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Mycoplasmas are members
of the class Mollicutes and comprise a large group of bacteria
which lack a cell wall, have small genomes, and a characteristically
low G+ C content. These diverse organisms are parasites for a
wide range of hosts including humans, animals, insects, plants,
and cells grown in tissue culture. Aside from their role as pathogens,
mycoplasmas are of interest because they are believed to represent
a minimal life form, having yielded to selective pressure to reduce
genome size. The species with the smallest genome size in this
class is Mycoplasma genitalium (580 kb), which was originally
isolated from urethral specimens of patients with non-gonoccocal
urethritis and has since been shown to exist in parasitic association
with ciliated epithelial cells of primate genital and respiratory
tracts .
Analysis of rRNAs and tRNAs from mycoplasmas suggests that this class of organisms arose through loss of genetic material from low G+C gram-positive ancestors of the Lactobacillus-Clostridium branch of the phylogenetic tree. Molecular characterization of the M. genitalium genome is hindered by the difficulty in applying classical genetics to the study of this and other mycoplasmas, and the lack of available auxotrophic mutants due to the requirement of this organism for complex media for growth in culture. This situation is further complicated by the fact that the UGA codon in several mycoplasmas species encodes tryptophan instead of a translational stop. This adaptation leads to premature termination of M. genitalium proteins when expressed in Escherichia coli or other hosts . An initial survey of the M. genitalium genome with random sequencing was performed by Peterson et al. in 1993. This study revealed that M. genitalium contains a limited number of genes involved in metabolic processes and suggested that the total number of genes encoded by the genome of this organism is on the order of 400. As a next step towards sequencing the chromosome of M. genitalium, Lucier et al. constructed a set of ordered cosmids which contain the entire genome of this organism. The M. genitalium genome is represented by 74 EcoRI fragments and a total genome size of 578 kb. Using a new approach for genome analysis based on whole chromosome shotgun sequencing and assembly, we completed the sequencing and assembly of the Haemophilus influenzae Rd genome and now report the complete genome sequence of M. genitalium. This organism represents an important system for determination of the minimal number of genes required for host-independent existence. |
