A genomic clone that specifies a single poypeptide precursor for ricin, a toxic lectin of Ricinus communis (castor bean), was isolated, sequenced and S1 mapped. The gene encodes a 64 kDa precursor which contains, in the following order: a 24 or 35 amino acid signal peptide, the A chain, a 12 amino acid linker peptide, and the B chain. The 5′-end of the ricin mRNA maps approximately 35 bases upstream from the first methionine codon. Two putative TATA boxes and a possible CAAT box lie in the 5′-flanking region. Two possible polyadenylation signals are found in the 3′ flanking region. No introns were found, which is typical of other lectin genes that have been sequenced. Southern blot analysis suggests that the castor bean genome contains approximately six ricin-like genes.
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