Euglena gracilis is a unicellular, free-living flagellate that inhabits various freshwater environments. Our research shows that exposure to UV-C light can trigger some form of programmed cell death. Cells exposed to UV-C light underwent delayed changes that were strongly reminiscent of apoptosis in mammalian cells, including cell shrinkage and DNA fragmentation that produced the characteristic ladder pattern commonly seen with apoptosis. DNA fragmentation could be inhibited by pretreatment with Z-VAD-FMK and also independently induced by exposure to staurosporine. In addition, Euglena possess proteins that cross-reacted with antibodies raised against human caspases 3 and 9. Given that Euglena are extremely easy to culture and represent a lineage positioned near the base of the eukaryotic tree, they will be an excellent model system for comparative analyses with apoptotic-like death processes in other eukaryotic microbes. 1. Introduction Apoptosis is a form of controlled cell death that is essential to animal life. This process plays a key role in embryological development, the homeostatic maintenance of organ systems, and the immune system [1, 2]. Given that apoptosis is tightly linked to so many crucial cell-cell interactions, it is not surprising that it is universally distributed among metazoans. It has been well documented in mammals, insects, and nematodes, and there is good evidence for its existence in cnidarians [3, 4] and sponges [5, 6]. In fact, both sponges and cnidarians possess caspases, the proteases that mediate apoptosis, and members of the Bcl-2 superfamily. This would suggest that the apoptotic pathway was already well developed by the time metazoans appeared. Because apoptosis would clearly impart an advantage in a multicellular context, allowing for the homeostatic control of cell numbers, it was originally believed to have coevolved with the appearance of metazoa. However, for more than a decade now, an apoptotic-like death process has been reported in a variety of unicellular organisms [7, 8]. Among these organisms, the process has been best described in Saccharomyces cerevisiae [9], where many of the characteristic cellular and biochemical changes associated with apoptosis [1, 2] have been detected: alterations in cell morphology (cell shrinkage), activation of a class of caspase-like proteases, externalization of membrane phosphatidylserine, condensation of the nucleus, and DNA fragmentation. Among the protozoa, a programmed cell death process has been most extensively characterized in the group Euglenozoa, specifically the
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