EN
Summary Plasmid survival relies heavily on the regulation of gene expression assuring the balance between the necessity of a certain level of plasmid genetic information being expressed and minimalization of the metabolic burden imposed on the host. The most commonly used regulatory mechanisms are autogenous repression and antisense RNA-mRNA interactions which provide the systems with economy, simplicity, sensitivity and possibility of rapid response to internal and external changes. IncP-1 plasmids are described in more detail as the paradigm of a multivalent regulatory network, responsible for tight repression, coordination and fine-tuning of almost all plasmid functions and providing simultaneously a high level of security to plasmid genome. Appropriate recognition and use of environmental stimuli as very important factors in plasmid biology is nicely exhibited by Ti plasmids of Agrobacterium tumefaciens and pheromone-responsive plasmids of Enterococcus faecalis.