Figure 1: A micrograph of Euglena in water. RIKEN researchers have shown that it uses unconventional markers to determine which sections of genetic code should be snipped. © FRANK FOX/SCIENCE PHOTO LIBRARY
Biology textbooks explain that cells follow a universal rule when processing gene transcripts to make proteins. Non-coding snippets of RNA are bracketed by a guanine-thymine (GT) nucleotide sequence on one end and an adenine-guanine (AG) sequence on the other-unmistakable signposts telling the cell exactly what sequences to leave out and what should stay in.
But a new RIKEN-led study has found that a common pond-dwelling microalga tosses that rule aside on a massive scale. Rather than relying on the usual GT-AG markers, Euglena agilis uses an entirely different set of cues to guide this excision step across most of its genetic messages1.
In this tiny green swimmer, more than 70% of the RNA segments that need to be snipped out-pieces called introns-ignore the standard pattern altogether. Instead, they follow their own sequence logic.
This hints that E. agilis runs a second processing system alongside the one found in almost every other complex organism.
"It seems to operate beyond the canonical rule in unexpected ways," says Keiichi Mochida of the RIKEN Center for Sustainable Resource Science. "As far as I know, no other organism has been reported to have such a high proportion of introns that so completely violate the conventional splicing rule."
To figure out how this rebel system works, Mochida's team inserted both natural and lab-made versions of these odd introns into another Euglena species. If the microalga could still make the protein normally encoded by the gene, the intron would be removed correctly; if not, the trimming machinery would have stumbled.
Those experiments revealed a simple but distinct set of sequence features-essentially a second genetic 'cut here' code-that the microalga's machinery reads instead of GT and AG.
The reasons for this parallel setup are still a mystery. "Why the Euglena species has evolved this dual RNA-processing machinery remains unknown," says Mochida. "To find out why will require more evolutionary and ecological investigation."
But even without knowing where the system came from, he and his collaborators are already thinking about how this unusual code might be put to work.
They have founded a biotechnology company that cultivates microalga at an industrial scale to produce nutritional supplements, cosmetics ingredients and experimental biofuel.
Understanding how the organism processes its genes should now make it easier to fine-tune those traits or build new ones. This could potentially turn this rule-breaking pond microbe into an even more versatile biological workhorse.
