Thermotoga maritima is an extremely thermophilic eubacterium and represents the deepest known branch and the most slowly evolving line within this kingdom. This bacterium possesses a sheath-like envelope which forms characteristic balloons (the ‘toga’) around the cell poles. We have shown by electron microscopy in conjunction with image processing and by biochemical analyses that this sheath is essentially an outer membrane. Its predominant 42 kDa protein, which is regularly arrayed in vivo, has all the characteristics of the trimeric porins known from the phylogenetically very distant proteobacteria. This was an unexpected finding, which has some interesting evolutionary implications.
