentalized. Thus, the metabolic flow is frequently inhibited or disrupted by undesirable reactions. Indeed, in the first attempt to synthesize THP using a single strain of engineered Escherichia coli, the yield was quite low (<5 μM), mainly because of the oxidation of THP by tyrosinase, an essential enzyme in our production system. To circumvent these problems, we constructed a stepwise (R,S)-THP production system, in which the dopamine-producing step and the subsequent THP-producing step were separated. The yield of (R,S)-THP reached 1.0 mM (287 mg/L), the highest yielding BIA production method using a microbe reported to date. Furthermore, we demonstrated that (R,S)-THP produced by stepwise fermentation is useful for the production of reticuline, an important BIAs intermediate. Based on these observations, applying the stepwise fermentation method is discussed.

Tyrosine phenol-lyase (Tpl), which can synthesize 3,4-dihydroxyphenylalanine from pyruvate, ammonia, and catechol, is a tyrosine-inducible enzyme. Previous studies demonstrated that the tpl promoter of Erwinia herbicola is activated by the TyrR protein of Escherichia coli. In an attempt to create a high-Tpl-expressing strain, we cloned the tyrR gene of E. herbicola and then randomly mutagenized it. Mutant TyrR proteins with enhanced ability to activate tpl were screened for by use of the lac reporter system in E. coli. The most increased transcription of tpl was observed for the strain with the mutant tyrR allele involving amino acid substitutions of alanine, cysteine, and glycine for valine-67, tyrosine-72, and glutamate-201, respectively. A tyrR-deficient derivative of E. herbicola was constructed and transformed with a plasmid carrying the mutant tyrR allele (V67A Y72C E201G substitutions). The resultant strain expressed Tpl without the addition of tyrosine to the medium and produced as much of it as was produced by the wild-type strain grown under tyrosine-induced conditions. The regulatory properties of the mutant TyrR V67A , TyrR Y72C , TyrR E201G , and TyrR V67A Y72C E201Gproteins were examined in vivo. Interestingly, as opposed to the wild-type TyrR protein, the mutant TyrR V67Aprotein had a repressive effect on the tyrP promoter in the presence of phenylalanine as the coeffector.Tyrosine phenol-lyase (Tpl) (EC 4.1.99.2) normally catalyzes the degradation of tyrosine into pyruvate, ammonia, and phenol (26-28, 56). However, this reaction is reversible, and if catechol is substituted for phenol, L-dihydroxyphenylalanine (L-DOPA) is produced (24, 57). L-DOPA is used in the treatment of Parkinson's disease, which afflicts 1 out of every 1,700 individuals. About 250 tons of L-DOPA is now supplied per year, and more than half of it is produced by an enzymatic method involving Tpl (24, 57).On an industrial scale, Erwinia herbicola cells with extremely high Tpl activity are prepared by cultivation in a medium containing L-tyrosine as an inducer of Tpl. The intact cells are then harvested by centrifugation and transferred to the reactor, as the catalyst, together with the substrate. This microbiological method is efficient; however, it actually has one serious drawback. Since Tpl is only synthesized under L-tyrosine-induced conditions (16, 49), the cells must be grown in medium supplemented with L-tyrosine. The extremely low solubility of L-tyrosine results in considerable carryover of it into the reactor, which severely complicates the separation of the final product, L-DOPA (hydroxyl derivative of L-tyrosine), from the remaining L-tyrosine. To avoid this drawback, the tpl genes of E. herbicola (17,20,50) and Citrobacter freundii (21) were cloned and expressed in Escherichia coli under the control of the tac promoter, respectively. In either case, Tpl was highly induced upon the addition of isopropyl-␤-D-thiogalactopyranoside (IPTG); however, the L-DOPA productivity of the cells was inferior to that of E. herbic...