IncU Plasmid pFBAOT6 - a brief introduction

What are plasmids and what is their function?

Plasmids are DNA molecules that are distinct from the main chromosome in a bacterial cell. They are capable of being stably inherited without being linked to the chromosome and can be transferred horizontally between cells. One of the most important aspects of bacterial plasmids is their carriage and spread of antibiotic resistance genes that ultimately have an impact on the treatment of diseases of animals and humans. They can also carry genes that code for a wide range of metabolic activities, enabling their host bacteria to degrade pollutant compounds, and produce antibacterial proteins (colicins). They can also harbour genes for virulence, that help to increase pathogenicity of bacteria causing diseases such as plague, dysentery, anthrax and tetanus.

Compatibility

Plasmids that can coexist within a bacterium are said to be compatible. Plasmids which cannot coexist are said to be incompatible and after a few generations one or other of these is lost from the cell. On a clinical level plasmids have been classified based upon their incompatibility grouping. Plasmids that encode their own transfer between bacteria are termed conjugative. Non-conjugative plasmids do not have these transfer genes but can be carried along by conjugative plasmids via a mobilisation site. This ability ensures that conjugative plasmids are highly promiscuous and can be found in a wide variety of bacteria (thereby having a broad host range; BHR).

We have studied environmental plasmids for a number of years and recently this has centred around a group of related tetracycline-resistant plasmids. Plasmid pFBAOT6 was originally isolated from a strain of Aeromonas caviae recovered from hospital effluent from Westmorland General Hospital, Kendal, United Kingdom in September 1997. It belongs to a group of related IncU plasmids that we have shown have global ubiquity.

Genetic coding revealed

Recently, in collaboration with scientists at the Sanger Centre, Cambridge, UK, we obtained the full nucleotide sequence pFBAOT6. The plasmid is 84,748 base pairs (bp) long and has 94 predicted coding sequences, only 12 of which do not have a possible function that has been attributed. Putative replication, maintenance, and transfer functions have been identified and are located in a region in the first 31 kb of the plasmid. The remaining 54 kb harbours the “genetic load” of the plasmid and includes resistance genes, a class I integron, an IS630 relative, and other transposable elements in a 43-kb region that may be a novel Tn1721-flanked composite transposon. This region also contains 24 genes that exhibit the highest levels of identity to chromosomal genes of several plant-associated bacteria. The features of the backbone of pFBAOT6 that are shared with this newly defined group of environmental BHR plasmids suggest that pFBAOT6 may be a relative of this group, but a relative that was isolated from a clinical bacterial environment rather than a plant-associated bacterial environment.

More information

Please see the following paper:

Rhodes, G., Parkhill, J., Bird, C., Ambrose, K., Jones, M., Huys, G., Swings, J., and Pickup RW. (2004). Complete Nucleotide Sequence of the Conjugative Tetracycline Resistance Plasmid pFBAOT6, a Member of a Group of IncU Plasmids with Global Ubiquity. Applied and Environmental Microbiology 70, 7497-7510.

A plasmid map of pFBAOT6 (c) Glenn Rhodes