Bacterium or organelle?

In the 7 September Nature Shigenobu et al. report the complete sequence of Buchnera, an obligate resident of aphid cells (Nature 2000, 407:81-86). The sequence suggests that this bacterium is on its way to becoming an organelle. Buchnera looks most like Escherichia coli, but with a genome one seventh the size. It lacks genes for most regulatory proteins and for the biosynthesis of nonessential amino acids, cell-surface components (including lipopolysaccharides and phospholipids), and crucial DNA repair, recombination, methylation and restriction enzymes. The aphid cannot survive without Buchnera, as Buchnera synthesizes several essential amino acids. But with Buchnera relying on the aphid for a membrane bilayer and defense mechanisms, the bacterium is starting to look more like an organelle.