Developmental regulation of gene expression in wheat embryos. Molecular cloning of a DNA sequence encoding the early-methionine-labelled (Em) polypeptide.

The early-methionine-labelled (Em) polypeptide is the most abundant single polypeptide found in the cytosolic fraction (30 000 X g supernatant) of dry wheat embryos. Synthesis of this polypeptide is readily detectable during the earliest stages (0-3 h) of wheat embryo germination, by labelling with [35S]methionine. Thereafter, synthesis of the Em polypeptide declines rapidly. A DNA sequence encoding a portion of this polypeptide has been isolated by the molecular cloning of DNA complementary to the messenger RNA from dry wheat embryos. The identity of this cloned sequence has been confirmed by hybrid-selected translation of the Em messenger, the product being analysed by one- and two-dimensional polyacrylamide gel electrophoresis, and by analysis of its partial proteolytic digestion products. Gel blot hybridisation of RNA isolated from embryos at successive stages of germination with this cloned DNA sequence confirms that the decline in the synthesis of the Em polypeptide, detected by labelling in vivo, is a consequence of the degradation of its corresponding messenger RNA.