Re: Do archaea have plasmids just like other bacteria?

hi Pedro,

Thanks for a great question. I'm not surprised that you found little information about Archaea in your books. Although these fascinating organisms were discovered almost 30 years ago, many biologists are still unaware of their significance in many fields of research. You say "like other bacteria" - in fact, the Archaea are a third domain of life, distinct from the Bacteria and the Eukarya (organisms with a cell nucleus). In some ways, Archaea are actually more like you and I than the Bacteria!

The quick answer to your question is yes - many (but not all) Archaea contain plasmids that are similar to those found in many (but not all) Bacteria. At least 41 plasmids from Archaea have been completely sequenced - that is, we know the complete DNA sequence of these plasmids. Sequences are stored in a large database by an research organisation in the USA called the NCBI (National Center for Biotechnology Information). If you like, you can have a look at some plasmid sequences from Archaea.

You'll see that some Archaea have typical small plasmids of a few thousand base pairs in length. Others have more than one plasmid and some contain much larger plasmids - up to around 400 000 base pairs long. This type of plasmid is sometimes called a megaplasmid, although as "mega" means "million", we should really only call them megaplasmids if they have more than 1 million base pairs.

You might like to click on some of the links from the NCBI web page and see what you find. There is information about the organisms, the plasmids and the genes and proteins encoded by the plasmid. We don't know much about the function of plasmids in Archaea. As in bacteria, they encode some plasmid- specific genes that control the replication and copy number of the plasmid. A lot of archaeal plasmid gene products are hypothetical (meaning that we don't know what they do) and other genes seem to be specific to certain organisms (e.g the gas vesicle proteins found in the salt-loving Halobacteria , which give them buoyancy in water). It's probable that plasmids and their genes can be transferred between different Archaea and perhaps even between some Archaea and Bacteria, but we don't know a lot about that process. There is still a lot to discover in the microbial world!