Circular RNAs can act like molecular 'sponges' binding to and blocking tiny gene modulators called microRNAs. These circular RNA molecules comprise "a hidden parallel universe" of unexplored RNAs that control gene expression. Typical RNA-sequencing methods did not detect this molecules unless they had a molecular 'tail'.

Erik Sontheimer, a molecular biologist at Northwestern University in Evanston, Illinois said: “It’s yet another terrific example of an important RNA that has flown under the radar,” you just wonder when these surprises are going to stop.” Previous accounts of circular RNAs in plants and animals were generally dismissed as genetic accidents or experimental artefacts. Nikolaus Rajewsky, the lead author of one of the studies and a systems biologist at the Max Delbrück Center for Molecular Medicine in Berlin focused on a circular behemoth, some 1,500 nucleotides around, that is expressed in the brains of mice and humans. They found that it contains about 70 binding sites for a microRNA that can block gene expression linked to cancer and Parkinson’s disease.

Circular RNAs could also be sponges for microRNA from outside the cell, notes Rajewsky. Some have possible binding sites for viral microRNAs, which can subvert immune responses. He hypothesizes that circular RNA could even interact with RNA-binding proteins. “They are so abundant, there are probably a multitude of functional roles.”

This discovery goes further to illustrate that life is such a rich tapestry of interconnecting elements that it truly challenges our ability to fathom its complexity.

Reference: Nature vol: 494, pg 415 (28 February, 2013)