Plasmids and Conjugation Systems Other Than F

Abstract

Not all plasmids are conjugative, and even those that are do not necessarily have chromosome-mobilizing ability (i.e., the ability to promote transfer of chromosomal markers to a recipient cell). Nevertheless, these plasmids can have major economic and genetic importance. The diversity of plasmids that have been identified is staggering, and they are ubiquitous. In one study, 34 of 87 hospital isolates of enteric bacteria or Pseudomonas carried at least one plasmid. Bacillus megaterium routinely has eight or more plasmids in its cytoplasm. All of these plasmids have only two things in common. They can be identified in cell lysates as autonomous DNA molecules, and they are capable of self-replication. If the presence or absence of a plasmid has no observable effect on the cell phenotype, it is a cryptic plasmid. Researchers often subdivide plasmids according to their capacity for self-transfer from one host cell to another using the F plasmid as the standard for comparison.