The minimal cellular genome of mycoplasma.

ABSTRACT

The mycoplasmas are the smallest and simplest self-replicating organisms, being built of a plasma membrane, ribosomes, and a circular double-stranded DNA molecule-the typical prokaryotic genome. The idea of using mycoplasmas as models for defining in molecular terms the entire machinery of a living cell was raised by Morowitz in 1984. The goal has been to prove the dogma of the completeness of molecular biology, that is, that the logic of life is finite, relatively simple and subject to full exploration. The recent complete sequencing of the genome of the human pathogen Mycoplasma genitalium brings us much closer to achieving this goal. The M. genitalium genome is only 580 kb long and contains only 470 predicted coding sequences(genes), as compared with 1727 in Haemophilus influenzae and about 4000 in E. coli. Thus, M. genitalium is apparently the simplest organism capable of independent life with a minimal set of genes. The drastic economization in genetic information must be associated with the parasitic mode of life of the mycoplasmas. Mycoplasmas evolved by reductive evolution from Gram-positive bacteria with low G + C genomes. During evolution the mycoplasmas have lost the cell wall and many biosynthetic systems involved in synthesis of macromolecule building blocks provided by their host. Thus, the M. genitalium genome carries only one gene involved in amino acid biosynthesis, and very few genes for vitamin and nucleic acid precursors; the lack of genes involved in fatty acid biosynthesis, leads to dependence on exogenous fatty acids, enabling the introduction of controlled variations in membrane acyl chains and the use of mycoplasmas as models in studying membrane fluidity. Moreover, the dependence of mycoplasmas on exogenous cholesterol for growth was exploited to show the role of cholesterol as a buffer of membrane fluidity. The mycoplasma genome carries the minimal set of energy metabolism genes, being content with a restricted supply of ATP needed for their parasitic mode of life. Being limited by a single permeability barrier enabled the saving of a considerable number of transport system genes. Nevertheless, these minimal organisms were shown to carry all the essential genes needed for DNA replication, transcription and translation, but even here gene saving is expressed in a minimal number of rRNA and tRNA genes. A genomic price had been paid to maintain parasitism, so that a significant number of mycoplasmal genes is devoted to adhesins, attachment organelles and variable membrane surface antigens directed towards evasion of the host immune system.