Part:BBa_K2260000
phaCBA operon with polyhistidine tags
Overview
The naturally-occuring phaCAB operon in R. eutropha H16 is involved in biosynthesis of poly[(R)-3-hydroxybutyrate] (PHB) (Hiroe et al., 2012). This operon was submitted to the Registry by Imperial College in 2013 (BBa_K1149051). It utilizes acetyl-coA as a primary substrate, which is produced from the degradation of carbohydrates and volatile fatty acids (Hiroe et al., 2012). Transcription of the phaCAB operon leads to expression of the following enzymes in the order: pha synthase (phaC), 3-ketothiolase (phaA), and acetoacetyl-CoA reductase (phaB). 3-ketothiolase converts acetyl-coA to acetoacetyl-CoA. The acetoacetyl-CoA reductase leads to conversion of acetoacetyl-CoA to (R)-3-hydroxybutyryl-CoA. Finally, pha synthase converts (R)-3-hydroxybutyryl-CoA to PHB (Hiroe et al., 2012).
We used this operon to convert carbojudrates and volatile fatty acids present in fermented human feces to produce PHB. However, literature has shown that the rearrangement of operon to phaCBA leads to higher amount of production of PHB (Hiroe et al., 2012). Thus, we obtained the operon from BBa_K1149051 and rearranged the construct from phaCAB to phaCBA. The construct also contains the RBS BBa_B0034 upstream of each coding region and histidine tags at the N-termini of each protein.
Weight of PHB Produced from Fermented Synthetic Feces Supernatant
We tested the production of PHB from our part using glucose and VFAs as feedstock. There were 9 replicates for our construct in the fermented synthetic feces supernatant and 3 replicates of the negative control (E. coli carrying the same vector without insert). The OD600 of bacterial overnights (O/Ns) was measured before inoculating the media with fermented synthetic feces supernatant. PHB was extracted using sodium hypochlorite extraction methods found here. After cells were resuspended in 1X PBS for PHB extraction, the OD600 was recorded to estimate the relative number of cells present. The following table summarizes the data:
Table 1. Recorded OD600 of O/Ns before inoculating the media containing "syn poo supernatant", OD600 of cells after growing in media for ~24 hours, centrifuged, and resuspended in (1x) PBS for extraction. Initial weight of 50 mL falcon tubes and final weights of tube containing PHB was recorded in grams.
Figure 1. Tubes after sodium hypochlorite extraction of PHB from cells. Negative control on left and phaCBA on right.
PHB extraction yielded a white powder from E. coli cells transformed with our phaCBA operon, but our next step was to confirm its identity as PHB. This was achieved using High Pressure Liquid Chromatography (HPLC).
HPLC of PHB Digested in Sulphuric Acid
Karr et al. (1983) showed that digestion of PHB in sulphuric acid leads to the production of crotonic acid. When run on an HPLC machine, the crotonic acid then produces an identifiable peak on generated spectra. The protocol we used for using this digestion method is included in our Experiments page.
0.581 g of white powder was obtained from our PHB extraction procedure. It was then diluted 32X in sulphuric acid and digested for 30 mins. Figure 2 shows the HPLC results obtained from this sample.
30 mins digestion, dilution factor 32
Figure 2. HPLC results from digestion of PHB in sulphuric acid for 30 mins.
We also generated standard curve using known concentrations of industrially-produced PHB to determine the conversion of PHB to crotonic acid (results not shown). The amount of crotonic acid from HPLC results was then used to calculate the concentration of PHB in the sample.
The HPLC results showed that crotonic acid and therefore PHB was present in our three samples. The dilution factor we selected was based on the predicted conversion of 80% from PHB to crotonic acid (Karr et al., 1983). The amount of crotonic acid detected in the sample with a dilution factor of 32 was 0.0282006 mM.
Discussion of HPLC results
The HPLC results confirmed that the product obtained was PHB. Although we were successful at detecting PHB using HPLC, the obtained amount of crotonic acid may be lower than the actual amount produced as the method of quantification of PHB using HPLC requires further development. In particular, an optimal digestion time needs to be selected so that all of the PHB in the sample digests to crotonic acid without further degradation of crotonic acid. The PHB extraction method can also be optimized to minimize the amount of PHB lost during extraction.
References
Hiroe A, Tsuge K, Nomura CT, Itaya M, Tsuge T. (2012). Rearrangement of gene order in the phaCAB operon leads to effective production of ultrahigh-molecular-weight poly[(R)-3-hydroxybutyrate] in genetically engineered Escherichia coli. Appl. Environ. Microbiol. 78:3177–3184. 10.1128/AEM.07715-11.
Karr DB, Waters JK, Emerich DW. (1983). Analysis of Poly-β-Hydroxybutyrate in Rhizobium japonicum Bacteroids by Ion-Exclusion High-Pressure Liquid Chromatography and UV Detection . Applied and Environmental Microbiology. 1983;46(6):1339-1344.
Sequence and Features
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