ScienceWeek

MICROBIOLOGY: ON MOLLICUTES

The following points are made by Shlomo Trachtenberg (Current Biology 2005 15:R483):

1) Mollicutes -- Spiroplasma, Mycoplasma and Acholeplasma -- are the smallest and simplest known free-living and self-replicating forms of life. They are bacteria of Gram-positive origin, as indicated by their 16S rRNA. But rather than being primitive, they diverged about 65 million years ago, by regressive evolution and genome reduction, from more complex ancestors on the branch of the bacterial phylogenetic tree containing the lactobacilli, bacilli, clostridia and streptococci. The mollicutes consist of about 200 systematically well-defined species.

2) The mollicutes depend completely on their hosts for nutrient supplies. Sixteen species of mollicutes have been reported to live in humans. Most are extracellular and colonize mucosal surfaces of respiratory and urogenital tracts. 35 species have been found in other animals, including cows, sheep, goats, poultry, dogs, cats and fish. Spiroplasmas are found mostly in plants and their insect vectors.

3) Research on these reduced organisms is leading to a clearer picture of the minimal set of genes that is sufficient, or required, to support independent life. Eight mycoplasma genomes have been fully sequenced, ranging in size from the ~580 kilobase genome of Mycoplasma genitalium to the ~1.4 megabase genome of M. penetrans. M. genitalium has the smallest known genome of any cellular (non-virus) life form; its entire genome is similar to the indispensable part of the 562-kilobase B. subtilis genome.

4) Whole genome comparisons suggests that the severe genome reduction in the mollicutes probably reflects their parasitic lifestyle. In phylogenetic analyses, they cluster with the parasitic eubacteria rather than with their ancestral Gram-positive bacteria. On a phylogenetic scale, the spiroplasmas are the closest to the mollicute's Gram-positive ancestors. Compared to the rest of the mollicutes, they have the least reduced genomes -- ranging from just below a megabase to more than two megabases -- which is no doubt related to their relative large size, spatial organization and structural complexity.

5) Mollicutes lack external appendages such as the flagella and pili responsible for motility of cell-walled bacteria. Spiroplasmas are active swimmers and respond chemotactically. They swim by propagating a slight deformation or segment with switched handedness along the helical cell body. Swimming direction is changed by flexing. The cell movements are driven by the cytoskeleton acting as a linear motor. Motile mycoplasmas glide on solid or semi-solid surfaces. In M. mobile, a fast glider, unique proteins located in the area between the cell's neck and body are thought to attach the cell to the surface and facilitate movement. The process is energized very likely by ATP hydrolysis.[1-5]

References (abridged):

1. Binnewies, T.T., Hallin, P.F., Staerfeldt, H.-H., and Ussey, D.W. (2005). Genome update: Proteome comparisons. Microbiol. 151, 1-3

2. Blanchard, A. and Brown, G. (2005). In: Mycoplasmas: Molecular Biology, Pathogenicity and Strategies for Control.. (2005). Horizon biosciences, Norfolk

3. http://www.ba.ars.usda.gov/mppl/research/kunkelii.html

4. Mayer, F. (2003). Cytoskeletons in prokaryotes. Cell Biol. Internat. 27, 429-438

5. Razin, S. and Herrmann, R. (2002). In: Molecular Biology and Pathogenicity of Mycoplasmas.. (2002). Kluwer Academic/Plenum Publishers, New York

Current Biology http://www.current-biology.com

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Related Material:

MOLECULAR BIOLOGY: A MINIMAL GENOME

Notes by ScienceWeek:

The field of molecular biology is currently engaged in the most extensive project of reverse engineering ever conceived: the elucidation of the molecular components and events that constitute living systems. One central question is simply stated: What is the minimal number of genes necessary to maintain the viability of a living system?

Apart from viruses, the smallest replicating biological systems known are the mycoplasmas (mollicutes). There are over 150 species of this class of cell-wall-free bacteria, some of which are human pathogens. Mycoplasmas apparently evolved from other bacteria by reduction of genome size: the smallest genome of the mycoplasmas is little more than twice the genome size of certain large viruses. Mycoplasmas are the smallest organisms that can be free-living in nature and self-replicating on laboratory media (viruses replicate only in bacteria and other cells). Mycoplasmas range from 125 to 250 nanometers in size. They change shape readily (pleomorphism) because they lack a cell wall, being bounded by a triple-layered lipoprotein membrane that contains a sterol.

The genome of Mycoplasma genitalium (an organism that causes one form of the urinary tract infection urethritis), which has been completely sequenced, consists of 580 kilobases comprising 517 genes (480 protein-coding genes; 37 genes for RNAs), and this is the smallest gene complement for any independently replicating cell so far identified.

The following points are made by C.A. Hutchinson III et al (Science 1999 286:2165):

1) The authors report the use of molecular genetic methods (global transposon mutagenesis) to identify nonessential genes in M. genitalium under laboratory growth conditions. The authors report their analysis suggests that 265 to 350 of the 480 protein-coding genes of M. genitalium are essential under laboratory growth conditions, including approximately 100 genes of unknown function.

2) The authors conclude: "The presence of so many genes of unknown function among the essential genes of the simplest known cell suggests that all the basic molecular mechanisms underlying cellular life may not yet have been described. The essential gene set is not the same as the minimal genome. It is clear that genes that are individually dispensable may not be simultaneously dispensable. The data presented here suggest some specific experiments that could be carried out as a first step in the engineering of a cell with a minimal genome in the laboratory environment."

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