Cloning of complementary DNAs encoding the amphibian bombesin-like peptides Phe⁸ and Leu⁸ phyllolitorin from phyllomedusa sauvagei: Potential role of U to C RNA editing in generating neuropeptide diversity

The bombesin-like peptides were originally characterized in frog skin, then later found to have a wide distribution and range of actions in mammals. The bombesin-like peptides have classically been divided into three subfamilies, the bombesin subfamily, of which gastrin-releasing peptide (GRP) is the mammalian form; the ranatensin subfamily, of which neuromedin-B (NMB) is the mammalian form; and the phyllolitorin subfamily, which to date has only been characterized in amphibians. As a first step in characterizing mammalian phyllolitorin-like peptides, we have cloned complementary DNAs (cDNAs) encoding Leu⁸ and Phe⁸ phyllolitorin from Phyllomedusa sauvagei. Sequence analysis revealed that the amphibian phyllolitorin messenger RNA (mRNA) encodes a precursor of 90 amino acids containing a signal peptide sequence, an aminoterminal extension peptide, the phyllolitorin peptide of nine amino acids, and a carboxy-terminal extension peptide. Northern blot, reverse transcriptasepolymerase chain reaction (PCR), and in situ hybridization analysis showed that the mRNA was present at highest levels in skin, at lower levels in brain, and at lowest levels in gut. Phylogenetic analysis of bombesin-like peptide prohormone sequences showed that the phyllolitorin prohormones are much more closely related to the bombesin and ranatensin prohormones than to the GRP and NMB prohormones. This analysis suggests that the bombesin-like peptides should be reclassified into the GRP subfamily, the NMB subfamily, and the skin peptide subfamily. Surprisingly, the cDNAs encoding Phe⁸ and Leu⁸ phyllolitorins were identical except for a single T to C difference in the codon coding for the Phe or Leu residue of phyllolitorin. PCR analysis of the skin, stomach, and brain phyllolitorin cDNAs showed that skin contained equal proportions of Phe⁸ and Leu⁸ phyllolitorin, that Phe⁸ phyllolitorin predominated in stomach, and that only Phe⁸ phyllolitorin could be identified in brain. PCR analysis of genomic subclones showed that all 52 subclones sequenced used the codon TTC encoding only Phe⁸ phyllolitorin. Genomic Southern blot analysis was consistent with a single phyllolitorin gene. This suggests that Leu⁸ and Phe⁸ phyllolitorins are encoded by the same gene and that a regulated posttranscriptional RNA-editing mechanism converts a U to a C to generate the two phyllolitorin mRNAs. This is the first report of potential U to C RNA editing in eucaryotes and suggests a new mechanism for generating neuropeptide diversity.