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Monday, September 2, 2013

No prokaryotes, you still don’t have nuclei

Everyone knows that the difference between prokaryotic cells, like bacteria, and eukaryotic cells, like the ones in our bodies, is that our cells have separate compartments for different functions. In particular, eukaryotic cells have nuclei in which they keep their DNA isolated from the rest of the cell. Yet, members of the bacterial superphylum Planctomycetes, Verrucomicrobiae, and Chlamydiae (PVC) have been observed with what seem to be nuclei housing their DNA. Should these bacteria be reassigned as quasi-eukaryotes? Not so fast, say Damien Devos and his colleagues from the European Molecular Biology Laboratory.

The authors constructed a detailed 3D model of one species (Gemmata obscuriglobus) of the PVC group. You can see an example below. 

3D reconstruction of bacteria with a complex endomembrane system.
Outer membrane (OM) is green
Inner membrane (IM) is cyan
DNA is yellow
OM invaginations are pink
Scale bar is 500 nm. 
Credit: Copyright Heidelberg, Universität
You can clearly see the green outer membrane surrounding at least some of the yellow DNA. However, other views of the same cell show that these internal membranes are in fact complex folds of the outer cellular membrane. That is, the would-be bacterial nucleus is contiguous with the cellular membrane and not a separate compartment at all. This was true for all the other perceived partitions.
Bacteria in this group may have long labyrinthine outer membranes, but they do not have separate distinct nuclei any more than any other bacteria do. Our categorizations are safe for another day.

Santarella-Mellwig R, Pruggnaller S, Roos N, Mattaj IW, & Devos DP (2013). Three-dimensional reconstruction of bacteria with a complex endomembrane system. PLoS biology, 11 (5) PMID: 23700385.