Engineering a conserved RNA regulatory protein repurposes its biological function <i>in vivo</i>.

Abstract

PUF (<u>PU</u>milio/<u>F</u>BF) RNA-binding proteins recognize distinct elements. In <i>C. elegans</i>, PUF-8 binds to an 8-nt motif and restricts proliferation in the germline. Conversely, FBF-2 recognizes a 9-nt element and promotes mitosis. To understand how motif divergence relates to biological function, we first determined a crystal structure of PUF-8. Comparison of this structure to that of FBF-2 revealed a major difference in a central repeat. We devised a modified yeast 3-hybrid screen to identify mutations that confer recognition of an 8-nt element to FBF-2. We identified several such mutants and validated structurally and biochemically their binding to 8-nt RNA elements. Using genome engineering, we generated a mutant animal with a substitution in FBF-2 that confers preferential binding to the PUF-8 element. The mutant largely rescued overproliferation in animals that spontaneously generate tumors in the absence of <i>puf-8</i>. This work highlights the critical role of motif length in the specification of biological function.