title: A eukaryotic (insect) tricistronic mRNA encodes three proteins selected by context-dependent scanning.
abstract: Eukaryotic mRNAs are generally considered monocistronic and encode only one protein. Although dicistronic mRNAs encoding two proteins were found in fungi, plants, and animals, polycistronic mRNAs encoding more than two proteins have remained elusive so far in any eukaryote. Here we demonstrate that a single mRNA from silkworm encodes the precursor of an insect cytokine paralytic peptide (PP) and two new cytokine precursor-like proteins, uENF1 and uENF2. RT-PCR analysis showed that this mRNA is widely conserved in moths. Western blot analyses and reporter assays using its modified mRNAs, created by replacing each one of the three ORFs with the firefly luciferase ORF, showed that all three proteins were translated from this mRNA in cell lines, larval tissues, and cell-free systems. Insertion experiments using the Renilla luciferase ORF or a stem loop ruled out the possible involvement of internal ribosome entry site in the three protein translation. On the other hand, systematic mutation analysis of the translation initiation sequence of the 5'-proximal uENF1 ORF suggested that the context-dependent leaky-scanning mechanism is involved in translation of the downstream uENF2 and PP ORFs. In vitro, a synthetic peptide corresponding to the putative mature form of uENF1 stimulated spreading of hemocytes as did the synthetic PP, whereas that of uENF2 antagonized the stimulating activities of PP and the uENF1 peptide, suggesting that the three proteins control cellular immunity interactively. Thus, eukaryotes have a cellular tricistronic mRNA that encodes three functionally related proteins as in an operon.
authors.affiliation: National Institute of Agrobiological Sciences, Tsukuba, Ibaraki 305-8634, Japan.
authors.forename: Yasushi
authors.initials: Y
authors.lastname: Kanamori
authors.forename: Yoichi
authors.initials: Y
authors.lastname: Hayakawa
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Matsumoto
authors.forename: Yuji
authors.initials: Y
authors.lastname: Yasukochi
authors.forename: Sachiko
authors.initials: S
authors.lastname: Shimura
authors.forename: Yuichi
authors.initials: Y
authors.lastname: Nakahara
authors.forename: Makoto
authors.initials: M
authors.lastname: Kiuchi
authors.forename: Manabu
authors.initials: M
authors.lastname: Kamimura
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D015152:Blotting, Northern; D015153:Blotting, Western; D012831:Bombyx; D003001:Cloning, Molecular; D018387:Codon, Initiator / Q000235:genetics / Q000378:metabolism*; D016207:Cytokines / Q000235:genetics* / Q000378:metabolism; D018507:Gene Expression Regulation, Developmental; D019476:Insect Proteins / Q000235:genetics* / Q000378:metabolism; D007814:Larva / Q000166:cytology / Q000235:genetics / Q000378:metabolism; D008156:Luciferases / Q000378:metabolism; D008969:Molecular Sequence Data; D009479:Neuropeptides / Q000235:genetics*; D016366:Open Reading Frames / Q000235:genetics*; D010442:Peptide Chain Initiation, Translational; D010446:Peptide Fragments / Q000138:chemical synthesis / Q000378:metabolism; D014176:Protein Biosynthesis; D012333:RNA, Messenger / Q000235:genetics*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012270:Ribosomes / Q000235:genetics*; D017386:Sequence Homology, Amino Acid; D014158:Transcription, Genetic
chemical_list: D018387:Codon, Initiator; D016207:Cytokines; D019476:Insect Proteins; D009479:Neuropeptides; D010446:Peptide Fragments; D012333:RNA, Messenger; C121924:paralytic peptide, insect; D008156:Luciferases