Regulation of nitrate reductase by molybdenum in chloronema cell cultures of the moss Funaria hygrometrica

Abstract

The relationship between NADPH: nitrate reductase (NR) activity and NR protein was investigated in the chloronema cell suspension cultures of the moss Funaria hygrometrica Hedw. When cells were transferred to fresh medium containing nitrate, other major elements and Heller's trace elements, the NR activity was induced. It increased to a maximum on the third day then decreased to undetectable levels by the ninth day. A single polypeptide of about 116 kDa (NR protein) was detected on the immunoblots probed with NR-specific monoclonal antibody. The increase and decrease of NR activity were associated with similar changes in the level of NR protein only during the first 6 days of culture. Subsequently, the NR protein increased again to reach a second peak on the ninth day. This increase seems to be due to lack of adequate molybdenum in the medium. Upon adding molybdenum, the nonfunctional NR did not accumulate and the increase and decrease in NR activity were mediated by the increase and decrease in NR protein. The profiles of partial activities of NR protein and of xanthine dehydrogenase activity also suggest that molybdenum becomes limiting five or six days after culture. The production of non-functional NR is suggested to be due to lack of end product repression.