The Tale of Caspases-homologs and their Evolutionary Outlook: Deciphering Programmed Cell Death in Cyanobacteria.

Programmed cell death (PCD), genetically orchestrated mechanism of cellular demise, paradoxically, required to support life. Like in lower eukaryotes and bacteria, PCD in cyanobacteria is poorly appreciated, despite, recent biochemical and molecular evidences which supported its existence. Cyanobacterial PCD is an altruistic reaction to the stressful conditions that significantly enhance genetic diversity and inclusive fitness of the population. Recent bioinformatic analysis revealed abundance of death-related proteases i.e. orthocaspases (OCA) and its mutated-varients in cyanobacteria, with larger genome containing morphologically complex strains harboring most of them. Sequence analysis depicted crucial accessory domains along with proteolytic p20-like sub-domain in OCAs, predicting their functional versatility. However, the cascade involved in sensing death signals, their transduction and downstream expression and activation of OCAs are still an assumption. Here, we have provided a comprehensive description of the attempts to identify mechanisms of PCD, existence and importance of OCAs based on in silico approaches and, further, reviewed the evolutionary and ecological significance of PCD in cyanobacteria. In future, the analysis of cyanobacterial PCD would identify novel proteins having varied functional dimensions in the signaling cascades and also help in understanding the incipient mechanism of PCD morphotype(s) from where eukaryotic PCD might have originated.