Team:UNIPV-Pavia/parts/characterized2.html

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UNIPV TEAM 2011

Parts

Contents

New parts

J23101x series

  1. BBa_K516130 (wiki name: J101-E5 ) J101-RBS30-RFP-TT
  2. BBa_K516131 (wiki name: J101-31 ) J101-RBS31-mRFP-TT
  3. BBa_K516132 (wiki name: J101-E7 ) J101-RBS32-mRFP-TT

BBa_J23101 is the reference standard promoter for the computation of RPUs. As discussed in 'data analysis' section, RPUs are relative units for the evaluation of promoter strength, based on a mathematical model of the transcription and the translation of a reporter gene.
The RPUs are supposed to be indepedent on the experimental setup, provided that the reference standard BBa_J23101 must be assayed in the same experimental condition of the studied promoter.
It means that if the studied promoter is in a low copy number plasmid and drives the expression of a reporter protein P, J23101 must be assembled in the same vector upstream of the same reporter P.
This approach is in accordance with the philosophy of synthetic biology, based on the concept of 'modularity' of the components. According to this approach, the assembly of basic well characterized modules to build complex circuits allows the prediction of the circuit behavior starting from the knowledge on the basic parts.
Salis et al. [Nat Biotec, 2009] stated that

'Identical ribosome binding site sequences in different genetic contexts can result in different protein expression levels'

and again

'It is likely that this absence of modularity is caused by the formation of strong secondary structures between the RBS-containing RNA sequence and one protein coding sequence but not another.'

For this reason, RPUs might not be reliable when comparing the same promoter with different RBSs because of the un-modularity of the RBS.
In order to asses what's the effect of RBS 'un-modularity' on RPUs reliability, we have built a set of four constitutive promoters (BBa_J23101) followed by one of the four RBSs tested. These parts were used to evaluate RBS efficiency. Data were collected and analyzed as described in 'Measurements' and 'Data analysis' sections. RPUs and Synthesis rate per cell [AUr] were computed and results are summarized in the table below.

NB: for the RPU computation, the J23101-RBS34-mRFP-TT construct has been considered as the reference standard. With this assumption, RPUs are identical to the estimated RBS efficiency.

If the hypothesis of RBS modularity depending on the promoter is accepted, the J23101-RBSx series we have provided can be used as a library of ready-to-use reference standard for RPU evaluation, that allows to depurate RPU measurement from RBS effect, thus providing only the promoter strength.

J23101 promoter with RBS Scell R.P.U.s
RBS30 150.97 [15.34] 3.17
RBS31 2.30 [0.37] 0.05
RBS32 17.60 [5.85] 0.37
RBS34 47.57 [0.82] 1
RPUs of J23101 promoter with different RBSs

AiiA expression cassette driven by aTc-inducible pTet promoter

LuxI expression cassette driven by aTc-inducible pTet promoter

Existing parts

Notes for promoter characterization

Inducible and constitutive promoters were assembled upstream of different coding sequences containing an RBS from the Community collection.

The assembled RBSs are:

BioBrick code Estimated efficiency
BBa_B0030 0,6
BBa_B0031 0,07
BBa_B0032 0,3
BBa_B0034 1

For an inducible device, the RBS variation has the purpose to stretch the induction curve, thus modulating its PoPs-OUT range.

The complex RBS-promoter acts as a whole regulatory element and determines the amount of translated protein. RBSs have been reported to have an un-modular behavior, since the translational efficiency is not independent on the coding sequences, but variates as an effect of different mRNA structure stability [Salis et al., Nat Biotec, 2009]. In addition, it is not possible to separate the effects of the sole promoter and of the sole RBS on the total amount/activity of gene product (in this case study, mRFP).

For this reason, every combination 'Promoter+RBS' was studied as a different regulatory element. Regulatory elements were characterized using mRFP reporter protein for different RBSs in terms of Synthesis rate per Cell (Scell) and R.P.U.s (Relative Promoter Units) as explained in measurements section.

RBSs

The complex RBS-promoter acts as a whole regulatory element and determines the amount of translated protein. RBSs have been reported to have an un-modular behavior, since the translational efficiency is not independent on the coding sequences, but variates as an effect of different mRNA structure stability [Salis et al., Nat Biotec, 2009]. In addition, it is not possible to separate the effects of the sole promoter and of the sole RBS on the total amount of mRFP produced. For this reason, every combination 'Promoter+RBS' was studied as a different regulatory element. The evaluation of RBS efficiency can be performed in a very intuitive fashion:

    • select the RBSs you want to study,
    • assemble them in a Promoter - XX - Coding sequence circuit,
    • measure the output of the circuits and calculate the RBS efficiency as the ratio of the output relative to the output of the circuit with the standard RBS.

This simple measurement system allows the quantification of RBS efficiency depending on the whole measurement system (i.e.: promoter and encoded gene). Today it has not still been completely validated the hypothesis that every functional module in a genetic circuit maintains its behavior when assembled in a complex circuits, even if many researchers implicitly accept this hypothesis when performing characterization experiments.
To rationally assess the impact that this hypothesis has on the genetic circuit design and fine tuning, several measurement systems were built to evaluate the dependance of RBS modularity from the promoter or the coding sequence separately.
In particular, in order to investigate if RBS efficiency depends on the promoter, the same coding devices (RBSx-RFP-TT) were assembled downstream of different promoters (J23101, pTet, pLux). Measuring the system output and evaluating the RBS efficiency. The results are summarized in the table below:
RBS η pLux η pTet η J23101
B00300.401.723.17
B00310.010.030.05
B00320.190.370.37
B0034111

On the other end, to investigate the dependance of RBS modularity on the coding sequence, the same regulatory elements (pTet-RBSx) were assembled upstream of different encoded gene (mRFP, AiiA and LuxI). RBS efficiency was assessed and the results are summarized in the table below:
RBS η mRFP η AiiA η LuxI
B00301.720.530.64
B00310.030.830.08
B00320.370.50N.D.
B0034111


The parts we used to characterize the RBSs are listed here:

pLux promoter

    • BBa_K516330 (wiki name: E17 ) pLambda-RBS30-LuxR-T-pLux-RBS30-mRFP-TT
    • BBa_K516331 (wiki name: E18 ) pLambda-RBS30-LuxR-T-pLux-RBS31-mRFP-TT
    • BBa_K516332 (wiki name: E19 ) pLambda-RBS30-LuxR-T-pLux-RBS32-mRFP-TT
    • BBa_K516334 (wiki name: E20 ) pLambda-RBS30-LuxR-T-pLux-RBS34-mRFP-TT
RBS αpLux δpLux ηpLux kpLux
BBa_B0030 438 [10] 0.05 [>100] 2 [47] 1.88 [27]
BBa_B0031 9.8 [7] 0.11 [57] 1.2 [29] 1.5 [26]
BBa_B0032 206 [3] 0 [>>100] 1.36 [10] 1.87 [9]
BBa_B0034 1105 [6] 0.02 [>100] 1.33 [19] 2.34 [18]
Data are provided as average [CV%]

pTet promoter

The protocols for the characterization of pTet promoter are reported in the pTet measurement section.
This promoter is widely studied and characterized usually using the strong RBS BBa_B0034. Here we have characterized its transcriptional strength as a function of aTc induction (ng/ul) for different RBSs. Four different induction curves were obtained and are reported in figure:



The data collected from the mRFP measurement systems were processed as described in data analysis section. The induction curves were obtained by fitting a Hill function as described in modelling section and the estimated parameters for pTet are reported in the pictures and in table below.



RBS αpTet δpTet ηpTet kpTet
BBa_B0030 215 [2] 0.001 [>>100] 4.3 [12] 8.3 [2]
BBa_B0031 0.7 [>>100] 0[>>100] 18.84[>>100] 47.51[>>100]
BBa_B0032 45.7 [11] 0.02 [>>100] 83 [>>100] 8 [>>100]
BBa_B0034 125 [12] 0.12 [61] 71.8 [>>100] 9.8 [>>100]
Data are provided as average [CV%]

While α parameter (representing the maximum trascriptional rate in the studied range of induction) varies as expected with the RBS variation , the K and η parameters (determining the switch-point of the induction curve) are quite constant among all the RBS variations.
This suggests that the RBS variation only modulates the amplitude of the induction curve, but doesn't affect the shape, i.e. the translational promoter activity. These results are quite encouraging, because suggest that, given the non-modular behavior of RBS dpending on the encoded gene, the RBS has a modular behaviour respect to the promoter.

Existing parts: sequence debugging

    • BBa_K516021 (wiki name: E10 ) RBS31-AiiA-TT Rebuilt existing part from BBa_I13914 (DNA planning)
    • BBa_K516022 (wiki name: E11 ) RBS32-AiiA-TT Rebuilt existing part from BBa_I13912 (DNA planning)
    • BBa_K516030 (wiki name: E5 ) RBS30-mRFP-TT Rebuilt existing part from BBa_S04180 (DNA planning)
    • BBa_K516032 (wiki name: E7 ) RBS32-mRFP-TT Rebuilt existing part from BBa_ J133000 (DNA planning)
    • BBa_K516214 (wiki name: E16 ) pTet-RBS34-LuxI Rebuilt existing part from BBa_S03623 (DNA available, only 2008 kit, inconsistent)
    • BBa_K516222 (wiki name: E26 ) pTet-RBS32-AiiA-TT Rebuilt existing part from BBa_ J22071 (2008 only, Bad sequencing)
    • BBa_K516224 (wiki name: E27 ) pTet-RBS34-AiiA-TT Rebuilt existing part from BBa_K077047 (Part deleted)
    • BBa_K516232 (wiki name: E23 ) pTet-RBS32-mRFP-TT Rebuilt existing part from BBa_I20252 (DNA planning)

Laboratory for Biomedical Informatics | Centre of Tissutal Engineering

University of Pavia
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