ISOLATION AND CHARACTERIZATION OF CDNA CLONES REPRESENTING THE GENES ENCODING THE MAJOR TUBER STORAGE PROTEIN (DIOSCORIN) OF YAM (DIOSCOREA-CAYENENSIS LAM)

cDNA clones encoding dioscorins, the major tuber storage proteins (M(r ) 32000) of yam (Dioscorea cayenensis) have been isolated. Two classes of clone (A and B, based on hybrid release translation product sizes and nucleotide sequence differences) which are 84.1% similar in their protein coding regions, were identified. The protein encoded by the op en reading frame of the class A cDNA insert is of M(r) 30015. The diff erence in observed and calculated molecular mass might be attributed t o glycosylation, Nucleotide sequencing and in vitro transcription/tran slation suggest that the class A dioscorin proteins are synthesised wi th signal peptides of 18 amino acid residues which are cleaved from th e mature peptide, The class A and class B proteins are 69.6% similar w ith respect to each other, but show no sequence identity with other pl ant proteins or with the major tuber storage proteins of potato (patat in) or sweet potato (sporamin). Storage protein gene expression was re stricted to developing tubers and was not induced by growth conditions known to induce expression of tuber storage protein genes in other pl ant species. The codon usage of the dioscorin genes suggests that the Dioscoreaceae are more closely related to dicotyledonous than to monoc otyledonous plants.