The Structure and Expression of Cereal Storage Protein Genes

Abstract

The cereal endosperm stores several types of protein. All cereals appear to store 7S globulins in their aleurone cells and prolamins in their starchy endosperm cells. In addition, the major storage proteins in the starchy endosperm of rice and oats are related to the 11S globulins of dicotyledonous seeds. Smaller amounts of proteins related to 11S globulins are also present in wheat while rice, maize and wheat also contain α-globulins. There are also clear gradients in protein amount and composition within the starchy endosperm, with higher protein contents in the subaleurone cells and in the vitreous parts of the maize endosperm. A number of mutant genes have been identified in maize, sorghum and barley which result in reductions in the accumulation of either all prolamins or specific prolamin groups leading to a high lysine phenotype. The prolamins are encoded by multigene families whose expression is regulated temporally and spatially. This regulation is determined by promoter elements such as the prolamin box which comprises endosperm (E) and nitrogen (N) elements, the opaque-2 (O2) binding site and the HMW glutenin subunit gene enhancer. Transcription factors that bind to these sites include prolamin box binding factor (PBF) (a Dof class Cys2Cys2 zinc finger DNA binding protein), opaque-2 (O2) and OHP1 (basic leucine zipper transcription factors) from maize, and ESBF-I and -II, SPA and BLZ1 from wheat. Prolamin genes may be silenced by several mechanisms including the presence of in-frame stop codons within coding sequences, promoter inactivity and reduced RNA stability.