The last piece of the poppy puzzle is now in hand: Plant geneticists have isolated the gene in the plant that carries out the last unknown step in converting glucose and other simple compounds into codeine, morphine, and a wide variety of other medicines. The discovery sets the stage for splicing the full suite of genes needed to produce these drugs into yeast, which could then produce safer and cheaper versions.
The new work is “solid,” says John Dueber, a synthetic biologist at the University of California, Berkeley, who is working on splicing morphine synthesis genes into yeast cells. Not only did the researchers figure out how poppies carry out this missing step, he says, but they also transferred the gene to yeast and showed it works in microbes as well.
People have harvested poppy plants for morphine and other drugs for thousands of years. Over the past decade, several research groups have been working to decipher the genes responsible for these chemical transformations. Their ultimate goal has been to transfer the complete synthetic pathway into yeast, making it possible to produce the medicines by fermentation. It could also make it far easier for medicinal chemists to tweak the genes to produce new versions of the drugs that might have fewer side effects and addictive properties. At the same time, the work has raised fears that drug dealers may soon be able to brew heroin as easily as home brewers now craft their own beer.
Last month, Dueber and his colleagues succeeded in splicing the first half of the full glucose-to-morphine pathway into yeast. These genes convert glucose into an intermediate compound called S-reticuline. Other groups had already supplied yeast with genes for the second half of the pathway, which converts a similar intermediate called R-reticuline into codeine, morphine, and the like. That left just one step missing in the middle: converting S-reticuline to R-reticuline.
Researchers have been searching for the biomolecules that carry out this conversion for decades. And some investigators had found leads. In 1992, for example, German biochemists discovered that the transformation from S-reticuline to R-reticuline likely went through another intermediate compound called 1,2-dehydroreticuline. But they were working with complex poppy plant extracts and never isolated the genes and enzymes responsible for carrying out this two-step process.
After earlier success on another part of the biosynthesis pathway, Ian Graham, a biochemical geneticist at the University of York in the United Kingdom, decided to go after the S-reticuline to R-reticuline step. The York team had seen previous work by other researchers that showed that giving an opium poppy a particular RNA molecule led a dramatic boost in S-reticuline. That RNA, known as an interfering RNA (RNAi), was designed to knock down the activity of a gene called COR, which works late in the biosynthesis pathway to convert precursors to codeine and morphine. It was unclear why knocking down COR would boost levels of S-reticuline far upstream. But Graham and his colleagues suspected that the RNAi was having unknown effects, inhibiting the expression of upstream genes closely related to COR. So they used the RNAi as bait to fish for any related genes to which it would bind.
They snagged a gene called STORR and hit the jackpot. After isolating the STORR gene, Graham and his colleagues showed that it encodes a single protein that carries out both the steps needed to convert S-reticuline to R-reticuline. They also analyzed mutant strains of poppy plants that had also been found to accumulate S-reticuline, and found that all had mutations in the STORR gene that made impossible for them to convert S- reticuline to R-reticuline. Finally, when they added the STORR gene to yeast, they found that they gained the ability to convert S-reticuline to R-reticuline, they report online today in Science.
With the last piece of the puzzle now in hand, Dueber says the challenge will be to express the genes for the full synthetic pathway into yeast. That’s a “considerable” hurdle, Dueber says, but likely doable within a few years. Even when that’s done, the microbes will still likely only make vanishingly small quantities of the final medicines. So then the task will be to increase the efficiency of each of the steps. That, too, is likely not an insurmountable challenge.