HbpR

Overview

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HbpR is a 63-kDa protein belonging to NtrC family of prokaryotic transcriptional activators. It shares a highly conserved homology to members of the XylR/DmpR subclass.

HbpR was found in Pseudomonas azelaica ^[1]. Pseudomonas azelaica can use 2-hydroxybiphenyl (2-HBP) and 2, 2’-dihydroxybiphenyl as sole carbon and energy sources through enzymes encoded by hbpCAD in meta-cleavage pathway.

The regulatory gene hbpR encodes a transcriptional activator HbpR, which activates PC and PD promoters when exposed to inducers (Fig. 1) . Thus three enzyme HbpCA, HbpD are expressed, to degrade 2-HBP via meta-cleavage pathway (Fig. 2).

The HbpR protein is composed of four domains, namely A-domain, B-domain C-domain and D-domain (Fig. 3) . C-domain contains an AAA ATPase motif ^[2]. It has the capacity to hydrolyze ATP and to interact with sigma54 RNA polymerase. D domain binds to DNA via a typical helix-turn-helix motif. A-domain is necessary for the recognition of chemical effector molecules to activate transcription.

Generally, these transcriptional activators act at a distance of 100 to 200 bp from the actual promoter, binding to the sequence, which are called enhancer-like elements or upstream activating sequences (UAS) ^[3]. HbpR binds to UAS C-1 and UAS C-2. The 32-bp space between the centers of UASs C-1 and C-2 is critical for cooperative interactions (Fig. 4) .

Most if not all of the transcriptional outputs from the hbpC promoter are mediated by the proximal UASs C-1/C-2. However, when the UASs C-1/C-2 are deleted and UASs C-3/C-4 are placed in an appropriate position with respect to the promoter region, the hbpC promoter is still inducible with 2-HBP, albeit at a lower level. The presence of UAS pair C-3/C-4 mediated a higher promoter activity for transcription of hbpR ^[4].

HbpR detects a limited range of effectors. Previous studies have revealed that HbpR detects 2-hydroxybiphenyl, 2, 2-dihydroxybiphenyl and structural analogs including 2-aminobiphenl and 2-hydroxybiphenylmethane (TABLE 1) ^[5].

A single mutation in the hbpR gene (T613C) causes a Trp-Arg substitution at amino acid residue 205. This residue is conserved among all members of XylR/DmpR subclass and significant for inter-domain repression ^[6].

We used PCR to get hbpR gene from bacterial strain and Pc was synthesized by Genscript Company. We construct biosensor circuit for 2-HBP and 2-ABP. The gene hbpR was controlled by a constitutive promoter on plasmid pSB4K5. Another plasmid pUC57 containing Pc-RBS-sfGFP was double transformed with pSB4K5 to construct a biosensor for substituted biphenyls (Fig. 5a). HbpR biosensors were cultured to appropriate OD600 and stored at -80℃ in 20% glycerol for induction test.

We firstly tested the ON-OFF response of biosensor HbpR (BBa_J23106-HbpR and Pc-BBa_B0034-sfGFP) when it was exposed to other aromatic compounds following test protocol 1. Unfortunately, the basal expression level is relatively high and only 2-HBP and 2-ABP showed slightly induction effect (Data not shown) possibly because the aromatics-responsive transcriptional regulator HbpR originally exists in Pseudomonas azelaica instead of E.coli.

Therefore, we then constructed a library of promoters to fine-tune the expression intensity. Results of the HbpR biosensor with different initiation strength showed that BBa_J23114 performed better in this construction (Fig. 5b). We chose BBa_J23114 rather than BBa_J23113 because of the relatively higher fluorescence intensity of biosensors containing the former promoter. We also created a library for the RBS of sfGFP to improve the performance of biosensor system. Experiment results showed that for inducers 2-ABP and 2-HBP, RBS BBa_B0032 had a higher induction ratio compared with RBS BBa_B0031 and RBS BBa_B0034. (Fig. 6c,d)

After we had determined the most appropriate expression level of HbpR Biosensor, the best HbpR biosensor was subjected to ON-OFF test via protocol 1. Response result showed that 2-HBP and 2-ABP had significant induction ratios compared with the other tested aromatic compounds (Fig. 6a,b).

We then tested a dose-response curve of HbpR when it was exposed to effectors 2-HBP and 2-ABP according to test protocol 1(Fig. 7).

In summary, we have constructed and fine-tuned the HbpR biosensor for 2-HBP and 2-ABP using HbpR protein, which add materials to practical biosensors for synthetic biology. We also want to acknowledge Professor van der Meer for his material assistance.

Figure 1. Operon of hbpR. Orange rectangle shows hbpR gene. Blue and green rectangles show hbpCA and hbpD genes controled by PC and PD respectively. The orange rectangle show hbpR gene which encodes HbpR protein. When exposed to effector 2-hydroxybiphenyl, HbpR will activate PC and PD.

Figure 2. Pathway for the primary metabolism of 2-hydroxybiphenyl and 2-propylphenol in P. azelaica HBP1. The enzymes for each step of the degradation are also indicated .

Figure 3. Schematics for domain organization of protein HbpR. N represents the N terminal of HbpR and C represent the C terminal of HbpR. A, B, C and D show 4 domains of HbpR, respectively, and the numbers below them show domain boundaries at amino acid level.

Fig. 4. Regions containing the binding sites for HbpR (UAS) are shown within boxes in the sequence. Sequence numbers refer to the locations of the transcriptional start sites of hbpC and hbpD.

Fig. 5. Construction of the HbpR biosensor and improvements of its performance. (a) Structure of plasmids for hbpR gene and the reporter gene sfGFP. There is a library for the constitutive promoter before HbpR and the RBS before sfGFP respectively, both of which function to fine-tune the expression level of HbpR. (b) Induction ratio of HbpR controlled by promoters with different expression intensity. The effectors 2-HBP and 2-ABP are plotted in different colors. Data were collected via ELIASA. (C) Induction ratio of HbpR when exposed to a series of concentration of 2-ABP. The reporter system includes Pc-RBS-sfGFP. Three lines represent sfGFP controlled by different RBS. Fluorescence intensity of sfGFP is detected and calculated to plot induction ratio. (D) Induction ratio of HbpR when exposed to a series of concentration of 2-HBP.

Fig. 6. On-Off test results for sensor strain 114-32 HbpR. (a) On/off response of strain HbpR to 78 aromatic compounds. (For the full name of the compounds, CLICK HERE(hyperlink is needed here)). The strain showed induction ratio more than 10 folds when exposed to 2-HBP and 2-ABP. (B) The detection range of sensor strain HbpR is profiled in yellow at the aromatics spectrum. The structure formula of typical inducer 2-HBP and 2-ABP is showed near its chemical formula.

Fig. 7. Dose response curves for the induction effect of 2-HBP and 2-ABP to the best-performed HbpR sensor strain (BBa_J23114-HbpR and Pc-BBa_B0032-sfGFP).

Reference:
[1] Jaspers, M. C., Suske, W. A., Schmid, A., Goslings, D. A., Kohler, H. P. E., & van der Meer, J. R. HbpR, a new member of the XylR/DmpR subclass within the NtrC family of bacterial transcriptional activators, regulates expression of 2-hydroxybiphenyl metabolism in Pseudomonas azelaica HBP1. Journal of bacteriology, (2000).182(2), 405-417.
[2] Neuwald AF, Aravind L, Spouge JL, Koonin EV AAA+: A class of chaperone-like ATPases associated with the assembly, operation, and disassembly of protein complexes. Genome Res (1999)9: 27–43
[3] Pe´rez-Martı´n J, de Lorenzo. VATP binding to the s54-dependent activator XylR triggers a protein multimerization cycle catalyzed by UAS DNA. Cell (1996) 86: 331–339
[4] Jaspers, M. C., Sturme, M., & van der Meer, J. R. Unusual location of two nearby pairs of upstream activating sequences for HbpR, the main regulatory protein for the 2-hydroxybiphenyl degradation pathway of ‘Pseudomonas azelaica’HBP1. Microbiology, (2001).147(8), 2183-2194.
[5] Jaspers, M. C., Suske, W. A., Schmid, A., Goslings, D. A., Kohler, H. P. E., & van der Meer, J. R.. HbpR, a new member of the XylR/DmpR subclass within the NtrC family of bacterial transcriptional activators, regulates expression of 2-hydroxybiphenyl metabolism in Pseudomonas azelaica HBP1. Journal of bacteriology, (2000)182(2), 405-417.
[6] Jaspers, M. C., Sturme, M., & van der Meer, J. R. Unusual location of two nearby pairs of upstream activating sequences for HbpR, the main regulatory protein for the 2-hydroxybiphenyl degradation pathway of ‘Pseudomonas azelaica’HBP1. Microbiology, (2001). 147(8), 2183-2194.