Expression of the circle “sponges up” miR-7 in the cell, effectively canceling out its expression and boosting miR-7 regulated genes. Kjems team showed that at least one other circular RNA can also do this: Sry contains 16 binding sites for miR-138, and its expression dampens miR-138 activity.
These papers provide the first evidence of function for a circular RNA, the first proof that the molecules are not mere molecular accidents. But that doesn't mean it's case closed. Rinn, for one, notes that if the circle can sop up microRNAs, so too can the linear molecules.
“We're pinning function way too fast,” Rinn says, rattling off a list of other possibilities that includes transcriptional memory, translational inhibition, and nuclear import/export. Maybe, he suggests, RNA circles could be packed into vesicles as a kind of transmissible transcriptome, “like little escape pods.”
Sharpless has a six-pack riding on the idea that some endogenous RNA circle will be shown to encode a novel run-off protein—an idea that Peter Sarnow at Stanford University showed to be possible in the 1990s using synthetic circular RNAs. Basically, if the number of base pairs in a circle is divisible by three, “you can make essentially a never-ending polypeptide,” Sharpless explains. “Even cooler, if [the transcript length] is not divisible by three, but does not have a stop codon in any reading frame, it will be translated around and around again, alternating between all three reading frames”—a possibility Sharpless and Jeck dub a “Möbius protein.”
The point, Rinn concludes, is that “the transcriptome continually surprises us with new forms of regulation in higher and higher complexity.” It won't be easy to identify them, of course—ascribing function to noncoding RNAs is always tough, and distinguishing between circles and their linear partners compounds the problem. But the fact is, microRNA inhibition could turn out to be RNA circles’ least interesting function.
Bottom line: When it comes to RNA, assume nothing.