Conjugative, mobile, non-mobile plasmids

Conjugative plasmids contain so-called tra-genes, which perform the complex process of conjugation, the transfer of plasmids to another bacterium.

Non-conjugative plasmids are incapable of initiating conjugation, hence they can only be transferred with the assistance of conjugative plasmids, by 'accident'.

An intermediate class of plasmids are mobilizable, and carry only a subset of the genes required for transfer. They can 'parasitize' a conjugative plasmid, transferring at high frequency only in its presence.

Anything else, reply back!

Thanks for your reply. And now applying this knowledge...


In a conjugation experiment, donor cells have a conjugative plasmid with tetR (tetracyline resistant) genes and has a mobile plasmid with CM^R (chloramphenicol resistant) genes.

Recipients have tetS (for sensitive) genes and have NA^R (naladixic acid resistant) genes.

Spread cells on plates that contain naladixic acid plus chloramphenicol.

Question one: Show the possibilities for the plasmid content of cells on this plate.

Question two: If we try to use these cells as donors in conjugation, what will happen?
A. They will all be donors
B. None of them will be donors
C. A few will be donors and most will not

I believe that the recipient cells will have the tetracyline resistant genes and chloramphenicol resistant genes since they are being transfered over by donor cells.

If we try to use these cells as donors in conjugation, what will happen?

Since we do not have anything that kills the donor cells - the ones mentioned with the resistant genes - they will continue to grow. Therefore, I'm guessing that they will all be donors because the recipients will have acquired the F plasmid along with genes that ensure resistance.

What are your thought?

The correct answer is actually C.

But I don't know the reason behind it.   

Would it have something to do with the mobile plasmids with CM^R genes?

I initially put C but then changed my mind . The ones that don't die are the recipients while the donors die off because of the naladixic acid. Thus, the ones that are dividing are those that are resistant to all.

Ok just to clarify... so the genes (ie in this case it's the NA^R gene) in the recipient will never be given to the donor?

Remember, a conjugative plasmid is able to promote the transfer of (mobilize) a second, otherwise non- conjugative, plasmid from the same donor cell. So the recipient gets the chloramphenicol resistant genes from the donor. The recipients has tetS (for sensitive) genes and have NA^R (naladixic acid resistant) genes. Once the cells are spread on the plate that contain naladixic acid plus chloramphenicol, the donor die off while the recipient survives because it have both genes resistant to these chemicals. The donor cells are not resistant to naladixic acid so the die off.