The role of opaque2 in the control of lysine-degrading activities in developing maize endosperm.

We have isolated a cDNA clone, designated ZLKRSDH, encoding the bifunctional enzyme lysine-ketoglutarate reductase/saccharopine dehydrogenase (LKR/SDH) from maize. The predicted polypeptide has an N-terminal LKR domain and a C-terminal SDH domain that are similar to the yeast LYS1 and LYS9 monofunctional proteins, respectively. The maize LKR/SDH protein is located in the cytoplasm of subaleurone endosperm cell layers. Transcripts and polypeptides as well as enzyme activities showed an upregulation and downregulation during endosperm development. The developmental expression of ZLKRSDH was examined in normal and opaque2 seeds. In the mutant endosperm, mRNA levels were reduced by >90%, with concomitant reductions in polypeptide levels and LKR/SDH activity. These results suggest that lysine levels in the endosperm are likely to be controlled at the transcriptional level by the Opaque2 transcription factor.