Locus Displays Features of a Type I Toxin-Antitoxin System.

Type I toxin-antitoxin (TA) systems consist of a protein toxin that exerts a cytostatic or cytotoxic effect and an antisense RNA antitoxin that prevents translation of the toxin. Although well studied, type I TA systems have so far only been discovered in bacteria from the phyla Proteobacteria, Firmicutes, and Tenericutes. We hypothesized that type I systems could also be present in Cyanobacteria. Through bioinformatic analysis of the PCC 7806SL genome, we discovered ten putative type I TA loci and characterized six of them experimentally. Two of the six putative type I toxins, BH695_0320 and MsoT1 (BH695_4017), were observed to negatively affect cell growth, with MsoT1 exerting a phenotype similar to SrnB, a known type I toxin. We focused on the MsoT1/MsoA1 TA system and confirmed the expression of MsoT1 and MsoA1 in our assay. Additionally, we found that MsoA1 delays the toxic effects of MsoT1, indicating its role as a cognate antitoxin of MsoT1. Our results suggest that MsoT1/MsoA1 represents a novel candidate type I TA system, the first to be discovered in the Cyanobacteria phylum.