Identification of a plastid-targeted RecQ-like helicase in the red alga Cyanidioschyzon merolae

Plastids retain their own genomic DNA (plastid DNA, ptDNA) which must be faithfully maintained for proper organelle function. However, the molecular mechanisms responsible for ptDNA maintenance remain poorly understood, particularly in red algae. Here, we identified a plastid-targeted RecQ-like helicase (ptRECQ) in the unicellular red alga Cyanidioschyzon merolae. Biochemical assays demonstrated that ptRECQ possesses ATP-dependent helicase activity and preferentially unwinds splayed DNA substrates that mimic replication fork structures. ptrecQ mutant cells were hypersensitive to nalidixic acid, a ptDNA gyrase inhibitor that induces replication stress, indicating a role for ptRECQ in plastid genome maintenance under replication stress. Phylogenetic analyses showed that ptRECQ is broadly conserved in red algae, absent from green algae and land plants, and phylogenetically distinct from cyanobacterial RecQ proteins. These results suggest that red algae employ a ptDNA maintenance system that differs from that of the green lineage.