Abstract

The expression of BBa_K1465223 (pSB1C3-T7:sap-csoS4AB-csoS1CA:gfp-csoS1B) in E. coli leads to the assembly of carboxysomes. We used a translational fusion of one shell protein with the CDS of the green fluorescent protein as an indicator of correct protein folding. The observed fluorescence in our E.coli cells is concentrated at different points and indicates the presence of functional carboxysomes.

Introduction

In nature there are plasmids which encode different proteins of the carboxysome in bacteria. One such plasmid is pHnCBS1D which was found in Halothiobacillus neapolitanis (Cannon and Shively, 1983). Different parts of this plasmid were used for the production of BioBricks. These BioBricks were used for the construction of a synthetic carboxysome-encoding plasmid. This bottom up approach allows to verify the essentiallity of the different components. We used a translational fusion of csoS1A and gfp (BBa_K1465222) as indicator of correct protein folding. A concentrated subcellular localisation ot the fluorescence shows the positions of carboxysoms. This reporter function of GFP was identified and used before (Waldo et al., 1999; Hsu et al., 2009).

Shell protein folding

The expression of csoS1A:gfp with all other known essentiell shell proteins (BBa_K1465222) did not yield any fluorescence (VERWEIS auf Ergebnisse von Birte). This indicates that the presence of a shell assosiated protein (SAP) encoded by csoS2 is needed for the correct folding of the shell proteins and the assembly of the whole carboysome. The combined expression of the sap and the cso coding sequences (BBa_K1465223) yields green fluorescence. This fluorescence was quantified by photometric measurements (Link auf Methode und auf Ergebnisse). The signal intensity of the E. coli KRX wildtype (negative control) was compared to cells carrying either a plasmid with (BBa_K1465223) or without sap(BBa_K1465222). E. coli KRX cells with pSB1C3-pTET:gfp (BBa_I13522) served as a positive control. From these results we deduce the correct folding of the shell proteins in presence of the shell assosiated protein.
[Tabelle / Grafik mit Ergebnissen vom GloMax]

Carboxysome assembly

To show the successfull assembly of the carboxysom we analysed the subcellular localisation of the green fluorescent protein. Pictures taken through a fluorescence microscope are shown in figure XXXX (fig = Mikroskopfotos von Birte). We assume that the concentrated green fluorescence results from GFP molecules which are fused to shell proteins of a successfully assembled carboxysome.
[pictures]
This assembly was possible without the expression of csoS1D another coding sequence which is located on pHnCBS1D. The resulting protein is probably responsible for the pore size in the carboxysome envelope (Kinney et al., 2011; Bonacchi et al., 2012). Nevertheless it seems that it has no essentiell function in protein folding.
[mikroskopische Aufnahmen => ggf. Evolution der Mikroskope]

Carboxysome application

The functional carboxysom could be used for the carbon dioxide fixation. Therefore three enzymes needs to be heterologous expressed in E. coli. We tried to combine the carboxysome encoding genes and two of the required enzymes on a single plasmid. One RubisCO (BBa_K1465202) and one phosphoribulokinase (BBa_K1465201) encoding sequence should be integrated upstream of the carboxysomal operon.

Summary and outlook

We report the construction of a synthetic plasmid encoding a functional carboxysome. Despite the fact that he have not shown the fixation of carbon dioxide in this compartiment yet we would like to emphasise the construction of a usefull microcompartment. This could be used for anarobic reactions in aerobic cultures of E.coli. One possible application beside the carbon dioxide fixation could be the fixation of atmospheric nitrogen which is essentiell for the growth of plants.

Carboxysome purification

Purification of recombinant proteins is one essential foundation of syntetic biology. For this reason, it was our first approach to purify the carboxysome from the chemolitoautotroph model organism Halothiobacillus neapolitanus, recombinant expressed in E. coli. After establishment of the methode, our constructed carboxysome should be purified with this methode. The work for purifying carboxysomes was nevertheless based on the plasmide pHnBCcsoS1D, which is coding for the H. neapolitanus carboxysome. For cultivation and recombinant expression of the carboxysome, we followed the method for cultivation described in Bonacchi et al., 2012 and the method for purification described by So et al., 2004. To proof the expression of the carboxysomal proteins, we analysed recombinant protein expression by SDS-PAGE and MALDI-TOF. The results of the SDS-PAGE are shown in Figure 1-3:

Figure 1:Proteinexpression of pHnCBcsoS1D induced with 0.5 mM IPTG

Figure 2: Proteinexpression of pHnCBcsoS1D induced with 2 mM IPTG

Figure 3: Proteinexpression of pHnCBcsoS1D induced with 5 mM IPTG

As this SDS-PAGE shows the cellular proteins at different times points in the cultivation, you can recognize, that 12 hours after induction there is a new band somewhat smaler than 55 kDa. Analytics via MALDI-TOF identified this band as the large subunit of RuBisCO (52.6 kDa) from Halothiobacillus neapolitanus with a sequence coverage (MS) of 8.9 %. When protein expression was induced with 0.5 mM IPTG, after 12 h increases a band with a size of propably 13 kDa. MALDI-TOF displayed that this is the small subunit of the RuBisCo (12.8 kDa) with a sequence coverage (MS) of 28.2 %. When IPTG was added to 2 or 5 mM, it seems like that the intensity of the band of the small subunit decreases in time of cultivation. It could be possible, that the small subunit is degrated, but inconsistent is the increase of the large subunit. Degradation of the small subunit is not described in the literature. The fact that RuBisCo composes nearly 70 % of the protein mass (by weight) in the carboxysome could explain why other proteins from the carboysome could not be found in this SDS-PAGE.

The purification of the carboxysome is carried out via sucrose gradient centrifugation. This method did not work in our realisation at all.