CH391L/S13/CleanGenomes

Introduction

A clean or minimal genome refers to the minimum set of genes that an organism needs to survive and reproduce. This implies that there are genes that are “nonessential” to the organism’s survival and can be removed without destroying the cell or disrupting its growth cycle. Examples of nonessential DNA would include duplicate genes, transposable elements and catabolic pathways used for the intake and breakdown of certain complex biomolecules and would be removed from a minimal genome.[1][2]

A reduced genome is..............................

Advantages of a Minimal Genome

As the complexity of synthetic biology projects increases, so too will the problems it will run into due to the somewhat inherent randomness inside each cell. A minimal genome would give synthetic biologists a reliable and predictable “chassis” that could provide an ideal platform for perusing new research.

Current model organisms used in modern research contain deleterious DNA and gene products (like insertion sequences and unforeseen molecular interactions) which could disable or prohibit future endeavors by interacting unfavorably with whatever the “subject of interest” is.[1][3] Smaller amounts of extraneous gene products would simplify and reduce the cost of extraction and purification of cell parts, biomolecules and pharmaceuticals, many of which would be more expensive and time consuming to separate from a population of cells by conventional means.[4]

Gene stability in reduced genomes has been improved by removing transposable elements (TEs), error prone DNA polymerases and the enzymes responsible for the SOS response. Spontaneous, random genetic changes would be extremely harmful toward a minimal cell lacking a number of redundant systems. A stable genome would be a very desirable trait for research and experiment replication.[4][5]

Future advances in genome replication, unnatural amino acids, drug development, fuel production and biomaterial synthesis could be accelerated by simplifying and streamlining the genome used to code for a cell.[1][3]

Genome Reduction

One approach to creating more reliable, efficient host organisms for synthetic constructs is the reduction of the genome to eliminate extraneous genes, mutagenic mobile elements, and other unnecessary or destabilizing factors. This can be viewed as a form of reverse engineering of extant strains.

Systematic Genome Reduction

One natural approach to engineering strains with a reduced genome is to systematically identify and delete regions of the genome not necessary for host cell survival. Posfai et al. created the MDS strains (multiple deletion strains) by aligning the genomes of multiple genomes of E. coli, identifying regions which were absent in multiple strains, and deleting them via Lambda Red recombination. All IS elements were removed as well, lowering the mutation rate and increasing the stability of genetic constructs introduced into the cell. The strain had comparable growth rate compared to wild type.[4][6]

Ara et al. constructed a minimal version of the B. subtilis genome in 2007. [7] This strain had slightly decreased growth rate compared to wild-type, but displayed normal morphology and similar protein production capabilities.

Selection for Reduced Genome

Long-term evolution of strains under the correct conditions could select for a genome of minimal size. Such conditions may include growth in rich media lacking sugars to favor the loss of biosynthetic pathways or sugar metabolism operons, growth in structured environments which favor a smaller cell, or growth under other conditions which favor the loss of unnecessary genes.

Mycoplasma mycoides genome synthesis strategy

Minimal Genome Synthesis

Another approach is the synthesis of a minimal, designed genome from scratch using DNA synthesis technology and the transformation of this genome into cells to create a viable, novel, synthetic organism. This approach can be viewed as forward engineering of a novel organism, but would likely be informed by studies which determine the minimal set of genes necessary for a living organism.

Mycoplasma mycoides Synthesis

Gibson et al synthesized the first artificial cell by generating the Mycoplasma mycoides genome from digitized genome information and transforming it into Mycoplasma capricolum cells devoid of genomic information. These cells were capable of continuous self-replication and were identified by "watermarks" inserted in the genome. This technology could be utilized in the future to create cells with novel and useful properties from scratch.[8][9]

References

1. Forster AC and Church GM. Towards synthesis of a minimal cell. Mol Syst Biol. 2006;2:45. DOI:10.1038/msb4100090 |
2. Hutchison CA, Peterson SN, Gill SR, Cline RT, White O, Fraser CM, Smith HO, and Venter JC. Global transposon mutagenesis and a minimal Mycoplasma genome. Science. 1999 Dec 10;286(5447):2165-9. DOI:10.1126/science.286.5447.2165 |
3. Jewett MC and Forster AC. Update on designing and building minimal cells. Curr Opin Biotechnol. 2010 Oct;21(5):697-703. DOI:10.1016/j.copbio.2010.06.008 |
4. Kolisnychenko V, Plunkett G 3rd, Herring CD, Fehér T, Pósfai J, Blattner FR, and Pósfai G. Engineering a reduced Escherichia coli genome. Genome Res. 2002 Apr;12(4):640-7. DOI:10.1101/gr.217202 |
4. Kolisnychenko V, Plunkett G 3rd, Herring CD, Fehér T, Pósfai J, Blattner FR, and Pósfai G. Engineering a reduced Escherichia coli genome. Genome Res. 2002 Apr;12(4):640-7. DOI:10.1101/gr.217202 |
5. Iwadate Y, Honda H, Sato H, Hashimoto M, and Kato J. Oxidative stress sensitivity of engineered Escherichia coli cells with a reduced genome. FEMS Microbiol Lett. 2011 Sep;322(1):25-33. DOI:10.1111/j.1574-6968.2011.02331.x |
6. Pósfai G, Plunkett G 3rd, Fehér T, Frisch D, Keil GM, Umenhoffer K, Kolisnychenko V, Stahl B, Sharma SS, de Arruda M, Burland V, Harcum SW, and Blattner FR. Emergent properties of reduced-genome Escherichia coli. Science. 2006 May 19;312(5776):1044-6. DOI:10.1126/science.1126439 |
7. Ara K, Ozaki K, Nakamura K, Yamane K, Sekiguchi J, and Ogasawara N. Bacillus minimum genome factory: effective utilization of microbial genome information. Biotechnol Appl Biochem. 2007 Mar;46(Pt 3):169-78. DOI:10.1042/BA20060111 |
8. Gibson DG, Benders GA, Andrews-Pfannkoch C, Denisova EA, Baden-Tillson H, Zaveri J, Stockwell TB, Brownley A, Thomas DW, Algire MA, Merryman C, Young L, Noskov VN, Glass JI, Venter JC, Hutchison CA 3rd, and Smith HO. Complete chemical synthesis, assembly, and cloning of a Mycoplasma genitalium genome. Science. 2008 Feb 29;319(5867):1215-20. DOI:10.1126/science.1151721 |
9. Gibson DG, Glass JI, Lartigue C, Noskov VN, Chuang RY, Algire MA, Benders GA, Montague MG, Ma L, Moodie MM, Merryman C, Vashee S, Krishnakumar R, Assad-Garcia N, Andrews-Pfannkoch C, Denisova EA, Young L, Qi ZQ, Segall-Shapiro TH, Calvey CH, Parmar PP, Hutchison CA 3rd, Smith HO, and Venter JC. Creation of a bacterial cell controlled by a chemically synthesized genome. Science. 2010 Jul 2;329(5987):52-6. DOI:10.1126/science.1190719 |