Expression of legumin and vicilin genes in pea mutants and the production of legumin in transgenic plants

Pea seeds contain two major storage proteins, legumin and vicilin, in propo rtions that are genetically and environmentally determined. They are synthe sized from at least 40 genes and at least 10 different genetic loci. Mutant alleles at loci involved in starch synthesis, which result in perturbation s in starch accumulation, also affect the expression of legumin genes, ther eby influencing the legumin :vicilin ratio within the total seed protein. E xamples of such alleles include r (starch-branching enzyme) and rb (ADP-glu cose. pyrophosphorylase), both of which result in a reduction in legumin sy nthesis; double mutants (rrb) show a particularly severe reduction in the a mount of legumin. The effects of such mutations are specific to legumins. T he amounts of vicilin are unaffected by mutations at r or rb. One of the co nsequences of the production of legumin from many genes is structural heter ogeneity that is believed to preclude the purification of homogeneous legum in for crystallization and 3D-structure determination. Expression of cloned legumin cDNA in E. coli can result in sequence homogeneity, but E. coli is unable to carry out the normal proteolytic processing of legumin precursor s and consequently such material is different from that produced in pea see ds. This paper describes the high-level synthesis, processing and assembly of pea legumin in transgenic wheat seeds, leading to the spontaneous in vit ro formation of paracrystalline arrays of legumin, which may be attributed to the fact that the legumin consists of a single type of subunit. Such mat erial might be used as, a source of single-sequence,. processed and assembl ed pea legumin for structural investigation.