transposons and insertion sequences and also contain a unique family of repetitive extragenic palindromes termed PhREPS for which no role is known. However, it is theorized that the stem-loop structures in PhREPS play a role in transcription termination or genome stability. Taxonomy Phytoplasmas belong to the monotypic order Acholeplasmatales. In 1992, the Subcommittee on the Taxonomy of Mollicutes proposed the use of 'Phytoplasma' rather than 'mycoplasma-like organisms' 'for reference to the phytopathogenic mollicutes'. In 2004, the generic name Phytoplasma was adopted and is currently of Candidatus (Ca.) status (used for bacteria that cannot be cultured). Phytoplasma taxonomy is complicated because the organisms cannot be cultured; methods normally used to classify prokaryotes are thus not available. Phytoplasma taxonomic groups are based on differences in fragment sizes produced by restriction digests of 16S ribosomal RNA gene sequences (RFLPs) or by comparisons of DNA sequences from 16S/23S spacer regions. The actual number of taxonomic groups remains unclear; recent work on computer-simulated restriction digests of the 16Sr gene suggested up to 28 groups, whereas others have proposed fewer groups with more subgroups. Each group includes at least one Ca. Phytoplasma species, characterized by distinctive biological, phytopathological, and genetic properties. Species , the following names and type strains are from LPSN,, the List of Prokaryotic names with Standing in Nomenclature. The associated diseases and 16Sr group-subgroup classifications are from various sources. Gallery See also Cherry X-disease Elm yellows Grapevine yellows List of taxa with candidatus status Milkweed yellows phytoplasma See also List of bacterial orders List of bacteria genera References External links Phytoplasma Classification Iphyclassifier First International Phytoplasmologist Working Group Meeting published in Vol. 60-2 2007 of Bulletin of Insectology Photo gallery about plants infected of phytoplasma Phytoplasma Resource and phytoplasma classification database Ohio State University publishes an informative site on this topic. First Internet Conference of Phytopathogenic Mollicutes includes several interesting articles on this topic. Phytoplasma Genome Projects. The Centre for Information on Coconut Lethal Yellowing with an associated Yahoo discussion group. Video of Melia yellows symptoms Video of maize bushy stunt symptoms Current research on Phytoplasmas at the Norwich Research Park Phytoplasma universal detection kit Nippon Gene Co., Ltd. Mollicutes Bacterial plant pathogens and diseases Candidatus taxa