Abundant embryonic mRNA in field bean (Vicia faba L.) codes for a new class of seed proteins: cDNA cloning and characterization of the primary translation product.

cDNA clones have been constructed bearing inserts for a specific mRNA class of high abundance in developing seeds of field bean (Vicia faba L.). Three full-length clones representing transcripts of different genes were sequenced and conceptually translated into a M=30 000 primary gene product. The structural analysis of the derived amino acid sequence revealed distinct domains: (i) a cleavable signal peptide; (ii) a hydrophilic N-terminal stretch possessing two serine clusters; (iii) a valine cluster and a hydrophobic domain in the C-terminal part of the polypeptide. The amino acid sequence of the polypeptide does not show homology with other known proteins.The corresponding mRNA could be isolated using cDNA clones and was efficiently translated in various systems. In a cell-free system the presence of a functional signal peptide was shown, which interacts with the signal recognition particle resulting in a cotranslational translocation across the membrane of the endoplasmic reticulum. If synthesized in Xenopus oocytes the translation product of the mRNA was secreted out of the cell. Homologous mRNA was found to be present also in developing cotyledons of pea (Pisum sativum L.) and french bean (Phaseolus vulgaris L.).