Genetic and physical map of plasmid NR1: comparison with other IncFII antibiotic resistance plasmids.

Plasmids are circular extrachromosomal genetic elements that are ubiquitous among the bacterial genera and are also found in some eucaryotic organisms (17, 39, 82, 95). Plasmids encode a number of specialized functions that are generally nonessential for their bacterial hosts. However, these plasmid-encoded functions provide their host cells with versatility and adaptability for growth and survival under a variety of conditions. Plasmids and the transposable elements that they often contain may also have played an integral role in the evolution of bacterial species by promoting the distribution and exchange of genetic information (5). The various plasmid-mediated functions include genetic transfer, bacteriocin production and resistance, production of and resistance to antibiotics, resistance to heavy metals and deoxyribonucleic acid (DNA) damaging agents, metabolism of carbohydrates and hydrocarbons, toxin production, virulence and colonization factors, hemolysin production, and tumorigenicity and nitrogen fixation in plants. In addition, plasmids encode genes and functions that make them capable of autonomous replication. In combination with their dispensable nature, this has made them attractive as model systems for the study of DNA replication and its coordination with cell growth and division. Recently, plasmids have also become indispensable components of the methods of modern molecular biology research. The three major classes of plasmids that have been characterized most extensively include plasmid F, colicinogenic (Col) plasmids, and drug resistance (R) plasmids (17, 39, 82, 95, 139). The F plasmid is the classic fertility factor that is capable of promoting its own transfer, the transfer of other plasmids, and the transfer of the host bacterial chromosome during bacterial conjugation. Col plasmids mediate the production of colicins, which are toxins that kill certain species of bacteria. Col plasmids also confer resistance to their bacterial hosts for the particular colicin that they specify. R