Plant protein blocks tumour growth

RNA-dependent RNA polymerase 1 (RDR1) is essential for siRNA-mediated antiviral defence in plants, but is absent in vertebrates. Now, Qi et al. report that expression of RDR1 in mammalian cancer cell lines blocks proliferation by targeting cell cycle genes; these effects were dependent on the miRNA pathway. miRNA duplex isoforms with 1-nt-shorter 3′ ends, which cannot be loaded onto the Argonaute complex, accumulated in human tumours. In cancer cell lines, RDR1, due to its nucleotidyl-transferase activity, restored the regular 2-nt overhang structure of miRNA duplexes, rescuing the defective miRNA pathway. In mouse xenograft models, RDR1 delivery increased miRNA expression and suppressed tumour growth.