Natural genetic competence is the ability of cells to take up extracellular DNA and is an important mechanism for horizontal gene transfer. Another potential benefit of natural competence is that exogenous DNA can serve as a nutrient source for starving bacteria because the ability to "eat" DNA is necessary for competitive survival in environments containing limited nutrients. We show here that eight Escherichia coli genes, identified as homologs of com genes in Haemophilus influenzae and Neisseria gonorrhoeae, are necessary for the use of extracellular DNA as the sole source of carbon and energy. These genes also confer a competitive advantage to E. coli during long-term stationary-phase incubation. We also show that homologs of these genes are found throughout the proteobacteria, suggesting that the use of DNA as a nutrient may be a widespread phenomenon.