NATURE CELL BIOLOGY VOLUME 9 | NUMBER 6 | JUNE 2007 617 NEWS AND VIEWS Ras activation 3 opens up new avenues to explore the biochemical mechanism underlying this critical immunological process. Exciting times are clearly ahead as previously unappreciated phosphatidic acid takes on new and increas- ingly important roles, and GTPase network diagrams are reassessed and rewired. COMPETING FINANCIAL INTERESTS The authors declare no competing financial interests. 1. Schlessinger, J. Cell 103, 211–225 (2000). 2. Zhao, C., Du, G., Frohman, M. A. & Bar-Sagi, D. Nature Cell Biol. 9, 706–712 (2007). 3. Mor, A. et al. Nature Cell Biol. 9, 713–719 (2007). 4. Hancock, J. F. Nature Rev. Mol. Cell. Biol. 7, 456–462 (2006). 5. Jacobson, K., Mouritsen, O. G. & Anderson, R. G. Nature Cell Biol. 9, 7–14 (2007). 6. Kusumi, A., Koyama-Honda, I. & Suzuki, K. Traffic 5, 213–230 (2004). 7. Di Fulvio, M., Lehman, N., Lin, X., Lopez, I. & Gomez-Cambronero, J. Oncogene 25, 3032–3040 (2006). 8. Plowman, S. 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Two previous studies of mRNA-like non-coding transcripts in Drosophila identified the locus, pncr001:3R or MRE29, as a polyA-containing RNA lacking open-reading frames of more than 100 amino acids 1,2 . On page 660 of this issue, Kondo et al. reveal that this mRNA codes not just for one, but four (albeit tiny) proteins 3 . Furthermore, they found that mutant embryos deficient for this gene lacked the ventral denti- cles and dorsal hairs that decorate the epidermis of wild-type embryos. Because mutant embryos had a smooth appearance similar to rice that has been milled to remove the husk and bran, the gene was renamed polished rice (pri). The pri story is of particular interest because of the novel structure of the gene — it encodes a polycistronic message that produces four pep- tides less than 100 amino acids in length — and because the small Pri peptides seem to function non-cell autonomously to promote changes in epithelial-cell morphology. It has become clear that eukaryotic genomes contain large numbers of transcribed sequences with no apparent coding potential 4 . These tran- scripts include thousands of polyA-containing RNAs that have only small open-reading frames (sORFs) and are often annotated as non-coding RNAs (ncRNAs). Computational approaches based on an analysis of codon bias and cross- species conservation suggest that approximately 5% of annotated genes in the genomes of yeasts, plants, flies, nematodes, mice and humans con- tain sORFs that may, in fact, be translated into exceptionally small proteins 5 . One example is that of the mille-pattes gene from the flour beetle Tribolium. Savard et al. reported that mille-pattes seems to encode four small peptides (10, 12, 15 and 23 amino acids) in a single polycistronic mRNA 6 . Fortuitously, the pri gene characterized by Kondo et al. is the Drosophila homologue of mille-patte, and they find that it has a similar gene structure, encoding four or five small peptides (three of 11 amino acids, one of 32 amino acids and a potential fifth sORF of 49 amino acids) 3 . The first four predicted sORFs of pri and the first three predicted sORFs of mille-pattes share a septomeric motif (LDPTGXY) present in one or two copies in each peptide. Savard et al. infer translation of four mille- pattes peptides from the conservation of the putative protein-coding sequences between Tribolium and Drosophila. Kondo et al. go one step further, showing that the Drosophila mes- sage can indeed be translated. They tagged each of the five putative peptides with GFP, and tested expression of the five pri–GFP fusion proteins in transformed S2 cells. GFP expres- sion was detected for all but the sORF5–GFP fusion, and western blot analysis confirmed that the fusion proteins were of the predicted size. Interestingly, the AUG of mille-pattes sORF4 (homologous to pri sORF5) overlaps with the stop codon of the preceding peptide and lacks a Kozak consensus sequence, suggesting that it may not be translated, or that translation may occur through an unusual read-through mechanism. Whether mille-pattes mRNA is normally translated has not been directly tested, although an embryonic requirement for mille-pattes transcript has been established by RNA interference (RNAi). Kondo et al. dem- onstrate that the four Pri peptides, although redundant with each other, are required for Drosophila development — expression of any one of the peptides rescues animals deleted for the endogenous locus, but a frame-shift muta- tion eliminates the rescuing activity. Polycistronic RNAs are rare in eukaryotes, where they comprise two general classes of RNA: polycistronic pre-mRNAs that are proc- essed into mature monocistronic mRNAs and mature polycistronic mRNAs for which trans- lation of downstream proteins depends on an internal ribosome entry site (IRES) or some sort of ribosome re-initiation mechanism 7 . Until now, all examples of this second class of message were dicistronic, coding for two, often functionally-related, proteins. The mille-pattes and pri genes define a novel subset of RNAs within this second class — unique because of Amin Ghabrial is in the Department of Biochemistry and Howard Hughes Medical Institute, Stanford University School of Medicine, 300 Pasteur Drive, Stanford, CA 94305, USA. e-mail: ghabrial@pmgm2.stanford.edu