The third major group of living systems besides bacteria and eukarya. Methanococcus jannaschii DNA (1.66 megabase) has been sequenced in (1996) and 1,738 predicted protein-coding genes have been identified. The organism has two, 58-kb and 16-kb, extrachromosomal elements. Only 38% of its genes appear similar to genes (by nucleotide sequences) of other fully sequenced bacteria or budding yeast. The metabolic genes bear similarities to bacterial genes whereas its genes involved in transcription, translation and replication bear greater resemblance to eukaryotic genes. Their genome displays nucleosomal structures. For a complete nucleotide sequence see World Wide Web at http://www.tigr.org/tdb. In halobacteria the genes are in linkage equilibrium, which is an indication of frequent genetic recombination (Papke RT et al 2004 Science 306:1928). Archaea in the rhizosphere of rice plants are responsible for an estimated 10 to 25% of the global methane emission. The genome of 3,179,916 bp of...