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Revision as of 17:02, 29 July 2008
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Contents |
Manipulating RNA attenuator sequences to generate genetic logic circuits in E. coli...
Terminator/Attenuator anti-sense Logic (TAasL)
The integrated circuit was a major milestone of modern technology that facilitated the entrance of society into the information age. Electrical circuits that once occupied an entire room are now able to be placed on a single micro-chip the size of a postage stamp. The implementation of Boolean logic (AND, OR, NOT, etc.) using integrated electronic circuits forms the basis for modern day computers.
When compared to electronics, similar levels of integration have yet to be realized in biological systems. While electronic devices are inherently connectible due to their common currency (flow of electrons), biological circuits lack similar connectivity. This poses a significant challenge to the development of truly programmable biosystems. The Logi-coli[i] project hopes to overcome this challenge by constructing connectible, extensible biological circuits based on antisense RNA regulation of transcriptional attenuation.
Transcriptional attenuation is a common biological mechanism for controlling transcriptional activity. As RNA strands are produced by RNA polymerase transcription of a DNA template, they often form secondary “stem-loop” structures due to base-pair complementarity of inverted repeat sequences. Inverted repeat sequences are short sequences in the transcript that are repeated at a later position in reverse order. These repeat sequences cause the RNA strand to fold back on itself due to base-pairing between complementary A-U and C-G nucleotide pairs in the different repeat regions. A terminator is formed when such a simple stem-loop is followed by a “UUUUUUU” sequence that causes the severing of the nascent RNA strand and the dislocation of the RNA polymerase from the DNA template strand. An attenuator is a terminator structure found at the start of a transcript and has the ability to prematurely truncate the transcript.
We hypothesize that transcriptional attenuation can be modulated by the action of an anti-sense RNA strand disrupting the normal base-pairing in the attenuator stem-loop structure. As a result of this disruption, the RNA polymerase can continue transcription past the attenuator sequence. Using software called RNAstructure4.5, our team hopes to be able to rationally design functional attenuator and corresponding input anti-sense RNA sequences. In this way, we hope to provide a means for connecting series of circuits by using the output RNA strand of one circuit to regulate the transcription of a second circuit. We hope to combine simple attenuators into more complex structures that act as biological equivalents to electronic Boolean logic gates (AND, OR, XOR, etc.)
In regards to the final output, a coding region of LacZ or GFP will be expressed to provide evidence of the functionality of the logic circuits. We hope to connect these basic logic gates into even more complex circuits such as half-adders or full-adders. This important step in the design of complex logic in biological circuits has eluded researchers for a number of years, but we believe our approach to have great potential for success.
Project Details
Terminators
A classic terminator found at the 3' end of an RNA transcript consists of a "stem" structure of length ~10 to 30 nucleotides followed by a "loop" of 4-6 nucleotides, then several complementary nucleotides forming the corresponding 3' portion of the stem. Finally a weakly-hybridizing sequence of about 9 nucleotides (mainly U's or A's) forms the portion that permits the nascent transcript to dissociate from the template DNA strand. The termination efficiency (TE) of this structure is determined by many complex factors, including its thermodynamic stability. For this structure (see right) this free energy is calculated by RNAStructure as -28.5 Kcal/mole.
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Results
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