Higher Order Taxa: Bacteria; Firmicutes; Mollicutes; Mycoplasmatales; Mycoplasmataceae; Mycoplasma
 Species Mycoplasma genitalium
Description and Significance Mycoplasma genitalium is a parasitic bacterium with the smallest known genome of any free living bacteria at 580,070 bp long (1998). They are believed to be simplest form of independent life with the minimal set of genes. M. genitalium are parasites of both plants and animals including humans. They are often found invading and adhering to the epithelial linings of the respiratory or urogenital tracts of animals and are associated with many urogenital tract infections, in both men and women. In fact M. genitalium was first isolated in two men with non-gonococcal urethritis. Besides its importance as a pathogen, its genome has been extensively studied and used in comparison with other small genome organisms, in order to determine the minimal set of genes essential for life.
Genome structure In 1995, the entire genome of M. genitalium was sequenced in less than 6 months using the random shotgun sequencing technique. It was found to have the smallest known genome of any free-living organism at about 580 kilobase pairs long, with 479 coding sequences for proteins. For comparison M. pneumoniae has 677 protein coding sequences, H. influenzae has 1703, and E. coli K-12 has 4,288. The genome-sequencing projects have shown that there is a 62% similarity between the coding regions of Bacillus subtilis and a 56% similarity to E. coli, however there is even closer similarity to the gram positive species of Lactobacillus and Clostridium. This provides strong evidence that the mycoplasmas are more closely related to gram-positive bacteria with low GC contents. Low GC content is a characteristic of all mycoplasmas. M. genitalium possess an average GC content of 32%. Genes coding for rRNA average 44% and 52% for tRNA. The GC content for rRNA and tRNA are much higher than the rest of the genome due to the importance of their secondary structures for these functional RNAs. The Mycoplasmas evolved through a process called reductive evolution from gram-positive bacteria. They have lost many genes involved in cell wall synthesis and biosynthetic systems as they adapted to a more parasitic lifestyle, as a result they have some of the smallest genomes of any self-replicating organism. In addition, their UGA codon is used as another codon for tryptophan instead of a stop codon found in other organisms. M. genitalium lack any genes for amino acid biosynthesis and few genes for nucleic acid, fatty acids, vitamins, and fatty acid biosynthesis. They must acquire most these products from their host or through an artificial medium. They also lack genes for oxidative metabolism (Kreb cycle, or Entner-Doudoroff pathway), gluconeogenesis, catalase, peroxidase, or other toxic oxygen protective enzymes. They also appear to possess few regulatory proteins, such as two-component systems and no identifiable transcription factors. They do however possess the genes necessary for DNA replication, transcription, and translation, but even these contain a minimal set of rRNA and tRNA genes. They also contain genes for glycolysis, phospholipids metabolism, and for converting vitamins to cofactors. They have less DNA repair genes compared to E. coli and H. influenzae, it can be assumed however that the genes detected such as uracil DNA glycosylase, exinuclease ABC genes, and recA must be essential for proper DNA repair. While M. genitalium have been able to reduce its genome during the course of its evolution as a parasite, it must maintain genes necessary for this parasitism. A significant portion of its genome is devoted to the transport of exogenous nutrients such as glucose and fructose, as well as genes for attachment organelles, adhesins, and antigenic variation to evade the host immune system. In fact it’s estimated that 5% of its total DNA are devoted to repeat fragments used for antigenic recombination and variation of cell pole adhesins. While M. genitalium possesses the minimum set of genes needed for protein synthesis and DNA replication, the result is also that protein synthesis and cell replication occurs much slower compared to E. coli. M. genitalium grow slowly with a generation time of 24 hours. E. coli and M. genitalium both have 12% of their genome occupied by intergenic non-coding regions such as control elements, promoters, and terminators. However, M, genitalium possess more operon systems, probably reducing the number of regulatory elements for the transcription of genes and further contributing to the reduction of its genome. This increases genes density with the average gene size for M. genitalium at 1,040 compared with 900 in H. influenzae. Describe the size and content of the genome. How many chromosomes? Circular or linear? Other interesting features? What is known about its sequence? Does it have any plasmids? Are they important to the organism's lifestyle?  Cell structure and metabolism Describe any interesting features and/or cell structures; how it gains energy; what important molecules it produces.  Ecology They are often found invading and adhering to the epithelial linings of the respiratory or urogenital tracts of animals. Describe any interactions with other organisms (included eukaryotes), contributions to the environment, effect on environment, etc.  Pathology They are often found invading and adhering to the epithelial linings of the respiratory or urogenital tracts of animals. They are associated with non-gonococcal urethritis found in men and possibly cervicitis in women. There is also evidence of upper genital tract infections in women. How does this organism cause disease? Human, animal, plant hosts? Virulence factors, as well as patient symptoms.  Application to Biotechnology Does this organism produce any useful compounds or enzymes? What are they and how are they used?  Current Research Enter summaries of the most recent research here--at least three required  References