Transfer of Conjugative Plasmids and Mobilizable Plasmids in Gram-Negative Bacteria

Summary Conjugation is one of the major mechanisms whereby genes (such as antibiotic resistance) can move between organisms in bacterial populations. This animation shows bacterial conjugation resulting in the transfer of a conjugative plasmid and a mobilizable plasmid. Introduction Conjugation is a major mechanism whereby genes can move between organisms in bacterial populations. Many species (but not all) can undergo conjugation, and some species can conjugate with other species of bacteria. This can be medically important as these new genes may include antibiotic resistance genes or genes that increase virulence. Conjugation involves a donor bacterium that contains a conjugative plasmid and a recipient cell that does not. A conjugative plasmid is self-transmissible, in that it possesses all the necessary genes for that plasmid to transmit itself to another bacterium by conjugation. Conjugation genes known as tra genes enable the bacterium to form a mating pair with another organism, while oriT (origin of transfer) sequences determine where on the plasmid DNA transfer is initiated by serving as the replication start site where DNA replication enzymes will nick the DNA to initiate DNA replication and transfer. In addition, mobilizable plasmids that lack the tra genes for self-transmissibility but possess the oriT for initiation of DNA transfer may also be transferred by conjugation if the bacterium containing them also possesses a conjugative plasmid. The tra genes of the conjugative plasmid enable a mating pair to form and the oriT of the mobilizable plasmid enable the DNA to replicate as it moves through the conjugative bridge. Methods Adobe Flash Professional CS5.5 was used in constructing this animation. Illustrations were drawn using Adobe Illustrator CS5.1 and imported into Adobe Flash Professional CS5.5. Discussion The most common mechanism for horizontal gene transmission among bacteria, especially from a donor bacterial species to different recipient species, is conjugation. This animation shows bacterial conjugation resulting in the transfer of a conjugative plasmid and a mobilizable plasmid. Frames 1, 2 and 3 show a donor bacterium with the larger chromosome (nucleoid) shown to the left and the smaller plasmid containing a tra gene which allows conjugation and the origin of replication which is used to initiate DNA replication and hence transfer (oriT) on the right; these sequences are labeled. You will note that the donor bacterium is also producing a conjugation or sex pilus composed of pilin; protein that is encoded by the conjugative plasmid. Frames 4 and 5 show the donor bacterium (with chromosome and conjugative plasmid) binding the recipient bacterium that contains a chromosome but lacks the conjugative plasmid. The conjugation pilus retracts and the two bacteria are pulled together. Frame 6 shows the membrane proteins encoded by the tra genes of the conjugative plasmid of the donor bacterium fusing the membranes of the two bacteria,  creating an opening between them. These two bacteria are now referred to as a mating pair. Frames 7, 8, 9 and 10 show the conjugative plasmid being nicked at the oriT by nuclease products of the tra gene, this nuclease also has helicase activity and will unwind the strand that is now being transferred to the recipient bacterium. The DNA is then replicated via rolling circle replication and DNA replication is actually occurring as the plasmid is transferred from the donor to the recipient bacterium. At the end of conjugation both bacteria contain the conjugative plasmid, and hence can serve as donors of the plasmid to other bacteria, as is noted by the ability of the recipient bacterium to now form a conjugative pilus. Frame 11 shows a donor bacterium with the larger chromosome (nucleoid) shown to the left and two smaller plasmids; one conjugative plasmid containing a tra gene which allows conjugation and the origin of replication which is used to initiate DNA replication and hence transfer (oriT), and a second plasmid that lacks the tra gene but contains the oriT sequence (mobilizable plasmid).  You will note that the donor bacterium is also producing a sex/conjugation pilus. Frame 12 shows the donor bacterium (with chromosome, conjugative plasmid and mobilizable plasmid) binding the recipient bacterium that contains a chromosome but lacks the conjugative plasmid. The sex pilus retracts and the two bacteria are pulled together and the membrane proteins encoded by the tra genes of the conjugative plasmid of the donor bacterium fuses the membranes of the two bacteria, creating an opening between them.  These two bacteria are now referred to as a mating pair. Frames 13, 14 and 15 show the mobilizable plasmid being nicked at the oriT by nuclease products of the tra gene of the conjugative plasmid. This nuclease also has helicase activity and will unwind the strand that is now being transferred to the recipient bacterium.  The DNA is then replicated via rolling circle replication and DNA replication is actually occurring as the plasmid is transferred from the donor to the recipient bacterium.  At the end of conjugation both bacteria contain the mobilizable plasmid. It should be noted, however, that the recipient bacterium will not be capable of serving as a donor of this plasmid to another bacterium unless it also contains a conjugative plasmid with a functional tra gene. References 1. Willetts N, Wilkins B. 1984. Processing of plasmid DNA during bacterial conjugation. Microbiol. Rev. 48: 24-41. 2. Grohmann E, Muth G, Espinosa M. 2003. Conjugative plasmid transfer in Gram-positive bacteria. MMBR. 67: 277-301.