Bright fluorescence of a novel protein from Vibrio vulnificus depends on NADPH and the expression of this protein is regulated by a LysR-type regulatory gene

The blue fluorescent protein from Vibrio vulnificus CKM-1 is a useful reporter for plant research

BACKGROUND

The mBFP is an improved variant of NADPH-dependent blue fluorescent protein that was originally identified from the non-bioluminescent pathogenic bacteria Vibrio vulnificus CKM-1. To explore the application of mBFP in plants, the mBFP gene expression was driven by one of the three promoters, namely, leaf-specific (RbcS), hypoxia-inducible (Adh) or auxin-inducible (DR5) promoters, in different plant tissues such as leaves, roots and flowers under diverse treatments. In addition, the expressed mBFP protein was targeted to five subcellular compartments such as cytosol, endoplasmic reticulum, apoplast, chloroplast and mitochondria, respectively, in plant cells.

RESULTS

When the mBFP was transiently expressed in the tobacco leaves and floral tissues of moth orchid, the cytosol and apoplast exhibited brighter blue fluorescence than other compartments. The recombinant mBFP-mS₁C fusion protein exhibited enhanced fluorescence intensity that was correlated with more abundant RNA transcripts (1.8 fold) as compared with a control. In the root tips of horizontally grown transgenic Arabidopsis, mBFP could be induced as a reporter under hypoxia condition. Furthermore, the mBFP was localized to the expected subcellular compartments, except that dual targeting was found when the mBFP was fused with the mitochondria-targeting signal peptide. Additionally, the brightness of mBFP blue fluorescence was correlated with NADPH concentration.

CONCLUSION

The NADPH-dependent blue fluorescent protein could serve as a useful reporter in plants under aerobic or hypoxic condition. However, to avoid masking the mitochondrial targeting signal, fusing mBFP as a fusion tag in the C-terminal will be better when the mBFP is applied in mitochondria trafficking study. Furthermore, mBFP might have the potential to be further adopted as a NADPH biosensor in plant cells. Future codon optimization of mBFP for plants could significantly enhance its brightness and expand its potential applications.

Advanced in vivo applications of blue light photoreceptors as alternative fluorescent proteins

The ultimate ambition in cell biology, microbiology and biomedicine is to unravel complex physiological and pathophysiological processes within living organisms. To conquer this challenge, fluorescent proteins (FPs) are used as versatile in vivo reporters and biosensors to study gene regulation as well as the synthesis, localization and function of proteins in living cells. The most widely used FPs are the green fluorescent protein (GFP) and its derivatives and relatives. Their use as in vivo reporter proteins, however, is sometimes restricted by different environmental and cellular factors. Consequently, a whole range of alternative, cofactor-dependent reporter proteins have been developed recently. In this perspective, we summarize the advantages and limitations of the novel class of cyan-green fluorescent flavoproteins in comparison to members of the GFP family and discuss some correlated consequences for the use of FPs as in vivo reporters.

Crystallization and preliminary X-ray diffraction analysis of a novel wild-type blue fluorescent protein from Vibrio vulnificus CKM-1

The use of green fluorescent protein (GFP) for non-invasive in vivo imaging is limited to aerobic systems, as chromophore formation requires oxygen. However, a novel NADPH-dependent blue fluorescent protein from Vibrio vulnificus CKM-1 (BFPvv) that emits blue fluorescence in both aerobic and anaerobic systems has recently been discovered. Wild-type BFPvv was overexpressed in Escherichia coli, purified and crystallized using the sitting-drop vapour-diffusion method. The resulting BFPvv crystals diffracted to a resolution of 1.9 Å and belonged to space group P3, with unit-cell parameters a = b = 96.62, c = 214.511 Å. Assuming the presence of eight molecules in the unit cell, the solvent content was estimated to be ~56.16%.

Fluorescent intensity of a novel NADPH-binding protein of Vibrio vulnificus can be improved by directed evolution

Blue fluorescent protein, BfgV, found from Vibrio vulnificus CKM-1, fluoresces through augmenting the intrinsic fluorescence of bound NADPH. Random mutagenesis and DNA shuffling were applied to increase the fluorescent intensity of BfgV. The wild type bfgV gene was subjected to four cycles of mutagenesis processes. A prominent D7 mutant protein had fluorescent intensity four times larger than wild type BfgV. The emission wavelength of this mutant protein appeared at 440 nm, which was 16 nm shorter than that of BfgV. There were eight amino acid substitutions in D7. As these substitutions were assigned to the modeled 3D structure of BfgV, three of them, V83M, G176S, and E179K, were shown to be located around NADPH-binding site. Time course analysis indicated the synthesis of D7 protein and fluorescent expression in Escherichia coli transformants were synchronic. This property was different from that of wild type GFP.