Identification and characterization of the mRNA for major storage proteins from radish

Precursors for radish storage proteins were identified by immunoprecipitation of cell-free protein-synthesis products. The 12S globulin polypeptides (21,000-33,000 Mr) are made from four larger precursors (47,500-60,000 Mr) and the 1.7 S albumin polypeptides (7,000-12,000 Mr) are synthesized as eight products of Mr about 20,000. From a cDNA library, several clones have been identified, using heterologous cDNA probes for rapeseed cruciferin and napin, and two of them (pAE10 and pBA3) used to hybrid-select the corresponding mRNA. The napin cDNA clone selects all translation products immunoprecipitated by anti-napin antibodies whereas the cruciferin clone selects only one polypeptide among the four recognized by 12S antibodies. The 12S globulin and the 1.7S albumin message sizes were estimated to be 1950 and 800 nucleotides respectively. Experiments carried out with mRNA populations from different maturation stages suggest a sequential gene expression for the two major storage protein families. In addition two napin subfamilies can be distinguished by their expression pattern. In dry seed, napin mRNA is not detectable and the amount of cruciferin mRNA decreased considerably so that it no longer represents a major fraction of dry seed mRNA.