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Designing and engineering evolutionary robust genetic circuits.

Sleight SC, Bartley BA, Lieviant JA, Sauro HM - J Biol Eng (2010)

Bottom Line: When there is no homology between terminators, the evolutionary half-life of this circuit is significantly improved over 2-fold and is independent of the expression level.We also found that on average, evolutionary half-life exponentially decreases with increasing expression levels.Inclusion of an antibiotic resistance gene within the circuit does not ensure evolutionary stability.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Bioengineering, University of Washington, Seattle, WA 98195, USA. sleight@u.washington.edu.

ABSTRACT

Background: One problem with engineered genetic circuits in synthetic microbes is their stability over evolutionary time in the absence of selective pressure. Since design of a selective environment for maintaining function of a circuit will be unique to every circuit, general design principles are needed for engineering evolutionary robust circuits that permit the long-term study or applied use of synthetic circuits.

Results: We first measured the stability of two BioBrick-assembled genetic circuits propagated in Escherichia coli over multiple generations and the mutations that caused their loss-of-function. The first circuit, T9002, loses function in less than 20 generations and the mutation that repeatedly causes its loss-of-function is a deletion between two homologous transcriptional terminators. To measure the effect between transcriptional terminator homology levels and evolutionary stability, we re-engineered six versions of T9002 with a different transcriptional terminator at the end of the circuit. When there is no homology between terminators, the evolutionary half-life of this circuit is significantly improved over 2-fold and is independent of the expression level. Removing homology between terminators and decreasing expression level 4-fold increases the evolutionary half-life over 17-fold. The second circuit, I7101, loses function in less than 50 generations due to a deletion between repeated operator sequences in the promoter. This circuit was re-engineered with different promoters from a promoter library and using a kanamycin resistance gene (kanR) within the circuit to put a selective pressure on the promoter. The evolutionary stability dynamics and loss-of-function mutations in all these circuits are described. We also found that on average, evolutionary half-life exponentially decreases with increasing expression levels.

Conclusions: A wide variety of loss-of-function mutations are observed in BioBrick-assembled genetic circuits including point mutations, small insertions and deletions, large deletions, and insertion sequence (IS) element insertions that often occur in the scar sequence between parts. Promoter mutations are selected for more than any other biological part. Genetic circuits can be re-engineered to be more evolutionary robust with a few simple design principles: high expression of genetic circuits comes with the cost of low evolutionary stability, avoid repeated sequences, and the use of inducible promoters increases stability. Inclusion of an antibiotic resistance gene within the circuit does not ensure evolutionary stability.

No MeSH data available.


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Loss-of-function mutations and evolutionary stability dynamics in two lacI-regulated circuits under constitutive vs. inducible expression. (A) Regulation of inducible R0011 + E0240 and R0010 + E0240 circuits. LacI represses transcription of GFP and IPTG de-represses the circuit to allow for GFP expression. (B) Top panel shows the evolutionary stability dynamics of constitutive R0011 + E0240 (open black circles) and inducible R0011 + E0240 (solid black circles). Bottom panel shows the evolutionary stability dynamics of constitutive R0010 + E0240 (open red circles) and inducible R0010 + E0240 (solid red circles). Error bars represent one standard deviation from the mean of nine independently evolved populations. (C) Types of mutations in nine independently evolved populations. For nine independently evolved populations, colored boxes correspond to the mutation legend below the table. The most common mutation for a particular type of mutation is labeled with "1" in the boxes above and less common mutations are labeled with increasing numbers. (D) Most common loss-of-function mutations that inactivated the R0011 + E0240 and R0010 + E0240 inducible and constitutive circuits. See Additional File 1, Supplementary Table S1 for mutation details.
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Figure 6: Loss-of-function mutations and evolutionary stability dynamics in two lacI-regulated circuits under constitutive vs. inducible expression. (A) Regulation of inducible R0011 + E0240 and R0010 + E0240 circuits. LacI represses transcription of GFP and IPTG de-represses the circuit to allow for GFP expression. (B) Top panel shows the evolutionary stability dynamics of constitutive R0011 + E0240 (open black circles) and inducible R0011 + E0240 (solid black circles). Bottom panel shows the evolutionary stability dynamics of constitutive R0010 + E0240 (open red circles) and inducible R0010 + E0240 (solid red circles). Error bars represent one standard deviation from the mean of nine independently evolved populations. (C) Types of mutations in nine independently evolved populations. For nine independently evolved populations, colored boxes correspond to the mutation legend below the table. The most common mutation for a particular type of mutation is labeled with "1" in the boxes above and less common mutations are labeled with increasing numbers. (D) Most common loss-of-function mutations that inactivated the R0011 + E0240 and R0010 + E0240 inducible and constitutive circuits. See Additional File 1, Supplementary Table S1 for mutation details.

Mentions: Overall, the use of a kanamycin resistance gene within the circuit does not significantly increase evolutionary stability. Although propagation with kanamycin in the media may put a selective pressure on the promoter, other mutational targets evidently can decrease expression of the circuit over time (Additional File 1, Supplementary Material). IS mutations are responsible for loss-of-function in the KanR circuits propagated without kan. Since there are relatively few IS mutations in circuits without kanR (Figures 3c, 4c, 5c, and 6c), this circuit may induce IS transposition bursts.


Designing and engineering evolutionary robust genetic circuits.

Sleight SC, Bartley BA, Lieviant JA, Sauro HM - J Biol Eng (2010)

Loss-of-function mutations and evolutionary stability dynamics in two lacI-regulated circuits under constitutive vs. inducible expression. (A) Regulation of inducible R0011 + E0240 and R0010 + E0240 circuits. LacI represses transcription of GFP and IPTG de-represses the circuit to allow for GFP expression. (B) Top panel shows the evolutionary stability dynamics of constitutive R0011 + E0240 (open black circles) and inducible R0011 + E0240 (solid black circles). Bottom panel shows the evolutionary stability dynamics of constitutive R0010 + E0240 (open red circles) and inducible R0010 + E0240 (solid red circles). Error bars represent one standard deviation from the mean of nine independently evolved populations. (C) Types of mutations in nine independently evolved populations. For nine independently evolved populations, colored boxes correspond to the mutation legend below the table. The most common mutation for a particular type of mutation is labeled with "1" in the boxes above and less common mutations are labeled with increasing numbers. (D) Most common loss-of-function mutations that inactivated the R0011 + E0240 and R0010 + E0240 inducible and constitutive circuits. See Additional File 1, Supplementary Table S1 for mutation details.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2991278&req=5

Figure 6: Loss-of-function mutations and evolutionary stability dynamics in two lacI-regulated circuits under constitutive vs. inducible expression. (A) Regulation of inducible R0011 + E0240 and R0010 + E0240 circuits. LacI represses transcription of GFP and IPTG de-represses the circuit to allow for GFP expression. (B) Top panel shows the evolutionary stability dynamics of constitutive R0011 + E0240 (open black circles) and inducible R0011 + E0240 (solid black circles). Bottom panel shows the evolutionary stability dynamics of constitutive R0010 + E0240 (open red circles) and inducible R0010 + E0240 (solid red circles). Error bars represent one standard deviation from the mean of nine independently evolved populations. (C) Types of mutations in nine independently evolved populations. For nine independently evolved populations, colored boxes correspond to the mutation legend below the table. The most common mutation for a particular type of mutation is labeled with "1" in the boxes above and less common mutations are labeled with increasing numbers. (D) Most common loss-of-function mutations that inactivated the R0011 + E0240 and R0010 + E0240 inducible and constitutive circuits. See Additional File 1, Supplementary Table S1 for mutation details.
Mentions: Overall, the use of a kanamycin resistance gene within the circuit does not significantly increase evolutionary stability. Although propagation with kanamycin in the media may put a selective pressure on the promoter, other mutational targets evidently can decrease expression of the circuit over time (Additional File 1, Supplementary Material). IS mutations are responsible for loss-of-function in the KanR circuits propagated without kan. Since there are relatively few IS mutations in circuits without kanR (Figures 3c, 4c, 5c, and 6c), this circuit may induce IS transposition bursts.

Bottom Line: When there is no homology between terminators, the evolutionary half-life of this circuit is significantly improved over 2-fold and is independent of the expression level.We also found that on average, evolutionary half-life exponentially decreases with increasing expression levels.Inclusion of an antibiotic resistance gene within the circuit does not ensure evolutionary stability.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Bioengineering, University of Washington, Seattle, WA 98195, USA. sleight@u.washington.edu.

ABSTRACT

Background: One problem with engineered genetic circuits in synthetic microbes is their stability over evolutionary time in the absence of selective pressure. Since design of a selective environment for maintaining function of a circuit will be unique to every circuit, general design principles are needed for engineering evolutionary robust circuits that permit the long-term study or applied use of synthetic circuits.

Results: We first measured the stability of two BioBrick-assembled genetic circuits propagated in Escherichia coli over multiple generations and the mutations that caused their loss-of-function. The first circuit, T9002, loses function in less than 20 generations and the mutation that repeatedly causes its loss-of-function is a deletion between two homologous transcriptional terminators. To measure the effect between transcriptional terminator homology levels and evolutionary stability, we re-engineered six versions of T9002 with a different transcriptional terminator at the end of the circuit. When there is no homology between terminators, the evolutionary half-life of this circuit is significantly improved over 2-fold and is independent of the expression level. Removing homology between terminators and decreasing expression level 4-fold increases the evolutionary half-life over 17-fold. The second circuit, I7101, loses function in less than 50 generations due to a deletion between repeated operator sequences in the promoter. This circuit was re-engineered with different promoters from a promoter library and using a kanamycin resistance gene (kanR) within the circuit to put a selective pressure on the promoter. The evolutionary stability dynamics and loss-of-function mutations in all these circuits are described. We also found that on average, evolutionary half-life exponentially decreases with increasing expression levels.

Conclusions: A wide variety of loss-of-function mutations are observed in BioBrick-assembled genetic circuits including point mutations, small insertions and deletions, large deletions, and insertion sequence (IS) element insertions that often occur in the scar sequence between parts. Promoter mutations are selected for more than any other biological part. Genetic circuits can be re-engineered to be more evolutionary robust with a few simple design principles: high expression of genetic circuits comes with the cost of low evolutionary stability, avoid repeated sequences, and the use of inducible promoters increases stability. Inclusion of an antibiotic resistance gene within the circuit does not ensure evolutionary stability.

No MeSH data available.


Related in: MedlinePlus