Heterologous expression in Pseudomonas aeruginosa and purification of the 9.2-kDa c-type cytochrome subunit of p-cresol methylhydroxylase

Development of efficient modules for recombinant protein expression and periplasmic localiszation in Pseudomonas bharatica CSV86T

Escherichia coli has been widely employed as a host for heterologous protein expression. However, due to certain limitations, alternative hosts like Pseudomonas, Lactococcus and Bacillus are being explored. Pseudomonas bharatica CSV86^T, a novel soil isolate, preferentially degrades wide range of aromatics over simple carbon sources like glucose and glycerol. Strain also possesses advantageous eco-physiological traits, making it an ideal host for engineering xenobiotic degradation pathways, which necessitates the development of heterologous expression systems. Based on the efficient growth, short lag-phase and rapid metabolism of naphthalene, Pnah and Psal promoters (regulated by NahR) were selected for expression. Pnah was found to be strong and leaky as compared to Psal, using 1-naphthol 2-hydroxylase (1NH, ∼66 kDa) as reporter gene in strain CSV86^T. The Carbaryl hydrolase (CH, ∼72kDa) from Pseudomonas sp. C5pp was expressed under Pnah in strain CSV86^T and could successfully be translocated to the periplasm due to the presence of the Tmd + Sp sequence. The recombinant CH was purified from the periplasmic fraction and the kinetic characteristics were found to be similar to the native protein from strain C5pp. These results potentiate the suitability of P. bharatica CSV86^T as a desirable host, while Pnah and the Tmd + Sp can be employed for overexpression and periplasmic localisation, respectively. Such tools find application in heterologous protein expression and metabolic engineering applications.

High-level expression and characterization of a highly functional Comamonas acidovorans xanthine dehydrogenase in Pseudomonas aeruginosa

An improved procedure is described for the high-level expression of Comamonas acidovorans XDH in Pseudomonas aeruginosa PAO1-LAC. The level of functional expression (56 mg protein/L culture) is found to be 7-fold higher than that observed in Escherichia coli and 30-fold higher than that induced in C. acidovorans. Co-expression of the xdhC gene is required for maximal level of functional expression. Comparison of purified preparations of XDH expressed in the absence of xdhC (XDH(AB)) with that expressed in its presence (XDH(ABC)) shows the increased level of activity due to the level of Mo incorporation. The Fe and FAD contents of expressed enzymes are independent of xdhC co-expression. Electron paramagnetic resonance spectroscopy, circular dichroism spectroscopy, metal analysis, and kinetic properties of recombinant purified XDH(ABC) are identical with those exhibited by the native enzyme. This expression system should serve as a valuable tool for further biophysical and mechanistic investigations of xanthine dehydrogenase by site-directed mutagenesis. A method is also described to evaluate the suitability of P. aeruginosa and other organisms as potential expression hosts for five different sources of xdh genes.

Insight into covalent flavinylation and catalysis from redox, spectral, and kinetic analyses of the R474K mutant of the flavoprotein subunit of p-cresol methylhydroxylase

Each flavoprotein subunit (PchF) of p-cresol methylhydroxylase (PCMH) has flavin adenine dinucleotide (FAD) covalently tethered to Tyr384. The PCMH structure suggests that Arg474 in PchF is required for self-catalytic covalent flavinylation and for substrate oxidation. The replacement of Arg474 with Lys was carried out to probe the subtleties of the role of Arg474 in these processes. In nearly all of the aspects examined, the mutant protein showed compromised properties relative to the wild-type protein, including the tenacity of noncovalent FAD binding to the apo-protein, the rate of covalent flavinylation, the affinity of the covalent flavoprotein for PchC (the cytochrome subunit), the k(cat) for substrate oxidation, and the affinity for substrate analogues in the formation of FAD-charge-transfer complexes (CT complexes). Nevertheless, because the mutant retains these attributes, the comparison allows for an examination of the role of this residue in the various properties of the enzyme. A correlation is proposed to exist between nu(m), the frequency for the absorbance maximum of the CT complex with a substrate analogue, and k(cat), the steady-state rate constant for oxidation of p-cresol by various forms of PCMH and PchF; both nu(m) and k(cat) can be expressed as functions of the ionization potential of the donor (I(D)) and the electron affinity of the acceptor (E(A)). This correlation is a better predictor of the rate constant for substrate oxidation than is the magnitude of the redox potential, E(m,7), of the bound FAD, which was determined for the various mutant enzyme species and compared with those of the wild type.

Cytochrome P460 of Nitrosomonas europaea. Formation of the heme-lysine cross-link in a heterologous host and mutagenic conversion to a non-cross-linked cytochrome c'

The heme of cytochrome P460 of Nitrosomonas europaea, which is covalently crosslinked to two cysteines of the polypeptide as with all c-type cytochromes, has an additional novel covalent crosslink to lysine 70 of the polypeptide [Arciero, D.M. & Hooper, A.B. (1997) FEBS Lett.410, 457-460]. The protein can catalyze the oxidation of hydroxylamine. The gene for this protein, cyp, was expressed in Pseudomonas aeruginosa strain PAO lacI, resulting in formation of a holo-cytochrome P460 which closely resembled native cytochrome P460 purified from N. europaea in its UV-visible spectroscopic, ligand binding and catalytic properties. Mutant versions of cytochrome P460 of N. europaea in which Lys70 70 was replaced by Arg, Ala, or Tyr, retained ligand-binding ability but lost catalytic ability and differed in optical spectra which, instead, closely resembled those of cytochromes c'. Tryptic fragments containing the c-heme joined only by two thioether linkages were observed by MALDI-TOF for the mutant cytochromes P460 K70R and K70A but not in wild-type cytochrome P460, consistent with the structural modification of the c-heme only in the wild-type cytochrome. The present observations support the hypothesized evolutionary relationship between cytochromes P460 and cytochromes c' in N. europaea and M. capsulatus[Bergmann, D.J., Zahn, J.A., & DiSpirito, A.A. (2000) Arch. Microbiol. 173, 29-34], confirm the importance of a heme-crosslink to the spectroscopic properties and catalysis and suggest that the crosslink might form auto-catalytically.

Expression of recombinant Pseudomonas stutzeri di-heme cytochrome c4 by high-cell-density fed-batch cultivation of Pseudomonas putida

The gene of the di-heme protein cytochrome c(4) from Pseudomonas stutzeri was expressed in Pseudomonas putida. High-yield expression of the protein was achieved by high-cell-density fed-batch cultivation using an exponential glucose feeding strategy. The recombinant cytochrome c(4) protein was purified to apparent homogeneity and analyzed by electronic absorption spectroscopy, nanoflow electrospray ionization time-of-flight mass spectrometry, and electrochemistry. Cyclic voltammograms and UV-vis electronic absorption spectra were indistinguishable from the equivalent data of native P. stutzeri cytochrome c(4). Furthermore, the calculated and experimentally determined molecular masses of recombinant cytochrome c(4) were identical. Biochemical characterization of both wild-type and mutant derivatives of the protein will be greatly enhanced and facilitated by the described high-yield fermentation and rapid isolation procedure.

Vectors to express foreign genes and techniques to monitor gene expression in Pseudomonads

Improved tools for Pseudomonas research include small, broad-host-range vectors that allow regulated expression from the lac operon and T7 promoters whose biology is well understood and adaptable to many bacteria. To facilitate studies on gene regulation, tracking and monitoring of bacteria in diverse environments, and the construction of biosensors, various reporter genes with versatile assay formats have been developed that can be delivered on plasmid, transposon and integration-proficient vectors.

The cytochrome c domain of dimeric cytochrome cd(1) of Paracoccus pantotrophus can be produced at high levels as a monomeric holoprotein using an improved c-type cytochrome expression system in Escherichia coli

Cytochrome cd(1) nitrite reductase from Paracoccus pantotrophus is a dimer; within each monomer there is a largely alpha-helical domain that contains the c-type cytochrome centre. The structure of this domain changes significantly upon reduction of the heme iron, for which the ligands change from His17/His69 to Met106/His69. Overproduction, using an improved Escherichia coli expression system, of this c-type cytochrome domain as an independent monomer is reported here. The properties of the independent domain are compared with those when it is part of dimeric holo or semi-apo cytochrome cd(1).

Effects of noncovalent and covalent FAD binding on the redox and catalytic properties of p-cresol methylhydroxylase

Each flavoprotein subunit (alpha or PchF) of the alpha(2)beta(2) flavocytochrome p-cresol methylhydroxylase (PCMH) from Pseudomonas putida contains FAD covalently attached to Tyr384. PCMH oxidizes p-cresol to 4-hydroxybenzyl alcohol, which is oxidized subsequently by PCMH to 4-hydroxybenzaldehyde. The Y384F mutant form of PchF (apo-PchF[Y384F]) displayed stoichiometric noncovalent FAD binding. PchF[Y384F]FAD associated with the cytochrome subunit (beta or PchC) (producing PCMH[Y384F]), although not as avidly as with wild-type PchF containing covalently bound FAD (PchF(C)). Dramatic increases in the two-electron E(m,7) (NHE) values for FAD were observed when it bound noncovalently to either apo-PchF or apo-PchF[Y384F], and the two-electron E(m,7) value for FAD was increased further by about 75 mV upon covalent binding to PchF, i.e., PchF(C). The E(m,7) values increased by approximately 20 and 45 mV, respectively, when PchF(C) and PchF[Y384F]FAD associated with PchC. The two-electron E(m,7) for covalently bound FAD in PCMH is 84 mV, the highest measured for a flavoprotein. The values for the one-electron redox potentials (E(m,7), NHE) for FAD were measured also for various forms of PchF. Under anaerobiosis, the reduction of PchF[Y384F]FAD by substrates was similar to that observed previously for PchF containing noncovalently bound FAD. Stopped-flow kinetic studies indicated a rapid substrate reduction of the FAD and heme in PCMH[Y384F] which produced PchF[Y384F]FAD(rad) x PchC, the mutant enzyme containing the flavin radical and reduced heme. These experiments also revealed a slow reduction of unassociated PchC(ox) by PchF[Y384F]FAD(rad) x PchC. Steady-state kinetic studies of the reaction of PCMH[Y384F] with p-cresol indicated that the K(m) for this substrate was unchanged relative to that of PCMH, but that the k(cat) was diminished by an order of magnitude. The data indicate that the covalent attachment of FAD to PchF assists catalysis by raising the E(m,7) of the flavin. Contributions to this effect likely result from conformational changes.