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J Bacteriol. Jun 2005; 187(11): 3869–3872.
PMCID: PMC1112040

Alginate Lyase (AlgL) Activity Is Required for Alginate Biosynthesis in Pseudomonas aeruginosa

Abstract

To determine whether AlgL's lyase activity is required for alginate production in Pseudomonas aeruginosa, an algLΔ::Gmr mutant (FRD-MA7) was created. algL complementation of FRD-MA7 restored alginate production, but algL constructs containing mutations inactivating lyase activity did not, demonstrating that the enzymatic activity of AlgL is required for alginate production.

The ability of Pseudomonas aeruginosa to produce chronic infection in cystic fibrosis (CF) patients' lungs is based in part on its biosynthesis of alginate, an exopolysaccharide copolymer composed of (1-4)-linked β-d-mannuronic acid blocks interspersed with its C5 epimer α-l-guluronic acid (10, 15). Acquisition of mucoid (alginate-producing) P. aeruginosa strains is associated with a decline in pulmonary function and a reduced survival rate among CF patients (2, 17). Alginate can restrict the diffusion of certain antibiotics into the cell and can also inhibit several of the host's major antibacterial defense mechanisms, making it very difficult for CF patients to clear mucoid P. aeruginosa from their lungs (see reference 15 for a review).

Most of the alginate biosynthetic genes, including algD, alg8, alg44, algK, algE, algG, algX, algL, algI, algJ, algF, and algA, are found in a chromosomal gene cluster which functions as an operon controlled by the algD promoter (1, 24). algC, which is also involved in lipopolysaccharide biosynthesis (9), is located outside of this cluster and is expressed from its own promoter (27). Alginate biosynthesis begins in the cytoplasm with fructose-6-phosphate, which is converted to GDP-mannuronic acid via a series of steps involving AlgA, AlgC, and AlgD (see reference 24 for review) and then transported across the periplasmic membrane, possibly by Alg8 and Alg44 (24). Alg8, due to its resemblance to β-glycosyltransferases (21), is considered to be a good candidate for the polymerization of GDP-mannuronic acid residues into a poly(M) chain, but AlgK might also be involved (12). Some of the mannuronic acid residues are acetylated through the action of AlgF, AlgI, and AlgJ (5-7), while others are epimerized into guluronic acid by AlgG (4). It has been hypothesized that these later stages of alginate synthesis occur via a protein complex or scaffold composed of alginate proteins AlgG, AlgK, and AlgX (8, 11, 12, 20). This scaffold is believed to assist in polymer formation by bringing the enzymes and mannuronic acid residues together in one location, facilitating the modification of these residues and guiding the movement of the developing polymer to the outer membrane secretin, AlgE, a protein that appears to form poly-uronic-specific channels for translocation out of the cell (19). During the later stages in the periplasm, AlgG (8, 11) and AlgX (20) protect the developing polymer from AlgL, a periplasmic alginate lyase that degrades alginate via β-elimination (16, 22). Interestingly, the AlgG protein, but not its mannuronan C5-epimerase, is required for alginate polymer formation (8, 11).

Previous work in our lab using transposon mutagenesis indicated that the AlgL protein is needed for alginate production in P. aeruginosa strain FRD1 (16). This study tested our working hypothesis that AlgL's enzymatic activity as an alginate lyase is required for the production of alginate in P. aeruginosa.

Construction of algLΔ::Gmr using FRD1::pJLS3.

A ~6.9-kb HindIII-BamHI fragment from pNLS42 (22) containing wild-type algL from P. aeruginosa strain FRD1 was cloned into pUC19 (New England Biolabs) to create pMA1. This vector was digested with BstZ17I and XhoI to delete most of algL and 162 bp downstream of algL. Plasmid pSJ12 (12) encodes a gentamicin resistance (Gmr) cassette with its promoter sequence intact but lacking its transcriptional stop sequence. This ~0.7-kb Gmr cassette was amplified via PCR with primers provided with restriction sites for BstZ17I and XhoI, at 5′ and 3′, respectively, so that the antibiotic resistance cassette could be cloned into pMA1 and the antibiotic resistance marker expressed from its own promoter in the appropriate orientation as the alginate biosynthesis genes, generating pMA2. The resulting ~6.6-kb HindIII-BamHI sequence containing algLΔ::Gmr from pMA2 was excised, blunt ended with Klenow polymerase, and cloned into the SmaI site of pEX100T, a gene replacement vector that does not replicate in P. aeruginosa and confers sucrose sensitivity from sacB (23); this generated pMA3. The ~0.24-kb XhoI fragment encompassing the algL terminus and 162 bp downstream of algL that was lost when deleting algL was isolated from pMA1 and cloned back into pMA3 in the proper orientation, determined via sequencing, giving rise to the algLΔ::Gmr allelic exchange vector pMA4. Triparental mating was used to mobilize pMA4 from GeneHog cells (Invitrogen, Carlsbad, CA) with the aid of helper plasmid pRK2013 (3) to P. aeruginosa strain FRD1::pJLS3 (Cbr Gms), which has the alginate biosynthetic cluster under Ptac control, allowing alginate production to be induced in the presence of 1 mM isopropyl-β-d-thiogalactopyranoside (IPTG). Merodiploids resulting from a single-crossover homologous recombination event were selected for on LA-PIA (a 1:1 mixture of LB agar and Pseudomonas isolation agar [both from Difco Laboratories]) plates with 250 μg/ml of gentamicin (20). Mucoid merodiploids were then grown in Luria-Bertani broth, Lennox (Difco) for 18 h and plated onto LA-PIA with 250 μg/ml of gentamicin containing 7.5% sucrose to select for colonies that had undergone double crossovers leading to gene replacement.

The algL-to-algLΔ::Gmr chromosomal exchange was confirmed via PCR and DNA sequencing in all of the nonmucoid recombinants tested, and one of these, designated FRD-MA7, was selected for further study. When induced with IPTG, FRD-MA7 displayed the same nonmucoid phenotype as its uninduced parent (Fig. (Fig.1).1). Complementation of FRD-MA7 with pNLS18 (22) and IPTG induction revealed that the mucoid phenotype could be restored in FRD-MA7 by algL provided in trans on pNLS18 (Fig. (Fig.1),1), demonstrating that the algLΔ::Gmr mutation was responsible for the observed nonmucoid phenotype.

FIG. 1.
FRD-MA7 (algLΔ::Gmr) is nonmucoid but is restored to the mucoid phenotype after complementation with a functional algL provided in trans by pNLS18. Each strain was grown on LA-PIA with the appropriate antibiotics and with or without IPTG as indicated ...

FRD-MA7 lacks alginate lyase activity and does not produce uronic acid.

FRD1::pJLS3, FRD-MA7, and FRD-MA7/pNLS18 were grown overnight at 37°C in 64 ml of modified alginate-producing (MAP) medium (4) supplemented with or without IPTG and the appropriate antibiotics. Cells were collected from 50 ml of the culture by centrifugation, washed twice, and resuspended in 30 mM Tris-HCl (pH 7.5) containing 0.2 M MgCl2. The periplasmic fraction was isolated from these cells by temperature shock (22), and the number of alginate lyase enzyme units per mg of protein present was determined as described previously (16, 22). As expected, FRD-MA7 did not have any detectable alginate lyase activity, whereas complementation of FRD-MA7 with pNLS18 restored lyase activity (165.4 ± 15.6 U/mg for FRD-MA7/pNLS18 versus 156.1 ± 40.0 U/mg for FRD1::pJLS3) (data reflect means ± standard errors of the means from IPTG-induced cultures).

Although the algLΔ::Gmr mutation in FRD-MA7 results in a nonmucoid phenotype, this does not exclude the possibility that some form of poly-uronic acid is being synthesized. For these studies, the remaining 14 ml of cultures from the experiment above was split into two 7-ml aliquots, and the cells were collected by centrifugation, dried, and weighed while the culture supernatants were placed in Spectra/Por 6 dialysis membranes (Fisher Scientific, Pittsburgh, PA). To examine the extent of polymerization, we used 1-kDa dialysis membranes, which would retain polymers of ≥5 subunits, and 10-kDa dialysis membranes, which would retain polymers with ≥50 subunits. The culture supernatants were dialyzed to equilibrium overnight against 7 ml of 10 mM Tris-HCl (pH 7.6), and then the two fractions corresponding to the dialyzed (inside the tubing) and dialysate (outside the tubing) were collected for analysis. The remaining supernatant was extensively dialyzed for an additional 48 h prior to collecting the dialyzed fraction. Concentrations of uronic acid in these samples were determined using the carbazole assay (14) and a standard curve based on Macrocystis pyrifera alginate (Sigma), and they are reported as mg of uronic acid/g (dry weight) of cells (Fig. (Fig.22).

FIG. 2.
Poly-uronic acid is not produced in the absence of AlgL. Supernatants from cultures of FRD1::pJLS3, FRD-MA7, FRD-MA7/pNLS18, FRD-MA7/pMAH202Q, and FRD-MA7/pMAY256F were obtained after 20 h of incubation, with or without IPTG induction, and dialyzed in ...

In the absence of IPTG, FRD1::pJLS3 produces 23.5 ± 3.8 mg/g of uronic acid, whereas in the presence of IPTG it produces 546.2 ± 35.1 mg/g. The difference between these values, 522.8 ± 33.4 mg/g, represents the amount of IPTG-inducible uronic acid made by FRD1::pJLS3, and this was high-molecular-weight, nondializable uronic acid (compare the values for inside versus outside the dialysis bag in Fig. Fig.2).2). In contrast, FRD-MA7 produces 12.3 ± 2.0 mg/g of uronic acid in the absence of IPTG and only 15.1 ± 2.2 mg/g in the presence of IPTG. These values are not significantly different (P > 0.05), indicating that alginate biosynthesis in FRD-MA7 is not induced in the presence of IPTG, due to the deletion of algL. Polymeric uronic acid production by FRD-MA7 was restored to ~70% of parental levels when complemented with pNLS18. The data obtained in similar experiments using 10-kDa dialysis membranes were essentially the same (data not shown). These findings demonstrate that FRD-MA7 does not produce extracellular poly-uronic acid.

To explore the possibility that FRD-MA7 produces uronic acid trapped within the cell, we measured undialyzed extracellular and intracellular uronic acid levels in FRD1::pJLS3 and FRD-MA7. After growth in MAP (with or without IPTG) for 20 h at 37°C, the cells were pelleted by centrifugation and the supernatants were collected. The cell pellets were then washed, weighed, resuspended in Tris-HCl (pH 7.6), and passed three times through a French pressure cell press (American Inst. Co. Inc., Silver Spring, MD); the cellular debris was removed via centrifugation. Analysis of samples from inside and outside the cell demonstrated that FRD-MA7 induced with IPTG produces only 3.0 ± 0.9 mg/g of extracellular uronic acid and 0.4 ± 0.2 mg/g of intracellular uronic acid above background levels (i.e., uronic acid levels found in uninduced samples), while FRD1::pJLS3 induced with IPTG produces 240.7 ± 7.0 mg/g of uronic acid above background levels, and this uronic acid was found exclusively outside the cell (data not shown). These data further suggest that the algLΔ::Gmr mutation blocks alginate biosynthesis from the IPTG-inducible alginate operon.

Alginate lyase's activity is required for alginate polymer formation.

To determine whether AlgL's enzymatic activity or physical presence is required for alginate biosynthesis, we created two mutant AlgL proteins using site-directed mutagenesis. These proteins were designated AlgL-H202Q and AlgL-Y256F, based on their respective mutations located within the active cleft. The 1.5-kb EcoRI-HindIII algL DNA fragment from pNLS18 was cloned into pUC19 to obtain pMA5. The basic amino acid histidine 202 (CAT), located in the NNHSYW conserved region of alginate lyase, and the hydrophobic amino acid tyrosine 256 (TAC), were targeted for mutagenesis due to their potential roles in the enzymatic cleavage of alginate (25, 26). Using the QuikChange XL site-directed mutagenesis kit (Stratagene, La Jolla, CA), H202Q (CAG) and Y256F (TTC) mutations were generated to obtain pMA6 and pMA7, respectively. These replacement residues were chosen since they are not ionizable and therefore should not interact with the alginate polymer. Furthermore, we would expect that these single point mutations would have no significant effect on the tertiary structure of AlgL. The mutagenized algL genes were cloned into pRK415 (13), sequenced to confirm the mutations, and designated pMAH202Q and pMAY256F, respectively. These vectors were electroporated into the algLΔ mutant, and transformants were selected for on LA-PIA plates with 100 μg/ml of tetracycline supplemented with IPTG and confirmed via PCR analysis and sequencing. Expression of the mutant lyase proteins within the periplasm was confirmed via Western blotting (Fig. (Fig.3)3) as previously described (20) with minor modifications, i.e., using periplasmic fractions from cells grown in 50 ml of MAP medium (with or without IPTG), anti-AlgL rabbit antiserum (18), and donkey anti-rabbit immunoglobulin G horseradish peroxidase-linked antibody (Amersham Biosciences, Piscataway, NJ).

FIG. 3.
Transformation and expression of algLΔH202Q and algLΔY256F in FRD-MA7 cultures were verified using Western blot analysis. Periplasmic fractions from FRD-MA7/pNLS18, FRD-MA7/pMAH202Q, and FRD-MA7/pMAY256F cultures were resolved on a 14% ...

FRD-MA7 complemented with AlgL-H202Q and AlgL-Y256F lacked lyase activity, was nonmucoid, and was phenotypically identical to FRD-MA7 (data not shown). Dialysis of culture supernatants revealed that the uronic acid concentrations obtained both inside and outside the 1-kDa dialysis bag with FRD-MA7/pMAH202Q and FRD-MA7/pMAY256F were not significantly above uninduced background levels (P > 0.05) (Fig. (Fig.2).2). Analysis of undialyzed extracellular and intracellular fractions from FRD-MA7/pMAH202Q and FRD-MA7/pMAY256F revealed that uronic acid levels in these strains were also not significantly above uninduced background levels (P > 0.05) (data not shown). These results suggest that in the absence of a lyase-active AlgL protein, P. aeruginosa produces only background levels of uronic acid, levels which are significantly reduced (P ≤ 0.01) relative to IPTG-induced FRD1::pJLS3 and FRD-MA7/pNLS18.

The present study supports the hypothesis that AlgL's lyase activity is critical to alginate biosynthesis. Although its exact role in the biosynthesis of alginate remains to be determined, we propose that AlgL functions as part of the scaffold complex with AlgG, AlgX, and AlgK.

Acknowledgments

We thank Dennis Ohman for providing P. aeruginosa strain FRD1::pJLS3 and plasmid pSJ12.

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