Inactivation of a chromosome- and plasmid-localised HEPN-MNT TA system alters expression of key regulators of nitrogen assimilation and arginine biosynthesis in Synechocystis sp. PCC 6803

In Synechocystis, two HEPN-MNT TA systems are located on the chromosome (mnt-hepn(c)) and pSYSM plasmid (mnt-hepn(p)). Although individual gene deletions were viable, simultaneous disruption of both systems was lethal, demonstrating their essential role in cell survival.

The partially segregated double mutant (∆∆mnt-hepn(cp)) showed slow growth and a pale-yellow phenotype. Sequence and structural analyses revealed conserved HEPN and MNT motifs similar to that of Shewanella, where MNT neutralizes HEPN ribonuclease activity through AMPylation. Physical association of TA components were demonstrated by overexpression co-elution assay.