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Complementary-DNA (cDNA) clones encoding thionin were identified as one of the most frequent types of clones in a cDNA library constructed from total polyadenylated RNA from young barley leaf cells. One full-length clone codes for a precursor protein that starts with a signal peptide (28 amino acids) followed by the mature thionin (46 amino acids) and terminated by a long acidic extension (63 amino acids). The amino-acid sequence of the leaf thionin is 52% homologous to thionins from barley endosperm and in the C-terminal extension the homology decreases to 41%. In contrast, the leaf thionin is 72% homologous to viscotoxin from mistletoe leaves. Leaf thionin is coded by a multigene family with an estimated nine to eleven genes and analysis of the cDNA clones showed that at least two extremely homologous genes are expressed. Northern hybridization experiments indicate that the leaf thionin genes are not expressed in endosperm and roots. In leaves, the expression of the thionin genes is strongly repressed by light.
Planta © 1987 Springer