Comparative role of polyamines in division and plastid differentiation of Euglena gracilis

Regulation of polyamine biosynthesis during growth and differentiation of Euglena gracilis was investigated. Increased activity of L-ornithine decarboxylase (EC 4.1.1.17), the enzyme which catalyzes the initial step in polyamine synthesis in Euglena, and accumulation of polyamines were observed prior to DNA replication in synchronous cultures of heterotrophically or photoautotrophically grown cells. In photoautotrophic cells three maxima of polyamine synthesis were observed during the light period of the cell cycle. The transition form quiescence to active growth was accompanied in heterotrophic Euglena by a very large stimulation of ornithine decarboxylase activity and polyamine synthesis; the decrease in growth potential of these cells was correlated with a decrease in polyamine levels. In contrast, differentiation of Euglena i.e. a shift from heterotrophic to photoautotrophic mode of living in the absence of division, led only to a minor stimulation of polyamine biosynthesis. Alpha-Methylornithine, an inhibitor of ornithine decarboxylase, blocked the growth of heterotrophic Euglena, and depletion of intracellular polyamines decreased the differentiation rate. Both events could be reversed by addition of putrescine to the growth medium. This study suggests that Euglena requires a minimal intracellular level of polyamines to grow and differentiate under optimal conditions. This requirement seems to be more stringent for cell division.