The Ssl2245-Sll1130 Toxin-Antitoxin System Mediates Heat-induced Programmed Cell Death in Synechocystis sp. PCC6803

Two putative heat-responsive genes, ssl2245 and sll1130, constitute an operon that also has characteristics of a toxin-antitoxin system, thus joining several enigmatic features. Closely related orthologs of Ssl2245 and Sll1130 exist in widely different bacteria, which thrive under environments with large fluctuations in temperature and salinity, among which some are thermo-epilithic biofilm-forming cyanobacteria. Transcriptome analyses revealed that the clustered regularly interspaced short palindromic repeats (CRISPR) genes as well as several hypothetical genes were commonly up-regulated in Δssl2245 and Δsll1130 mutants. Genes coding for heat shock proteins and pilins were also induced in Δsll1130. We observed that the majority of cells in a Δsll1130 mutant strain remained unicellular and viable after prolonged incubation at high temperature (50 °C). In contrast, the wild type formed large cell clumps of dead and live cells, indicating the attempt to form biofilms under harsh conditions. Furthermore, we observed that Sll1130 is a heat-stable ribonuclease whose activity was inhibited by Ssl2245 at optimal temperatures but not at high temperatures. In addition, we demonstrated that Ssl2245 is physically associated with Sll1130 by electrostatic interactions, thereby inhibiting its activity at optimal growth temperature. This association is lost upon exposure to heat, leaving Sll1130 to exhibit its ribonuclease activity. Thus, the activation of Sll1130 leads to the degradation of cellular RNA and thereby heat-induced programmed cell death that in turn supports the formation of a more resistant biofilm for the surviving cells. We suggest to designate Ssl2245 and Sll1130 as MazE and MazF, respectively.