Cell-cycle-dependent regulation of ornithine decarboxylase activity in the unicellular green alga Chlamydomonas reinhardtii

Abstract

Polyamines play an important role in the control of cell growth and cell division. In the unicellular green alga Chlamydomonas reinhardtii as in animal cells, biosynthesis of the 3 commonly occurring polyamines (putrescine, spermidine and spermine) is dependent on the activity of ornithine decarboxylase (ODC, EC 4.1.1.17) catalyzing the formation of putrescine, which is the precursor of the other two polyamines. Therefore, we have investigated the regulation of ODC activity during the cell cycle of Clamydomonas reinhardtii using synchronized cultures. A 2.5-3-fold increase in ODC activity was observed during the transition to the cell division phase. This up-regulation of ODC activity was not due to an increased level of ODC-mRNA as revealed by northern-blot analyses, but correlated with an increased half-life of this particular enzyme (from 1.1 to 3.2 h). Addition of the DNA topoisomerase II inhibitor nalidixic acid during the second half of the growth period caused a transient decrease of ODC activity and a considerable delay of cell divisions. After cell division, a down-regulation of ODC activity was observed which was faster in the dark than in the light and also correlated with changes of the ODC half-life.