EPISOME

in bacteria, one of a group of extrachromosomal genetic elements called plasmids, consisting of deoxyribonucleic acid (DNA) and capable of conferring a selective advantage upon the bacteria in which they occur. Episomes may be attached to the bacterial cell membrane (such a cell is designated F+) or become integrated into the chromosome (such a cell is designated Hfr). F+ and Hfr cells act like males during conjugation, a mating process in certain bacteria (e.g., Escherichia, Salmonella, Serratia, Pseudomonas). During conjugation, cells lacking the episome (called F- cells) may receive either the episome (from an F+ cell) or the episome plus the genes to which it is attached (from an Hfr cell). Experiments involving gene transfers from Hfr cells have been used to determine the locations of genes on the chromosome of Hfr cells. Some bacterial viruses, called temperate phages, carry DNA that can act as an episome. A bacterial cell into whose chromosome the viral DNA has become integrated is called a prophage. See lysogeny.