Molecular organization of a gene in barley which encodes a protein similar to aspartic protease and its specific expression in nucellar cells during degeneration.

摘要

A barley gene which encodes an aspartic proteinase-like protein, and is specifically expressed in nucellar cells during degeneration, was cloned and characterized. The gene contains 8 exons and 7 introns and encodes a polypeptide of 410 amino acid residues. The deduced polypeptide, designated nucellin, is characterized by 2 aspartic proteinase catalytic site motifs, Asp-Thr-Gly-Ser in the N-terminal and Asp-Ser-Gly-Ser in the C-terminal region, and 2 other regions nearly identical to 2 regions of plant aspartic proteinases. However, it shares <20% overall sequence identity with known plant aspartic proteinases, and does not contain other characteristic features involved in enzyme activation and targeting. In northern analyses nucellin transcriptswere most abundant in ovaries 3-4 days after pollination, and only marginally detectable before pollination or 10 days after. RNA in situ hybridization showed that before pollination the nucellin gene was expressed at low levels only in a cluster of nucellar cells close to the embryo sac at the chalazal end, but after pollination it was highly expressed in most nucellar cells surrounding the entire embryo sac. Furthermore, no nucellin transcripts were detectable in anther, leaf or root tissue. Temporaland spatial patterns of nucellin gene expression are synchronal with nucellar cell degeneration, indicating nucellin is involved in nucellar cell death. Nucleotide sequence data reported here have been submitted to the EMBL/GenBank/DDBJ database under accession numbers U87148 and U87149.