It is known from eukaryotic world that cell death can be executed by several mechanisms which all have some characteristics of a regulated programme. Even necrosis which was once believed to occur in a non-regulated manner, is now considered as a programmed cell death mechanism. Other types of programmed cell death are pyroptosis, autophagy and apoptosis. Apoptosis was studied in detail in a range of different organisms and in each of them activity of specific protein-degrading enzymes was observed. These enzymes were called caspases. They are different in different organisms, so we now know  caspases (like in vertebrates) and caspase-like enzymes, such as metacaspases in plants. In bacteria, it has been shown at the DNA level that some similar proteins should exist, but it was unclear whether they could be active at all. And this is where we step in...

Orthocaspase

Understanding cell death mechanisms in cyanobacteria is important for several reasons. We might develop approaches towards improved biosafety, by eventually triggering cell death intrinsically. Also, it would be interesting to stimulate cell death in harmful algal blooms which are often composed of cyanobacteria such as Microcystis aeruginosa.
By bioinformatics approaches we found out that our model cyanobacterium Synechocystis sp. PCC 6803 which was our chassis in synthetic biology has a caspase homologue gene that could never yield an active enzyme because the putative protein would not have a full active site. We then moved to Microcystis in which there was no mutation in the active site. We amplified the gene and prepared a recombinant protein. Then we found conditions under which the protein is converted from an inactive to an active form and we showed that the enzyme was in fact active. This was the first demonstration that bacterial caspase homologues possess proteolytic activity. Due to their relative simple structure, we called these enzymes Orthocaspases. Results were published in 2015 in Molecular Microbiology.

Link to Toxins

As next, we checked the genomic context in which the orthocaspase gene is present. Surprisingly, we found an unusually high presence of genes (but also incomplete genes, likely) coding for toxin-antitoxin pairs. There are several possible explanations for this observation. One is that orthocaspase and toxin-antitoxin modules might work in concert. This is a very intriguing idea, but first we would have to check whether these TA modules are in fact active or not. We do have quite some experience with toxins and antitoxins, so we feel we could come behind this story in the near future.