Understanding Bacterial Conjugation & the F Factor Plasmid

[Another God]

Firstly, some textbook definitions:
Conjugation : The form of gene transfer and recombination in bacteria that requires direct cell to cell contact.
Plasmid : A circular Double Stranded DNA molecule that can existand replicate independently of the chromosome or may be integrated with it. A plasmid is stably inherited, but is not required for the host cell's growth and reproduction.
F Factor : The fertility factor, a plasmid that carries genes for bacterial conjugation and makes its E. coli host cell the gene donor during conjugation.

Conjugation is where an F+ bacterium (A bacterium with the F Factor in it) encounters an F- bacterium, and a temporary bridge (called the Sex Pilus) forms between the two cells where the F Factor Plasmid copies itself into the F- bacterium. Thereby resulting in two F+ bacteria.

That is essentially what conjugation is. What makes it more interesting is the fact that other genes can be put onto the plasmid, allowing propagation of those genes, horizontally through the bacterial population as well as vertically (ie: Passed on to brothers and sisters, rather than just down to the kids). Another thing which makes it more interesting/confusing, is the fact that some F Plasmids can integrate themselves into the chromosome of their host bacteria. These results in 1. Sometimes during excision from the host chromosome, the plasmid is removed with host chromosome in it. in this way host chromosome can be spread to other bacteria. and 2. Sometimes the conjugation occurs while the plasmid is in the chromosome, and so the cell attempts to copy its whole chromosome across to the F- recipient cell. This is either never, or at the extreme, highly unlikely to occur, but what does happen, is some of the plasmid gets copied, and a chunk of the host chromosome gets copied. The recipient cell therefore gets bits and pieces, but nothing complete.

If you want me to go into any more detail on any of these points, just ask for it.

 

[sceeth0]

How about the formation of the Sex Pilus? And what happens when two F+ bacteria meet? What keeps both of them from trying to copy into the other?

 

[Monique]

I remember one of my collegue-students, we have to work in a lab for 8 mo. in order to write a dissertation, give a presentation and a defense.. in order to get a BS.

He worked in a lab that was looking into the dangers of genetically engineered bacteria.. where they had a mechanical model of the human digestive tract, they put in cow stomach acid, and some bacteria with a marker, and they were interested to see if the intestinal bacteria were able to take up the marker DNA through these processes.. unfortunately.. all the information was private.

 

[E8]

Originally posted by sceeth0
How about the formation of the Sex Pilus? And what happens when two F+ bacteria meet? What keeps both of them from trying to copy into the other?

Those are really good questions by the way and were all ask during the discovery of conjugation. My research is in this exact field but I work w/ Hfr (High Frequncy)not F+ strains.

Sex pili are short, stiff, rod shaped structures prevalent throughout the outer membranes of bacteria but have no role in locomotion whatsoever.

The formation of the pilus are coded on the F plasmid and only produced when temp. is above 33 degrees C and 'fall off' at temps lower. These temps are consistant w/ the growth of the bacteria. Typically at a high growth rate sex, such as the case for E.Coli @ 37 degrees C, pili are present but during the stationary phase, at say rm temp, they are not. Both cells, donor (F+) and recipient (F-), must be metabollically active for conjugation to take place or for the sex-pilus to even be present.

The tips of the pili are vital, and experiments where the tips are bound (via f1 phage) demonstrate prevention of mating aggregrates (F+ attached to an F- or a 'bacterial couples'). From the top of my head the pili tips recognize specific surface markers located on the surface of recipients(F-) and the recipient sends some signal back to the donor(F+).

The reason why two donor (F+) strains can't mate is due to 2 proteins. One acts on the the surface of the membrane (TraT) and the other w/n the inner membrane as well as the cytoplasm (key word: surface exclusion). So the presence of those 2 proteins, peptidoglycan layer, and the pili collectively prevent 2 F+ from mating.

Once the Pili attaches to the recipient, it is thought that it 'pulls' the recipient towards itself and membrane fusion actually takes place. Generally people believe that the DNA transfers via through the pilus but this is NOT the case since the pili does not have enough girth for DNA to travel through and experiments show that genetic transfer is NOT disturbed when the pili is experimentally removed. What actually happens is not known yet or it is very recent and I haven't heard it yet.

Generally The reason why they don't 'copy one another' is b/c the Donor(F+), is the only one of the two that has the genes to produce the protens required for the mobilization of dna from the donor on to the recipient as well as allow for 'rolling-circle' type replication. W/O those proteins the dna is 'static' as is the case w/ the recipient (F-). So you see the transfer is typically one-way b/c only the donor has the ability to do so genetically.

I'll leave any corrections or additions for AG.

 

[Another God]

Oh no, don't leave it to me at all!

I'm only learning this stuff. You're actually involved in it! I'll post what I know about, but your explanation was great. Thank you for helping!