Eubacteria, halobacteria, and the origin of photosynthesis: the photocytes.

The halobacteria and the photosynthetic members of the eubacteria have previously been classified in two separate urkingdoms--the archaebacteria and the eubacteria, respectively. They were thought to be no more closely related to each other than they each were to the eukaryotes. In accord with this earlier classification, photosynthesis was thought to have originated twice by independent events--once within the eubacteria and once within the archaebacteria. In this paper, however, using three-dimensional ribosome structure as a probe of evolutionary divergences, we show that the eubacteria and the halobacteria are more closely related to each other than they are to any other known organisms. The simplest interpretation of our data is that all extant photosynthetic cells are descended from a single common ancestor that possessed a primeval photosynthetic mechanism. Numerous data on the occurrence of related biochemical processes in halobacteria and eubacteria support this theory. Essential components of the photosynthetic apparatus, such as carotenoids, are present in both halobacteria and in eubacteria, including the nonphotosynthetic eubacteria, suggesting that photosynthesis could be a primitive property of both groups. Our data indicate that together the eubacteria and the halobacteria form a monophyletic group for which we propose the name "photocytes." If other techniques of phylogenetic analysis confirm this evolutionary tree, we propose that the photocytes be given urkingdom status.