In article <3ojv8a$71u at gaia.ucs.orst.edu>, lanoilb at ava.bcc.orst.edu
(Brian_D Lanoil) wrote:
> I am trying to find references and put together a list of all bacteria
> that have megaplasmids. Although I am not exactly sure where the
> dividing line between a plasmid and a megaplasmid is, I would say any
> genetic unit above 100-200 kb is probably a megaplasmid. Do you agree?
>> I know about Alcaligenes, Agrobacterium, Rhizobium, and a few others. If
> anyone knows of any other bacteria or Archaea that carry megaplasmids,
> could you please write me at lanoilb at bcc.orst.edu?
>> Thanks, all
>> Brian
As a Rhizobium person and long-time student of large plasmids I thought
I'd give you my opinion. Mega means million so a plasmid should be around
a million base-pairs in size before it is called a megaplasmid. With
generous rounding off that would make 500 kb as a minimum. In practise
this is more or less what we use in Rhizobium - the megaplasmids in R.
meliloti are 1200-1700 kb in size whereas the "large plasmids" in other
species run around 200-600 kb. This type of border line also makes
practical sense as you can isolate meaningful amounts of DNA from a
200-500 kb plasmid, while megaplasmids behave more or less like
chromosomes and you may as well work with total DNA.
By this definition Agrobacterium spp. usually don't have megaplasmids.
Although there was an interesting article in J. BAct last year showing
that at least some strains of Agrobacterium have two chromosomes, one
circular, the other linear.
Some Pseudomonads DO have megaplasmids. Eg. OCT plasmid , plasmids in
Burkholderia solanacearum (formerly Pseudomonas) which have been shown by
Christian Boucher and coworkers to be involved in plant pathogenesis.
Michael Hynes
