Difference between revisions of "Part:BBa K819005"

Revision as of 08:16, 25 September 2012

Constitutive LuxBrick Generator

a fusion protein consisting of E.coli endogenous SOS system repressor LexA and fungus N.crass photosensor protein VVD with LexA carrying mutation at position 40-42 and VVD carrying mutation N56K, C71V and M135I. This part serves as an ultra sensitive photoreceptor(which can sense light as weak as moonlight) and will induce a light-dependent repression of genes with their promoter containing mutated 408 form of SOS box.

Luminesensor is designed by following the general principle of optogenetics fusion protein design--attaching a physiologically functional domain to a photoreceptor domain.

FIG.1 The general design of optogenetic fusion protein.

In order to circumvent the potential problems of current optogenetic approaches, we concluded that our Luminesensor should be highly sensitive, should incorporate no chromophore that cannot be synthesized by bacteria, and should be modular in structure and function. We gladly found that among all these three groups of commonly used light-sensitive domains, phototropins possesses the most distinct modular structure. Phototropins have a structurally conserved light sensor domain, termed LOV (light, oxygen and voltage) domain, which is easily discernable and often precedes, within a single reading frame, a sequence of an enzymatically functional domain, connected by the sequence of a linker domain. So far our good phototropin has satisfied two of our requirements: modularity and compatibility. So how about its sensitivity? To date, scientists have, for example, attached Rac1 protein to LOV2 domain from phototropin1 originated from Avena sativa to achieve photoactivatable cell motility, or shuffled the histidine kinase domain of FixL protein, which belongs to a two-component system, to the downstream of the LOV domain of B. subtilis YtvA protein to create a light-regulated histidine kinase, and so on. Among these designs, a particular one that utilizes the photosensor domain of a Vivid (VVD) protein, which originated from Neurospora. Crassa, camptured our attention. The VVD-GAL fusion protein thus designed achieved a rather high light-sensitivity—about 0.04W/m2. This is definitely thrilling news. Moreover, VVD was shown to form a rapidly exchanging homodimer upon blue-light activation, which means, like the Phy-PIF system, the general theme of protein-protein interaction might also be applied to this particular protein. Adding more promise to this VVD protein is its size—the smallest LOV domain containing protein known. This feature makes it easy to engineer and more likely to be stably expressed in bacteria systems.

FIG.2 Illustration of function mechanism of phototropin VVD.

Based on the discussion above, we have finally chosen the smallest LOV protein (or phototropin) VVD protein’s photosensor domain as our photoreceptor domain. Due to its small size, structural and functional modularity, bacteria compatibility, previous experience of being highly sensitive and its photoswitching mechanism, we reasoned that VVD photosensor domain would be an enabling tool in our coming design of a novel optogenetic module.

Our next step is to choose a physiologically functional domain for our new optogenetic module.

Based on the function mechanism of phototropin VVD, we can envisage a rough blueprint of our design: the DNA binding domain of a transcription inhibitor shall be fused to the downstream of VVD photosensor domain, and upon blue-light activation, VVD domain will dimerize, helping the DNA binding domain dimerize at the same time to enable its DNA binding activity and inhibition of transcription initiation. Based on these criteria, a transcription repressor,LexA, came into our sight. LexA is a transcription repressor of all the genes in the SOS system in E.coli, and its crystal structure has been resolved at high-resolution. LexA protein consists of a N-terminal DNA binding domain and a C-terminal dimerization domain with a short hydrophilic linker linking the two separate domains, making the structure impressively modular. Under normal physiological conditions, two LexA proteins will form a homodimer through the dimerization domain interface and bind to the SOS box target gene sequence in promoters of genes in SOS system and create significant steric hindrance for transcription polymerase binding and thus inhibit transcription initiation.

FIG.3 Illustration of function mechanism of LexA transcription repressor.

From these facts, we can conclude that the LexA repressor is the suitable physiological functional domain for our light-sensitive module, as it matches all of our criteria. First of all, a high-resolution crystal structure clearly shows its structural modularity. Beside the apparent structural modularity, various researches have utilized LexA to create versatile gene expression regulating modules that have convincingly demonstrated its functional modularity, further adding to its promise. (For example, a LexA-based genetic system has been devised for monitoring and analyzing protein heterodimerization in Escherichia coli.) Moreover, the DNA binding domain of LexA rigorously requires dimerization to bind efficiently to its targeting site (SOS box), and DNA binding domain itself has no dimerization ability. Last but not least, LexA is bacteria endogenous protein, which means it is likely to be efficiently expressed and function normally in bacteria systems.

FIG.4 Illustration of the function mechanism of our luminesensor.

By choosing the DNA binding domain of a bacteria endogenous transcription repressor, we must use reporter genes that will also be repressed by endogenous LexA protein. If we do not introduce changes into the DNA binding domain of our luminesensor, the only way to rule out the interference of endogenous LexA would be knocking out the genomic sequence of LexA protein. This will indubitably diminish its applicability in the future, since it will not work in any strain with endogenous LexA remaining. So we set out to find a solution to this important problem. The most natural way would be creating a mutated form of LexA so that it will only recognize a target sequence different from the sequence the wild-type LexA would recognize. We gladly found that such a thing actually exists. It is called the LexA408 variant, which carries mutations PA40, NS41 and AS42 and has been shown much higher affinity for a symmetrically altered target sequence CCGT (N)8 ACGG, different from the wild type SOS box CTGT (N)8 ACAG, which is specifically recognized by wild-type LexA. Thus by introducing the 408 mutations into the DNA binding domain of our luminesensor, we can insulate our light-sensitive system from the interference from the genetic context of the host strain. This outstanding character that have ensured the orthogonality of our system to the endogenous SOS system will greatly enhance the compatibility of our module to those synthetic modules that have hitherto been designed and thus add to the promise of future application.

FIG.5 Our solution to the problem of orthogonality.

Sequence and Features