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e.g. the systematic inactivation of each individual gene present in a genome. Still, all these methods have
limitations because they uncover several possibilities of misclassification of genes (Moya et al. 2009).
The simplification of modern cells has also been approached in a third way, by searching for the
biochemical
description of well-defined pathways that are needed to perform essential functions. Forster and Church
(2006) described a minimal genome that contains just genes needed for informational processes but does not
include genes for intermediate metabolism (e.g. lipid metabolism and glycolysis), representing the main
component needed to synthesise a minimal self-replicative system. Self-reproductive vesicles that are defined
in the minimal genome allow transmembranous small molecule transport. Such a system is entirely composed
of genes with
well-defined functions, which are designed by modules and allow a system-by-system debugging
to attain self-replication. Moya et al. (2009) propose that this approach could be a good start to synthesise a
useful,
near-minimal, self-replicating system dependent on added energised substrates.
Bacteria with small genome sizes (Moya et al. 2009):
-
Mycoplasma genitalium
: the 580kb genome is the smallest among organisms that can be grown as pure
cultures, nevertheless it carries all essential genes for informational processes (replication, transcription,
translation, and protein folding and processing) plus a severely limited metabolism. It still represents the
smallest known genome of a bacterium that can be grown in the laboratory and therefore must be close to a
minimal autonomous genome.
-
Nanoarchaeum equitans
is the only known archaeon exhibiting a symbiotic lifestyle. Its genome size is of 490
kb presents a complete information-processing system and a highly simplified metabolic apparatus.
-
Gammaproteobacteria, Buchnera aphidicola
BCc and Candidatus
Carsonella ruddii are three bacterial
endosymbionts of insects with a very small microbial genome. Their role in their respective symbiosis is to
supply the nutrients that lack in the insect host diet, mainly essential amino acids and vitamins.
-
The smallest genome with 1.308 Mb of any cell known to replicate independently in nature corresponds to
the photoheterotrophic marine bacterium Pelagibacter ubique
.
3.2.2
De novo construction of a cell – bottom-up approach
Synthetic cells
Contrary to top-down applications, the bottom-up approach starts from scratch: an entity is built by self-
assembling of molecular components (Sole et al. 2007). These can be of biological nature or instead
completely ad hoc chemical components. Another approach is the synthesis of a cell from its basic elements.
Within this approach the comprehensive understanding of three essential components of cellular life – the
DNA
information, the compartment and the metabolism – is a prerequisite.
The synthetic cell, built from scratch, is a unique compartment with a structure and an organisation similar to
a bacterium. ATP and GTP are used as energy sources in the first stages of the development. For the
information part, the synthetic DNA programmes would be expressed with the transcription and translation
machineries extracted from an organism. The physical boundary of the artificial cell would be a phospholipid
bilayer. Lipid bilayers are also the natural template for membrane proteins.
The cell wall, anchored to the lipid membrane, provides the structural strength to the bacterium. The
fabrication of a stable compartment with an active interface is one of the most challenging steps in the
synthesis of a DNA-programmed artificial cell. The efficient insertion of membrane proteins into the bilayer is
the real current limitation to the development of an active interface. Membrane proteins can be also
expressed and integrated into phospholipid bilayers (Noireaux et al. 2011).
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A living organism is an open system made of hundreds of chemical reactions whose properties go beyond the
DNA programme. A continuous uptake of energy and a continuous elimination of reaction by-products are
critical for any living systems as well as for a synthetic cell. The construction of an artificial cell requires the
development of an artificial environment. The external medium has to be an isotonic non-dissipative feeding
solution that maintains physiological conditions by exchanges of low molecular mass nutrients, nucleotides
and amino acids through the phospholipid membrane (Noireaux et al. 2011). Selective exchanges, osmotic
pressure problems, and efficient transcription/translation are among the issues that must be solved to obtain
an initial workable microscopic vesicle.
Chassis
By far the most common chassis in use is
Escherichia coli. However, also other hosts as listed below are used.
-
Escherichia coli:
1.
Is a prominent example whose genome has been reduced up to 15% in various projects without any
noticeable effect on the investigated physiological properties (Heinemann and Panke 2006).
2.
Has the advantage of the availability of numerous vectors (plasmids, cosmids, BACs) and promoters for
heterologous expression, and can express genes with G + C codon usage as high as 73%, but does not
recognise promoters from
Streptomyces (Zotchev et al. 2012).
3.
Escherichia coli is one of the most common chassis used in synthetic biology because it can be grown easily
and has relatively simple genetics which can be easily manipulated. Furthermore non-infective lab-strains
can be constructed.
4.
To date
Escherichia coli is the best understood cellular life form; its genome has
over four million base pairs
of DNA and encodes about 5.000 genes (Glass 2012).
Mycoplasma species:
Mycoplasma genitalium has only 525 genes, about the tenth the number of genes encoded by
Escherichia coli
(Glass 2012). It has a very slow growth rate compared to e.g.
Mycoplasma capricolum.
Mycoplasma capricolum
was used as a chassis by Gibson et al. (2010) for the transplantation of a digitised genome sequence of
Mycoplasma mycoides.
-
Bacillus subtilis:
Bacillus subtilis can easily be manipulated in relation to genetic changes. It is sometimes used in the place of
Escherichia coli because it has certain properties that are more amenable to some specific forms of genetic
manipulation related to synthetic biology, as for example DNA circuits can be easily integrated into the
Bacillus subtilis genome (Zotchev et al. 2012).
Minimum genome engineering substantially facilitated the optimisation of
Bacillus subtilis as a production
host. Bacillus species are mainly used to produce important enzymes (e.g. alpha-amylase) and biochemicals
(e.g. biosurfactants) used in industry (Liu et al. 2013).
Several Bacillus species have evolved as an important platform for the production of various enzymes and
chemicals. A vast range of cellular phenotypes can be obtained in Bacillus species by regulation and
modification of the corresponding metabolic pathway at the global and gene-specific level. The genome
sequence data of Bacillus species is available in specified databases (Liu et al. 2013).
“Bacteria have the functional equivalent of an immune system that evolved to protect them from
bacteriophages, the viruses that infect bacteria. The bacterial immune system is a complex of enzymes called
restriction-modification systems that enable bacteria to recognise and destroy foreign DNA, which would
usually come from an infecting bacteriophage. One of the enzymes is a methylase that chemically modifies