Paradigms of plasmid organization

Plasmids are extrachromosomal elements built from a selection of generally quite well understood survival and propagation functions, including replication, partitioning, multimer resolution, post‐segregational killing and conjugative transfer. Evolution has favoured clustering of these modules to form plasmid cores or backbones. Co‐regulation of these core genes can also provide advantages that favour retention of the backbone organization. Tumour‐inducing and symbiosis‐determining plasmids appear to co‐regulate replication and transfer in response to cell density, both being stimulated at high density. Broad‐host‐range plasmids of the IncP‐1 group, on the other hand, have autogenous control circuits, which allow a burst of expression during establishment in a new host, but a minimum of expression during maintenance. The lessons that plasmids have for clustering and co‐regulation may explain the logic and organization of many small bacterial genomes currently being investigated.