Evolutionary relationships among photosynthetic bacteria.

To understand the evolution of photosynthetic bacteria it is necessary to understand how the main groups within Bacteria have evolved from a common ancestor, a critical issue that has not been resolved in the past. Recent analysis of shared conserved inserts or deletions (indels) in protein sequences has provided a powerful means to resolve this long-standing problem in microbiology. Based on a set of 25 indels in highly conserved and widely distributed proteins, all main groups within bacteria can now be defined in clear molecular terms and their relative branching orders logically deduced. For the 82 presently completed bacterial genomes, the presence or absence of these signatures in various proteins was found to be almost exactly as predicted by the indel model, with only 11 exceptions observed in 1842 observations. The branching order of different bacterial groups as deduced using this approach is as follows: low G+C Gram-positive (Heliobacterium chlorum) <--> high G+C Gram-positive <--> Clostridium-Fusobacterium-Thermotoga <--> Deinococcus-Thermus <--> green nonsulfur bacteria (Chloroflexus aurantiacus) <--> Cyanobacteria <--> Spirochetes <--> Chlamydia-Cytophaga-Flavobacteria-green sulfur bacteria (Chlorobium tepidum) <--> Aquifex <--> Proteobacteria (delta and in) <--> Proteobacteria (alpha) <--> Proteobacteria (beta) and <--> Proteobacteria (gamma). The Heliobacterium species, which contain an Fe-S type of reaction center (RC 1) and represent the sole photosynthetic phylum from the Gram-positive or monoderm bacteria (i.e., bounded by only a single membrane), is indicated to be the most ancestral of the photosynthetic lineages. Among the Gram-negative or diderm bacteria (containing both inner and outer cell membranes) the green nonsulfur bacteria, which contain a pheophytin-quinone type of reaction center (RC 2), are indicated to have evolved first. The later emerging photosynthetic groups which contain either one or both of these reaction centers could have acquired such genes from the earlier branching lineages by either direct descent or by means of lateral gene transfer.