Although Taxol® (paclitaxel) offers significant benefits to cancer patients, its initial isolation from the bark of the Pacific yew tree (Taxus brevifolia) raised serious ecological concerns since the trees are killed in the harvesting process. These concerns led to the development of both synthetic and semisynthetic routes to paclitaxel, but the drug is now manufactured more efficiently using plant cell cultures.
Scientists at MIT and Tufts University have now engineered a new strain of E. coli bacteria that can produce taxadiene and taxadien-5-α-ol, key precursors to paclitaxel. Although E. coli does not naturally produce taxadiene, it does produce isopentenyl pyrophosphate (IPP), a compound that is two steps back in the plant biosynthetic pathway. The team identified four bottlenecks in the eight-step E. coli biosynthesis of IPP and engineered the bacteria to produce multiple copies of the genes encoding the enzymes responsible for carrying out these four steps. To enable the bacteria to carry out the two additional steps needed to covert IPP to taxadiene, the researchers added the plant genes for the appropriate enzymes. By altering the copy numbers of genes to find the most efficient combination, the team were able to produce a strain of E. coli that produces more than 1000 times more taxadiene (ca 1g/L) than any other engineered strain. They then added an extra step towards the synthesis of paclitaxel, achieving the first microbial conversion of taxadiene to taxadien-5-α-ol.
There are another fifteen to twenty steps to go to achieve a microbial synthesis of pactitaxel but, if these can be achieved, as well as producing pactitaxel, the engineered bacteria should allow access to a variety of terpenoid natural products.
The study is published in the journal Science.