Paradigms of plasmid organization

Plasmids are extrachromosomal elements built from a selection of generally quite well understood survival and propagation functions, including replica tion, partitioning, multimer resolution, post-segregational killing and con jugative transfer. Evolution has favoured clustering of these modules to fo rm plasmid cores or backbones. Go-regulation of these core genes can also p rovide advantages that favour retention of the backbone organization. Tumou r-inducing and symbiosis-determining plasmids appear to co-regulate replica tion and transfer in response to cell density, both being stimulated at hig h density. Broad-host-range plasmids of the IncP-1 group, on the other hand , have autogenous control circuits, which allow a burst of expression durin g establishment in a new host, but a minimum of expression during maintenan ce. The lessons that plasmids have for clustering and co-regulation may exp lain the logic and organization of many small bacterial genomes currently b eing investigated.