Cooccurrence of ScDSP Gene Expression, Cell Death, and DNA Fragmentation in a Marine Diatom, Skeletonema costatum

Abstract

A novel death-specific gene, ScDSP, was obtained from a death stage subtraction cDNA library of the diatom Skeletonema costatum. The full length of ScDSP cDNA was 921 bp in length, containing a 699-bp open reading frame encoding 232 amino acids and two stretches of 66 and 156 bp in the 5 and 3 untranslated regions, respectively. Analysis of the peptide structure revealed that ScDSP contained a signal peptide domain, a transmembrane domain, and a pair of EF-hand motifs. When S. costatum grew exponentially at a rate of 1.3 day ؊1 , the ScDSP mRNA level was at 2 mol ⅐ mole 18S rRNA ؊1 . In contrast, when the culture entered the death phase with a growth rate decreasing to 0.5 day ؊1 , ScDSP mRNA increased dramatically to 668 mol ⅐ mole 18S rRNA ؊1 , and a high degree of DNA fragmentation was simultaneously observed. Under the influence of a light-dark cycle, ScDSP expression in both exponential and stationary phases clearly showed a diel rhythm, but the daily mean mRNA level was significantly higher in the stationary phase. Our results suggest that ScDSP may play a role in the molecular mechanism of self-destructive autolysis in phytoplankton under stress.Events of mass cell loss usually occur after algal blooms. In addition to physical damage, such as sedimentation or herbivore grazing, the self-destructive lysis of stressed cells is also considered to be a main cause for the decline of phytoplankton blooms (6, 12). Brussaard et al. (6) first used esterase activity to demonstrate that the self-destructive lysis of cells was the major loss factor accounting for 75% of the decline of a Phaeocystis bloom. Dissolved organic material released by lysed phytoplankton is subsequently utilized by heterotrophic bacteria and enters the microbial loop to support regenerated production in marine ecosystems (1,3,6). Such massive autolysis of phytoplanktonic cells is usually triggered by external stress factors such as nutrient starvation (4), light limitation (4, 39), and pathogenic virus infection (15). Nevertheless, the molecular regulatory mechanisms involved in cell lysis have seldom been investigated in algal cells.Several genes are known to have a close association with self-destructive lysis in unicellular organisms. When the budding yeast Saccharomyces cerevisiae enters the stationary phase, a series of genes, including doa4, ctt1, ppn1, sod1, sod2, and so on, are expressed to maintain intracellular homeostasis, and a deficiency in any of these gene functions can cause the loss of viability (14, 16). Additionally, apoptosis-like death has also been observed in aging yeast cells. The cause of this event is considered to be facilitated by a novel metacaspase encoded by yca1 (19,30). In phytoplankton, increasing numbers of studies have pointed out that light limitation, nutrient starvation, or the accumulation of reactive oxygen species can induce an apoptosis-like syndrome, such as cell shrinkage, blebbing, chromatin condensation, and the formation of nuclear DNA fragmentation. Through biochemical and immunological assays...

Characterisation of algicidal bacterial exometabolites against the lipid-accumulating diatom Skeletonema sp.

“…6). This indicates that EC-1 may induce a mechanism of programmed cell death which is consistent with previous studies demonstrating that Skeletonema encodes a cell death mechanism involving a gene product (Death Specific Protein, DSP1) that induces autolysis when the cells are stressed [20]. Further analysis of DSP1 indicated its expression is modulated by the messenger molecule nitrous oxide and that light intensity is one trigger of stress-induced autolysis [21].…”

Hydrogen peroxide induces apoptotic-like cell death inMicrocystis aeruginosa(Chroococcales, Cyanobacteria) in a dose-dependent manner

“…Similar results are reported for Anabaena sp. (Ning et al 2002), Microcystis aeruginosa (Bouchard & Purdie 2011), Dunaliella viridis Teodoresco (Jimenez et al 2009) and Skeletonema costatum (Greville) Cleve (Chung et al 2005). However, we found only diffuse low molecular weight bands on agarose gels at 48 h following 250 mM H 2 O 2 exposure, not DNA laddering.…”

Death-specific protein in a marine diatom regulates photosynthetic responses to iron and light availability

“…DSPs are restricted to the red algal lineage (12) and have the highest similarity to Ca 2+ binding proteins, in part because of the highly conserved nature of the Ca 2+ binding EF-hand motif (a motif found in numerous proteins involved in a wide array of cellular functions, many of which are unrelated to each other). However, we found partial and distant similarity to the thylakoid-associated proton gradient regulator-5 (PGR5; BLASTp e value < 4 × 10 −4 ) protein found in higher plants.…”

“…Their associated protein sequences had strongest homology to so-called death-specific proteins (DSPs) from the diatom Skeletonema costatum (ScDSP1) (12) and thus, were denoted T. pseudonana DSP1 (TpDSP1; BLASTp e value < 1 × 10 −76 ) and TpDSP2 (BLASTp e value < 3 × 10 −26 ) (Fig. S1A).…”

“…S1A). ScDSP1 expression is induced by senescence, low light, chemical inhibition of photosystem II (PSII)/cytochrome b 6 f, and high intracellular levels of nitric oxide (12,13 ). In silico analysis gave conflicting predictions for the subcellular localization of TpDSP1.…”