The trouble with mRNAs, at least from a translation point of view, is that they have extensive secondary structure. The ribosomal slot through which the mRNA passes is too narrow to allow anything wider than a single strand. Yet no one had identified the relevant RNA helicase activity. “It's kind of a long-overlooked problem,” says Noller. “No one got down to how the structure is unwound.”
Now, he shows that the ribosome itself does the unwinding. Using purified proteins and labeled oligos bound to mRNAs (to mimic helices), the group demonstrates that the ribosome alone can release the oligo (unwind the helix) during translation.
This activity was mapped to the downstream tunnel, where the RNA enters the 30S subunit. Since externally supplied energy was not needed, the ribosome may use energy released during translocation. In vivo, GTPases such as elongation factors may make the helicase more efficient.
Noller suggests two models for how the helicase works. In one scenario, normally temporary melting at the end of the helix may be made permanent by the pulling of the unattached RNA into the tunnel. Alternatively, the head of the 30S subunit has been seen to ratchet against the 30S body. If one helical strand attaches to the head, and the other to the body, this ratcheting might rip apart the helix base pairs.