The tra locus includes the pilin gene and regulatory genes, which together form pili on the cell surface, polymeric proteins that can attach themselves to the surface of F-bacteria and initiate the conjugation.
The existence of the tra region of a plasmid genome was first discovered in 1979 by David H. Figurski and Donald R. Helinski[1] In the course of their work, Figurski and Helinski also discovered a second key fact about the tra region – that it can act in trans to the mobilization marker which it affects.
[1] This finding suggested that there were two basic aspects necessary for a plasmid to move from one cell to another: The tra genes encode proteins which are useful for the propagation of the plasmid from the host cell to a compatible donor cell or maintenance of the plasmid.
A plasmid with a mob site (like many found in Rhodococcus species) cannot be transferred via transfer genes which normally interact with the oriT site (which is common in E. coli)[3] Each of the individual genes in the tra operon codes for a different protein product.
These products may perform a number of tasks including interaction with one another to perform mating pair functions and regulation of different regions of the tra operon itself,[6] or conjugative DNA metabolism and surface exclusion.