Practice question

Low copy plasmids require faithful segregation to avoid loss during division that would be frequent if distribution random. Actin like protein ParM encoded by many IncE plasmids R1 provides pushing mechanism. ParM ATPase polymerizes into left handed double helical filaments structurally resembling F actin despite low sequence identity, two protofilaments wound. Filaments exhibit dynamic instability both ends grow ATP bound, hydrolysis induces catastrophe. In plasmid context, centromere site parC contains repeat sequences bound by adaptor ParR which forms helical oligomer encircling parC forming ring. ParR ring caps plus ends ParM filaments stabilizing them promoting elongation that pushes two plasmid ParR parC complexes apart towards opposite cell poles prior septation. Thus ParM plays role bacterial plasmid segregation via actin like spindle. FtsZ tubulin homolog cell division, MreB maintains rod shape via elongasome, MamK positions magnetosome chain chain. Discovery ParM demonstrated bacteria possess true cytoskeleton capable force generation similar eukaryotic mitosis but simplified, target for understanding plasmid stability biotechnology maintaining industrial plasmids without antibiotic selection pressure.