Subcellular localization and light-dark control of ornithine decarboxylase in the unicellular green alga Chlamydomonas reinhardtii

Abstract

The polyamines putrescine, spermidine and spermine are required for cell proliferation of both eukaryotic and prokaryotic organisms. In plant cells, putrescine, which is also the precursor of spermidine, is derived from ornithine by the action of ornithine decarboxylase (ODC, EC 4.1.1.17) or from arginine by arginine decarboxylase (ADC, EC 4.4.4.19). In the unicellular green alga Chlamydomonas reinhardtii, ODC activity exceeds ADC activity by orders of magnitude as shown in the present communication, indicating that biosynthesis of polyamines is controlled by ODC activity in this organism. ODC was localized in the cytosol and was found to consist of 50 kDa subunits as revealed by affinity labeling with [ 3 H]difluoromethylornithine and subsequent SDS-PAGE and fluorography. Its activity was much higher in light- than in dark-adapted (starved) cells. After transfer from darkness to light, a rapid increase in ODC activity was observed. This light-induced increase in ODC activity could be completely abolished by addition of cycloheximide, an inhibitor of protein biosynthesis, but not by actinomycin D inhibition of transcription. Up-regulation of ODC activity after onset of illumination could be prevented by addition of the photosystem II inhibitor 3-(3'-4'-dichlorophenyl)-1,1-dimethyl-urea. ODC activity was found to be correlated with the ATP level both in light- and dark-adapted cells. ODC half-lives of 42 and 83 min were determined for dark- and light-adapted cells, respectively.