Orthocaspase and toxin-antitoxin loci rubbing shoulders in the genome of Microcystis aeruginosa PCC 7806

Programmed cell death in multicellular organisms is a coordinated and precisely regulated process. On the other hand, in bacteria we have little clue about the network of interacting molecules that result in the death of a single cell within a population or the death of almost complete population, such as often observed in cyanobacterial blooms. With the recent discovery that orthocaspase MaOC1 of the cyanobacterium Microcystis aeruginosa is an active proteolytic enzyme, we have gained a possible hint about at least one step in the process, but the picture is far from complete. Interestingly, the genomic context of MaOC1 revealed the presence of multiple copies of genes that belong to toxin–antitoxin modules. It has been speculated that these also play a role in bacterial programmed cell death. The discovery of two components linked to cell death within the same genomic region could open new ways to deciphering the underlying mechanisms of cyanobacterial cell death.