Size Limits of Very Small Microorganisms Proceedings of a Workshop (1999) / Chapter Skim
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de Duve, opened the workshop by commenting on some theoretical calculations of the lower limits of cell size based on the assumptions and calculations shown in Box 1 (see pp.8-9~. The representative results (Tables 1 and 2)
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Dr. Moore's presentation emphasized that, as cell volume decreases, the fraction of volume occupied by the genome increases greatly, and eventually becomes a major determinant of minimal cell size.
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· The minimal viable cell diameter is expected to lie in the range of 250 to 300 nm. · The number of ribosomes required for adequate genome expression is a significant constraint on minimal cell size.
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4) is daunting, but can be thought of as built up from Figure 1, with the key intermediates serving as starting materials for dedicated routes to the amino acids of the proteins, the constituents of polysaccharides, nucleic acids and lipids, and to cofactors.
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WaN. Life of normal sized bacteria in dilute solutions requires a wall to protect cells from lysis by osmotic pressure differences.
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2 SIZE LIMITS OF VERY SMALL MICROORGANISMS Formate~ 0xalate= ~/~ in Formate~ 0xalate= co2 H+ H2 :_ ,, 2 H+ in out Fumarate 2 ~H+ Succinate hV`, cyt-c ~ 4 H+ ~,~ in (reaction center~ (bc~ complex)
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1995 by Oxford University Press. cling mechanism to avoid vulnerability during cell division (20 genesis.
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, which is used by many bacteria with access to carbohydrates, also contribute ATP by cytoplasmic reactions ("substrate level phosphorylation"~; when growth depends solely on the latter reactions, the ATPase energizes the membrane rather than vice versa. Catabolism without respiration can involve massive wasting of organic metabolites as fermentation products.
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The essential set of genes for autonomous growth of modern bacteria in a minimal medium with a single organic carbon source or CO2 is, from the above crude estimates, 400 or 500, and, as mentioned, fewer in an enriched medium.
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are unlikely to exist on Earth today. In addition, the limitations on genome size that accompany small cell size guarantee that cells of that size will have to depend on larger, more complex organisms for supplies of most of the small organic molecules required for their growth.
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, the protein mass in the cell will have just doubled by the time cell division begins. The number of copies of each of the enzymes involved in intermediary metabolism is similarly regulated to balance supply and demand.
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T4 DNA becomes metabolically active only when injected into bacterial cells where the total DNA concentration is much lower. I take it as axiomatic that DNA packed as densely as it is in bacteriophages will always be inert metabolically.
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A genome that size is big enough to encode enough enzymes to explain the intermediary metabolism of these organisms, and intracellular DNA concentration is certainly not a problem. The free-living nanobacteria that Kajander and his colleagues have isolated from mammalian cell cultures are harder to understand at this point (Kajander et al., 1997~.
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1995. The minimal gene complement of Mycoplasma genitalium.
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(>150 min doubling time) DNA 3% 4% RNA 35 14 Protein 60 80 Cell size Larger Smaller
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Thus, ribosomes are a prominent feature determining minimum cell size in any life system using ribosomes as an obligatory element. However, we can imagine circumstances not requiring as many ribosomes as E
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Proteins Although some proteins of E cold are the ribosomal proteins, most are soluble enzymes.
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Primitive cells with slow growth characteristics could have had far fewer ribosomal RNA molecules and could have used many fewer individual proteins to build primitive ribosomes. Because with slow growth conditions protein (the sum of enzymes and ribosomal protein)
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Neidhardt, R Curtiss III, E.C.C.
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The energy required to bend a flat fluid membrane into the shape of a cell is comparatively small, such that closed spherical shapes are energetically favored for radii greater than about 20 nm, depending upon composition. Further, the membrane of a small cell could withstand the osmotic pressures typical of many bacteria without the aid of a cell wall.
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We demonstrate that: · In simple models of fluid membranes, the bending energy of a spherical shell is independent of its radius, so that it takes the same amount of energy to bend a flat membrane into a small spherical shell as
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In order to code sufficient genetic information in a linear sequence, small cells would need very flexible molecules with perhaps half the mass per unit length of DNA, a requirement that is consistent with the idea that RNA or some other single-stranded molecule is the evolutionary precursor of DNA as the genetic template.
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Membrane Rupture . 29 The fluid membrane not only resists bending, but also resists in-plane stretching.
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surface stress n can result from the osmotic pressure difference P between the cell's interior and its external environment. For a spherical shell of radius R
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It is more likely that small cells would use RNA or another flexible molecule to carry genetic information, consistent with the idea that RNA predated DNA in evolution. Many biopolymers display a persistence length that varies as the square of the mass per unit length along the polymer, a scaling behavior consistent with the theoretical expectation that the persistence length varies as the fourth power of the radius for uniform cylindrical rods (Doi and Edwards, 1986; Landau and Lifshitz, 1986~.
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Intraspecific recombination is more common than horizontal exchange, allows for the removal of deleterious mutations, and helps maintenance of species identity. Recombination enables organisms to maintain maximum genome sizes that are larger than those capable without gene exchange (escape of Muller's ratchet)
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From page 33... ...
, horizontal transfer of DNA can result in the exchange of phenotypic capabilities among organisms, as all genes required for a particular function may be mobilized between organisms. Although this process allows for rapid adaptation of organisms to changing environments, horizontal transfer does not intrinsically limit genome size, nor does it enable substantially smaller genome sizes to be achieved (however, see selfish operons below)
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Below, I will develop a context-independent model describing how rapid gene transfer predicts the maintenance of very small genome sizes when cells are constrained to small sizes. Model of Minimal Genome Size Minimum Genome Composition and the Cellular Environment What is represented by the smallest collection of 256 essential genes described by Mushegian and Koonin (1)
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For gene exchange to affect this minimal number of genes, whose number and nature depend entirely on the properties of the minimal cell, we must speculate how constraints on cell size permitfewer than this minimal number of genes to be present in a cell at any one time. To do this, we must model the assembly of the cellular consortium, and devise a mechanism whereby constraint on cell size permits cells to replicate with fewer than the minimal number of genes required to form the cellular consortium.
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and D Gene 4 Synthesizes compound D; replicates when provided with compounds A, B and C Compound A Required for gene replication; synthesized from precursors by Gene 1 Compound B Required for gene replication; synthesized from precursors by Gene 2 Compound C Required for gene replication; synthesized from precursors by Gene 3 Compound D Required for gene replication; synthesized from precursors by Gene 4 Micelle Enclosed environment maintaining compounds A, B
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Perspective on the Minimum Genome Size The meta-cell model is a stable means of propagating replicons using high rates of gene transfer among micelles that cannot support the entirety of the cellular consortium. In this case, the rate of
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From page 38... ...
, the average genome size may be less than one gene. One may consider the meta-cell to be a single-celled organism whose genome is distributed through a network of nacelles.
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