Prokaryotic genome sequencing is providing insights into microorganisms that are important for agriculture and medicine. Scientists who analyze the sequences have discovered previously unknown genes and proteins that can be targeted for isolation and functional study. They have also discovered surprising relationships between some organisms, suggesting that genes may be transferred between different groups.
Rhizobium species are bacteria that form symbiotic associations with plants, living inside the roots of legumes such as beans, peas, and clover. The bacteria fix atmospheric nitrogen from the air and convert it into forms usable by the plants, reducing the need for nitrogen-
E. coli strain O157:H7 causes illness (sometimes severe) in at least 70,000 people a year in the United States. Its genome has 5,416 genes, of which 1,387 are different from those in the familiar (and harmless) laboratory strains of this bacterium. Many of these unique genes are also present in other pathogenic bacteria, such as Salmonella and Shigella. This finding suggests that there is extensive genetic exchange among these species, and indeed, “superbugs” that have acquired multiple genes for antibiotic resistance are already a major problem. Sequencing of a particular strain of E. coli causing food-
Severe acute respiratory syndrome (SARS) was first detected in southern China in 2002 and rapidly spread in 2003. There is no effective treatment, and 10 percent of infected people die. Isolation of the causative agent, a virus, and the rapid sequencing of its genome revealed several novel proteins that are possible targets for antiviral drugs or vaccines. Research is under way on both fronts, since further outbreaks of this and other newly revealed pathogens are anticipated.
Some archaean prokaryotes, such as Methanobrevibacter, produce methane gas (CH4) in the stomachs of cows and other ruminants. By contrast, certain bacterial species, such as Methylococcus, remove methane from the air and use it as an energy source. The genomes of both of these prokaryotes have been sequenced. Understanding the genes involved in methane production and consumption is stimulating thinking about ways to slow the progress of global warming, given that methane is the second most prevalent greenhouse gas emitted in the United States and a significant contributor to climate change (see Key Concept 53.1).