Complete genome sequences of cellular life forms: glimpses of theoretical evolutionary genomics.

The availability of complete genome sequences of cellular life forms creates the opportunity to explore the functional content of the genomes and evolutionary relationships between them at a new qualitative level. With the advent of these sequences, the construction of a minimal gene set sufficient for sustaining cellular life and reconstruction of the genome of the last common ancestor of bacteria, eukaryotes, and archaea become realistic, albeit challenging, research projects. A version of the minimal gene set for modern-type cellular life derived by comparative analysis of two bacterial genomes, those of Haemophilus influenzae and Mycoplasma genitalium, consists of approximately 250 genes. A comparison of the protein sequences encoded in these genes with those of the proteins encoded in the complete yeast genome suggests that the last common ancestor of all extant life might have had an RNA genome.