Photosynthesis transforms solar energy into oxygen and sugar and first arose in bacteria, but the molecular mechanisms involved in the origin and evolution of the existing bacterial phototrophs have been unclear. In November 22 Science, Jason Raymond and colleagues at Arizona State University, Tempe, USA, show that horizontal gene flow has played a major role in the evolution of bacterial phototrophs and that many of the essential components of photosynthesis are among these horizontally transferred genes (Science, 298: 1616-1620, November 22, 2002).

Raymond et al. performed a whole-genome comparison of representatives from all ?ve groups of photosynthetic prokaryotes. They observed that the genomes of these organisms represent mosaics of genes with very different evolutionary histories. In addition, they describe a subset of "photosynthesis-speci?c" genes that were elucidated through a differential genome comparison.

"These ideas suggest further tests of estimating times of divergence and lateral gene transfer...

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