Talmudic Question #28

The eukaryotic nuclear membrane is something of a mysterious structure and its origins are the subject of much debate. It would be interesting to hear a microbiological view on the question: why is there a nuclear membrane?

Comments

[Dorothy Lewis]

A global view might be that the nuclear membrane prevents mitochondria from invading the nucleus--it allows separation of different kinds of DNA, which there is some selection pressure to keep separated.

[Matt Hirsch]

I'm a graduate student doing my thesis research on anammox bacteria (related to planctomycetes), so I've spent some time thinking about the membrane-bound 'organelles' in these organisms. Anammox bacteria have an organelle (the aptly named anammoxosome) where it is thought that the toxic intermediates (N2O and N2H4) which are present in anammox metabolism. Perhaps the 'nucleoid' was initally developed as another way to protect DNA from mutagens coming either from some prehistoric organism's own metabolism or from a harsh environment. van Niftrik et al. (JB 190:708-717) have some great micrographs and 3d models of 4 of the anammox genera (and some supplemental movies!)

[Robert Murray]

It is about time that someone said something to the question : Why is there a nuclear membrane. There are many nucleated microbes and at least one of them, Yeast, has had as close an inspection of nuclear membrane as modesty will allow. From the work (see Nature 29 Nov 07) by Albers et al. it iis clear that the membrane is a very integrated structure devoted to isolating its contents by a double membrane with very highly stuctured pores devoted to providing for export and import. The pores are a complex doughnut shaped insert (over 30 proteins) with internal spokes to support the movement of the special transport proteins that do the work of moving goods.So it is clear that the real membrane is designed to maintain an internal environment to an even more specialised level than the wall and cytoplasmmic membrane does for the cell as a whole. Maybe the work of replicating genomes deserves a more organized enviironment than cytoplasm to be sure of the right cations for ionic needs and the proteins for supportive, protective and enzymic purposes. Whether or not the Planctomyces with either single or double membranes enclosing the nucleoids gain much by the device is yet to be shown. No doubt J.A. Fuerst and colleagues are working on it. Lets wish them luck for we may learn a bit of how this functional membrane started out.

[Mark O. Martin]

The important point, I think, is to remember that we are far, far too focused on the genetics and physiology of the organisms which we can culture easily in the laboratory. Our first thoughts are "coli-centric" much of the time. I can't blame anyone for that: it worked!

But many, many years ago I was trying to work with bacteriophages of Rhizobium, and had some correspondence with a Hungarian microbiologist named Tibor Sik (who was very kind and patient with a feckless graduate student). I was frustrated with much of the genetics of different biovars of Rhizobium meliloti---now Sinorhizobium meliloti. The strain we worked with was wonderful---conjugation was effective, transposon mutagenesis worked, and so on. But other strains---ones that interested me mightily---did not! Why not?, I thought in my inexperience. Aren't they the same species? At the time, we were not quite yet starting to see that the term "species" was slippery in microbiology!

Tibor Sik wrote me a nice letter during that period, and it had a quote that I cut out and posted over my lab bench. It read:

"Mark, please remember that not all bacteria are E. coli---nor should they be!"

I think that "extreme" examples of microbes teach us the most about "normal" bacteria, paradoxically. I'm not sure if planctomycetes have a nucleus per se, no, but I am waiting to hear what the intrepid "Diversity Explorers" discover in the coming months and years.

It is truly a great time to be a microbiologist! It's not quite Indiana Jones with a microscope, truth be told, but there are such wonders to find and describe! In so many ways, I wish I could start over again, now---just to see what unfolds and be part of it!

[elio schaechter]

Bill,
Right you are, there is lots to be learned from that fascinating group of bacteria, including about the question at hand. However, I will reveal that I am not entirely convinced that the intracytoplasmic membrane seen in of some of the plactomycetes corresponds to a nuclear membrane. For one thing, there are ribosomes to be seen within, which best I can tell, is not the case for eukaryotic nuclear membranes. Let's agree, nonetheless, that this is a wide open question and that G. obscuriglobus may end up telling us something of great interest.

[William Margolin]

A good discussion of this topic is in a recent review by John Fuerst in Annual Review of Microbiology, 2005. Gemmata obscuriglobus, a member of the planctomycetes family of bacteria, has a double membrane surrounding its nucleoid, resembling the eukaryotic nucleus. Planctomycetes and relatives also lack FtsZ, the division protein that most other bacteria have. Further study of these bacteria may shed light on the origins of the eukaryotic nucleus and why the nucleoid in some bacteria is surrounded by a membrane.