Caulobacter crescentus Adapts to Phosphate Starvation by Synthesizing Anionic Glycoglycerolipids and a Novel Glycosphingolipid.

Bacteria adapt to environmental changes in a variety of ways, including altering their cell shape. Caulobacter crescentus adapts to phosphate starvation by elongating its cell body and a polar stalk structure containing both inner and outer membranes. While we generally think of cellular membranes being composed largely of phospholipids, cellular elongation occurs when environmental phosphate, and therefore phospholipid synthesis, is limited. In order to adapt to these environmental constraints, C. crescentus synthesizes several glycolipid species, including a novel glycosphingolipid. This finding is significant because glycosphingolipids, while ubiquitous in eukaryotes, are extremely rare in bacteria. In this paper, we identify three proteins required for GSL-2 synthesis and demonstrate that they contribute to phage resistance. These findings suggest that bacteria may synthesize a wider variety of lipids in response to stresses than previously observed.

membrane fluidity. For bacteria such as Escherichia coli, cells incorporate an increasing proportion of unsaturated fatty acids as temperatures decrease (1,2); the kinks introduced by acyl chain unsaturation decrease membrane viscosity to counteract the effects of lower temperature. Similarly, a variety of Gram-positive and Gram-negative bacteria alter the ratio of phospholipid headgroups in response to osmotic shock (3); E. coli increases the ratio of cardiolipin:phosphatidylethanolamine when osmotically stressed (4).
Oligotrophic bacteria require adaptations to stresses associated with nutrient availability. For example, nutrient levels in freshwater lakes experience seasonal fluctuations, and phosphate concentration has been shown to be a limiting factor for bacterial growth (5). The oligotrophic Gram-negative bacterium Caulobacter crescentus responds to phosphate limitation by dramatically elongating its cell body and a polar stalk structure, a thin extension of the cell envelope, consisting of an inner membrane, a peptidoglycan cell wall, an outer membrane, and a surface layer (6) (Fig. 1A). The stalk has been hypothesized to serve as means to increase phosphate uptake (7), since all four members of the PstSCAB high-affinity phosphate import pathway are found in the stalk (7,8). Additionally, analytical modeling of nutrient diffusion suggests that stalk elongation is the most efficient method of increasing nutrient flux to the cell while minimizing cell surface area and volume (8). Under phosphate-rich growth conditions, cells are approximately 1 m in length, stalks are very short (ϳ100 nm), and phosphatidylglycerol (PG) accounts for approximately 30% of total lipids (9). Upon phosphate starvation, cell bodies and stalks can grow up to 3.5 m and 15 m in length, respectively, which requires significant production of new lipids to build the inner and outer membranes. When phosphate is limited it is unlikely that this new membrane contains phospholipids; therefore, we hypothesized that C. crescentus synthesizes alternative lipids for cellular and stalk elongation.
Several alphaproteobacteria adapt to phosphate limitation by increasing the production of glyceroglycolipids and ornithine lipids. For example, Agrobacterium tumefaciens synthesizes monoglucosyl diacylglycerol (DAG), glucuronosyl diacylglycerol, and diacylglycerol trimethylhomoserine (DGTS) (10,11), while Mesorhizobium loti produces di-and triglycosyldiacylglycerols, DGTS, and ornithine lipid (12). Glycolipids make up a large proportion of the C. crescentus membrane even in phosphate-rich growth media (45% to 62%) (9), but phosphate-mediated changes in lipid composition have not been characterized. We hypothesized that during phosphate limitation C. crescentus either (i) increases the proportion of existing glycolipids or (ii) synthesizes novel lipid species to replace phospholipids.
Analysis of total membrane composition following phosphate limitation revealed that both hypotheses were correct. C. crescentus increases the amount of monohexuronosyl DAG (MHDAG) and synthesizes a novel hexosyl-hexuronosyl-ceramide glycosphingolipid (HexHexA-Cer). This glycosphingolipid (GSL) represents a novel bacterial lipid species. In this report we characterize this GSL, identify the enzymes responsible for initiating ceramide synthesis and its sequential glycosylation, and address the physiological importance of ceramide-based lipids.

RESULTS
Phosphate limitation induces changes in membrane composition. Phosphate starvation induces elongation of both the cell body and stalk in C. crescentus (Fig. 1A). If we approximate the shapes of the cell body and stalk as cylinders, we can estimate that the total surface area of the cell, and thus the synthesis of membrane lipids, increases 6-to 7-fold upon phosphate limitation. This significant increase in membrane area suggests that C. crescentus must be able to produce alternatives to phospholipids during phosphate starvation. Indeed, when we compared the total lipid composition under phosphate-rich (1 mM phosphate) and phosphate-starved (1 M phosphate) conditions by using normal phase liquid chromatography-tandem mass spectrometry (LC-MS/MS), we saw a dramatic decrease in phosphatidylglycerol and a corresponding increase in several species of glycolipids (Fig. 1B). In particular, phosphate starvation   1B). These lipid species were identified by exact mass measurement in conjunction with collision-induced dissociation (CID) tandem mass spectrometry. The presence of a GSL was unexpected since sphingolipids, while highly abundant in eukaryotes, are rarely found in bacteria. Some species in the Bacteroides, Porphyromonas, and Prevotella genera are capable of synthesizing the phosphosphingolipids ceramide phosphorylethanolamine and/or ceramide phosphoglycerol (13). Non-phosphate-containing GSLs have been described only for the family Sphingomonadaceae, in which they can function as a substitute for lipopolysaccharide (LPS) (14). Structural analyses of Sphingomonas GSLs have revealed carbohydrate moieties containing 1, 3, or 4 sugar units ( Fig. 1C) (14,15). In contrast, the GSL found in C. crescentus has two sugars and thus represents a novel bacterial GSL species, which we named GSL-2 (Fig. 1C).
Ceramide synthesis in C. crescentus. Sphingolipid synthesis begins with the production of a ceramide molecule which is then modified with various polar groups. In eukaryotes, ceramide synthesis is a four-step process that begins with the condensation of serine and a fatty acyl coenzyme A (acyl-CoA) to produce 3-oxo-sphinganine in a reaction catalyzed by oxoamine synthase (16). While each of the enzymatic steps in ceramide synthesis has been well characterized in eukaryotes (17), only the oxoamine synthase enzyme required for the first step appears to be conserved in bacteria. Indeed, bacterial species have been identified encoding one or more oxoamine synthases, including serine palmitoyltransferase (Spt), 8-amino-7-oxononanoate synthase (BioF), 5-aminolevulinate synthase (HemA), and 2-amino-3-oxobutyrate coenzyme A ligase (Kbl) (18). C. crescentus encodes three putative oxoamine synthases: CCNA_01220 (BioF), CCNA_01417 (HemA), and CCNA_01647 (BioF).
To assess the role of the candidate oxoamine synthases in ceramide synthesis, total lipid composition was analyzed in wild-type, Δccna_01220, and Δccna_01647 cells. We were unable to obtain a deletion of ccna_01417 since this is an essential gene (19); however, recent biochemical analysis of CCNA_01417 reveals that it is most likely involved in the production and regulation of heme cofactors (20). Ceramides were completely absent in the Δccna_01220 strain (Fig. 2); therefore, we refer to this gene as ccbF, for Caulobacter crescentus BioF. Complementation of the ccbF deletion restored ceramide synthesis (see Fig. S1A in the supplemental material). Interestingly, deletion of BioF homolog CCNA_01647 had no effect on ceramide synthesis despite the fact that the protein has 51% similarity and 35% identity to CcbF (Fig. 2). We note that while the ccna_01647 deletion did not affect ceramide levels, we observed a reduction in MHDAG synthesis (Fig. 2), though the mechanism for this decrease is unknown.
Glycosphingolipid synthesis requires two sequential glycosyltransferases. GSL-2 has a novel glycosylation pattern consisting of a hexose and a hexuronic acid (Fig. 1C). Lipid glycosylation, in both eukaryotes and prokaryotes, is performed by GT-4 family glycosyltransferases; C. crescentus encodes 11 putative GT-4 glycosyltransferases (21). To identify the glycosyltransferases required for GSL-2 synthesis, we narrowed down the list of candidate genes using the following criteria: (i) genes upregulated upon phosphate starvation (22), (ii) nonessential genes (19), and (iii) genes without a direct homolog in E. coli, since E. coli does not produce GSLs. Of the 11 initial candidates, only 2 genes fit all three criteria: ccna_00792 and ccna_00793 ( Fig. 3A and Table S4). Both candidate genes are part of the PhoB regulon, which contains genes upregulated under phosphate starvation conditions (  (Fig. 3D). Complementation of these deletions recovered ceramide glycosylation ( Fig. S1B and C). These data support a model of sequential ceramide glycosylation by CCNA_00793 and CCNA_00792, which we are naming sphingolipid glycosyltransferases 1 and 2 (Sgt1 and -2), respectively (Fig. 3E). Sgt1 functions as a glucuronosyltransferase, adding a hexuronic acid, while Sgt2 is a glycosyltransferase responsible for adding a hexose sugar. Sgt1 and -2 appear to specifically glycosylate sphingolipids, as neither deletion affected the glycosylation of DAG-based lipids ( Fig. 3C and D). The specificity of Sgt1 and -2 was further confirmed by heterologous expression in E. coli, in which we could detect only nonglycosylated lipids ( Fig. S2A and B).
While GSL-2 synthesis occurs in response to phosphate starvation, neither ceramides nor GSL-2 appears to be necessary for stalk biogenesis or cell elongation (Fig. S3A). This is likely due to the sufficiency of upregulated DAG lipids under low-phosphate conditions (Fig. 1B). Unlike GSL-2, nonglycosylated ceramides are produced across a wide range of phosphate concentrations, albeit at lower levels in the presence of excess phosphate (Fig. S3B). Surprisingly, ccbF mRNA levels are reduced during phosphate starvation, despite higher levels of ceramide production (Fig. S3C). Restriction of ceramide glycosylation to growth environments in which phospholipid synthesis is limited appears to be critical for membrane homeostasis. Overexpression of Sgt1 and Sgt2 in high-phosphate media results in the production of both HexA-Cer and GSL-2 ( Fig. 4A to C). Unlike physiological glycosphingolipid production during phosphate starvation, Sgt1 and -2 overexpression leads to an accumulation of HexA-Cer (compare Fig. 1B to Fig. 4A). Furthermore, we did not detect HexHexA-DAG under these conditions, providing additional evidence that Sgt1 and -2 specifically glycosylate ceramide lipids (Fig. 4C). The production of glycosphingolipids under high-phosphate conditions leads to cell lysis, as assessed by propidium iodide staining (Fig. 4D). This phenomenon may be due to an   Fig. 5A and B). GSL-producing Sphingomonas species are resistant to the antibiotic polymyxin B (24); therefore, we tested whether ceramides or GSL-2 conferred similar resistance on C. crescentus. Surprisingly, the deletion of ccbF increased resistance to polymyxin B under both high-and lowphosphate conditions (Fig. 5A and B). Under low-phosphate conditions, the Δsgt1 strain did not have increased polymyxin B resistance, demonstrating that the resistance phenotype was specifically due to the presence of ceramide lipids regardless of their glycosylation (Fig. 5B). Under both phosphate conditions, complementation of ccbF restored sensitivity to polymyxin B (Fig. 5A and B).  Modifying the bacterial envelope structure and composition can also affect cellular interactions with bacteriophages. Phage ⌽Cr30 infects C. crescentus by attaching to the extracellular surface layer (S-layer) (25). Growth curves of wild-type and ccbF deletion strains infected with ⌽Cr30 in peptone-yeast extract (PYE) demonstrated that ceramides are important for increasing phage resistance (Fig. 5C); this initial screen was performed in PYE because ⌽Cr30 infections are generally inhibited in minimal media (26). The increased susceptibility of the ΔccbF strain could be attributed to either enhanced phage adsorption or an increased viral burst size. We tested the ability of ⌽Cr30 to adsorb to wild-type, ΔccbF, and Δsgt1 cells in Hutner base-imidazole-glucoseglutamate medium (HIGG)-1 M phosphate. Both of the mutant strains had an enhanced rate of phage adsorption which was restored to normal upon complementation (Fig. 5D), suggesting that mature GSL-2 is required to inhibit phage adsorption. Measurements of burst size did not reveal any differences between the strains in low-phosphate medium (Fig. 5E); however, the ccbF and sgt1 deletion strains appear to have shorter latent periods, consistent with faster phage adsorption to these cells (Fig. 5E).
Phage ⌽Cr30 attaches to C. crescentus by binding to the cell envelope S-layer, a crystalline lattice composed of the protein RsaA (27). The S-layer is, in turn, anchored to the cell through interactions with the O-antigen domain of lipopolysaccharide (LPS) (28). A comparison of LPS and S-layer production in wild type and GSL mutants did not reveal any remarkable distinctions between the strains (Fig. S4A and B), suggesting that the enhanced phage adsorption observed in the GSL mutants is not due to an increase in S-layer production. The accessibility of the S-layer to phage is restricted by the production of an exopolysaccharide (EPS) capsule (29). To test whether the absence of ceramides disrupts EPS production, C. crescentus strains were grown in HIGG-1 M phosphate and incubated with fluorescein isothiocyanate (FITC)-labeled dextran (29). The wide zone of exclusion around the wild-type and ΔccbF cells show that they produce EPS, in contrast to the non-EPS-producing ΔMGE strain (29) (Fig. S4C).

DISCUSSION
C. crescentus adapts to phosphate limitation, in part, by dramatically elongating both its cell body and polar stalk appendage (7,30) (Fig. 1A), requiring a significant amount of lipid synthesis. Without the environmental phosphate required for phospholipid synthesis, C. crescentus upregulates the production of several glycolipid species, including a novel glycosphingolipid, GSL-2 (Fig. 1B). In this study, we identified three enzymes involved in GSL production: CcbF is responsible for the first step of ceramide synthesis (Fig. 2), while Sgt1 and Sgt2 sequentially glycosylate ceramide to yield GSL-2 ( Fig. 3C to E).
Upregulation of glycolipid synthesis in response to phosphate limitation has been previously described for Agrobacterium tumefaciens and Mesorhizobium loti (11,12). In these species, cells produce nonphosphorus glycosyl-DAGs. While C. crescentus also produces mono-and diglycosyl-DAGs, this is the first demonstration of bacterial GSL synthesis in response to phosphate starvation. While GSLs are found ubiquitously in eukaryotic organisms, their presence in bacteria was thought to be limited to species of the family Sphingomonadaceae. In Sphingomonas species, GSLs are used as a substitute for LPS in the outer membrane and contain 1, 3, or 4 sugar units (14,15). Sphingomonas wittichii strain RW1 produces two different monoglycosylated GSLs in place of LPS (31). Not surprisingly, the gene for serine palmitoyltransferase, which catalyzes the first step of ceramide synthesis, is an essential gene in S. wittichii (32). In contrast, C. crescentus GSL synthesis genes are nonessential and GSL-2 is produced even in the presence of LPS (Fig. S4A). Furthermore, ablation of ceramide or GSL-2 has no effect on proliferation (Fig. 5A and B) or cellular elongation (Fig. S3A). Thus, we conclude that while GSL production occurs under conditions of phosphate limitation, it is dispensable for cell elongation, stalk synthesis, and survival. This is likely due to the presence of sufficient glycosylated DAGs to compensate for the loss of GSLs.
This study is the first to identify bacterial glycosyltransferase enzymes required for ceramide glycosylation. As expected, BLAST homology searches (33) demonstrate that outside the Caulobacteraceae family, Sgt1 and Sgt2 are most homologous to glycosyltransferases in the GSL-producing Sphingomonadaceae family. Unlike many other bacterial glycosyltransferases which demonstrate a high degree of promiscuity regarding sugar acceptors (34,35), Sgt1 appears to have a high degree of specificity toward ceramide glycosylation. Deletion of sgt1 in C. crescentus has no effect on glycosyldiacylglycerol production (Fig. 3C), and heterologous expression of Sgt1 and Sgt2 in E. coli does not lead to lipid glycosylation (Fig. S2).
While Sphingomonas species use GSLs to replace LPS, the role of GSLs in C. crescentus is less clear. Ceramide synthesis occurs over a wide range of phosphate concentrations, yet mature GSLs are produced only during phosphate starvation. Complete deletion of ceramides appears to alter the function of C. crescentus membranes, resulting in increased resistance to the lipid-interacting antibiotic polymyxin B and increased sensitivity to phage-mediated killing (Fig. 5A to C). These effects occur despite the absence of gross changes to LPS, S-layer, or EPS production ( Fig. S4A to C). Resistance to cationic antimicrobial peptides like polymyxin B often occurs by reducing the negative charge of the membrane to prevent binding; for example, in E. coli, lipid A is modified with 4-amino-4-deoxy-L-arabinose (36) to neutralize charge. In C. crescentus, the impact of ceramide or GSL deficiency on total membrane charge is less clear; nonglycosylated ceramides are neutral, while the hexuronic acid found in GSL-2 is anionic. Therefore, the relative abundances of all lipid species would be required to assess the role of membrane charge in antibiotic resistance.
The increased susceptibility of ceramide-depleted cells to phage lysis appears to be due to enhanced phage adsorption to the ccbF and sgt1 deletion strains (Fig. 5D). Increased adsorption reduces the phage latency period without affecting the phage burst size (Fig. 5E). Although the abundance of S-layer protein was not affected in the GSL mutants (Fig. S4B), recent biophysical studies have shown that the S-layer protein RsaA can exist on the cell surface in either a crystalline or aggregated state (37). This is consistent with cryo-electron tomography showing distinct regions of S-layer organization in intact C. crescentus cells (38). While we do not know exactly how phage ⌽Cr30 binds to the S-layer, it is possible that GSLs affect S-layer organization, rather than production, thereby regulating phage interactions.

MATERIALS AND METHODS
Bacterial strains, plasmids, and growth conditions. The strains, plasmids, and primers used in this study are described in Tables S1, S2, and S3, respectively. Details regarding strain construction are available in the supplemental materials. C. crescentus wild-type strain NA1000 and its derivatives were grown at 30°C in peptone-yeast extract (PYE) medium (39) for routine culturing. To control phosphate levels, C. crescentus was grown in Hutner base-imidazole-glucose-glutamate media (HIGG) with variable amounts of phosphate (1 to 1,000 M) (40). E. coli strains were grown at 37°C in LB medium. When necessary, antibiotics were added at the following concentrations: kanamycin, 30 g/ml in broth and 50 g/ml in agar (abbreviated 30:50) for E. coli and 5:25 for C. crescentus; ampicillin, 50:100 for E. coli; tetracycline, 12:12 for E. coli and 1:2 for C. crescentus; gentamicin, 15:20 for E. coli and 0.5:5 for C. crescentus; and spectinomycin, 50:50 for E. coli and 25:100 for C. crescentus. Gene expression was induced in C. crescentus with either 0.3% (wt/vol) xylose or 0.5 mM vanillate. E. coli gene expression was induced with isopropyl-␤-D-1-thiogalactopyranoside (IPTG; 1 mM). Phage titering was performed by adding 1 to 10 l of ⌽Cr30 to 100 l of an overnight culture of NA1000 in PYE. This mixture was added to 4 ml of soft agar (0.3% [wt/vol] agar in PYE) and overlaid on a PYE-agar plate. After solidifying, the plate was incubated overnight at 30°C and plaques were counted.
Microscopy and image analysis. Cells were spotted onto 1% agarose pads made in the corresponding growth medium. Phase microscopy was performed on a Nikon TiE inverted microscope equipped with a Prior Lumen 220PRO illumination system, Zyla sCMOS 5.5-megapixel camera, CFI Plan Apochromat 100ϫ oil immersion objective (numerical aperture [NA] of 1.45 and working distance [WD] of 0.13 mm), and NIS Elements software for image acquisition. Cell and stalk dimensions were measured using Morphometrics (41) and ImageJ v. 1.48q (NIH), respectively. To measure membrane permeability, cells were grown in the presence of 1 g/ml of propidium iodide. EPS production was assessed as previously described (29). Briefly, 500 microliters of cells grown in HIGG-1 M phosphate were collected by centrifugation (14,000 ϫ g, 5 min), and the pellet was resuspended in 30 l of 0.5ϫ phosphatebuffered saline (PBS). Ten microliters of the cell suspension was mixed with 5 l of FITC-dextran (10 mg/ml; molecular weight [MW], 2 MDa; Sigma) and 1 l of SlowFade Diamond mountant (Thermo Scientific). Two microliters of this mixture was spotted onto a glass slide, coverslipped, and sealed with vaseline-lanolin-paraffin (VALAP; 1:1:1) for imaging.
qRT-PCR. RNA was extracted from bacterial cultures using the Qiagen RNeasy kit. Following DNase digestion, RNA (5 ng/l) was reverse transcribed using a high-capacity cDNA reverse transcription (RT) kit (Applied Biosystems). One microliter of cDNA was used as a template in a 10-l quantitative RT-PCR (qRT-PCR) performed with Power SYBR reagent (Applied Biosystems). qRT-PCR was performed on an ABI QuantStudio 6 using the threshold cycle (ΔΔC T ) method. rpoD expression was used as the loading control.
Lipid extraction. C. crescentus strains were grown in 500 ml of HIGG with either 1 mM or 1 M phosphate until reaching stationary phase. Sgt1 and -2 E. coli expression strains were grown overnight in 500 ml of LB medium with 1 mM IPTG to induce protein expression. Lipids were extracted by the method of Bligh and Dyer (42). Cells were harvested in glass tubes at 10,000 ϫ g for 30 min, and the majority of the supernatant was removed; stalked C. crescentus organisms are very buoyant and do not form tight pellets, preventing the complete removal of supernatant. The cells were resuspended in the residual supernatant, 3.75 volumes of 1:2 (vol/vol) chloroform-methanol was added, and the samples were mixed by vortexing. Chloroform (1.25 volumes) and water (1.25 volumes) were added sequentially with vortexing to create a two-phase system, and the samples were centrifuged at 200 ϫ g for 5 min at room temperature. The bottom, organic phase was transferred to a clean tube with a Pasteur pipette and washed twice in "authentic" upper phase. Subsequently, the residual organic phase with the lipids was collected and dried under argon.
LC-ESI-MS/MS. Methods for liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) have been described previously (43,44). Briefly, normal phase LC was performed on an Agilent 1200 quaternary LC system equipped with an Ascentis Silica high-performance liquid chromatography (HPLC) column, 5 m, 25 cm by 2.1 mm (Sigma-Aldrich, St. Louis, MO). The LC eluent (with a total flow rate of 300 l/min) was introduced into the ESI source of a high-resolution Triple-TOF5600 mass spectrometer (Applied Biosystems, Foster City, CA). Instrumental settings for negative-ion ESI and MS/MS analysis of lipid species were as follows: IS, Ϫ4,500 V; CUR, 20 lb/in 2 ; GSI, 20 lb/in 2 ; DP, Ϫ55 V; and FP, Ϫ150 V. The MS/MS analysis used nitrogen as the collision gas. Data analysis was performed using Analyst TF1.5 software (Applied Biosystems).
Growth curve analysis. For polymyxin B sensitivity assays, C. crescentus cells were diluted to an optical density at 660 nm (OD 660 ) of 0.05 in HIGG with 1 mM or 1 M phosphate, treated with 30 g/ml of polymyxin B, and incubated with shaking (250 rpm and 30°C). Complementation gene expression was induced with 0.3% xylose. Samples were taken at the desired times for absorbance measurements (OD 660 ). For phage sensitivity assays, cells were grown in PYE and diluted to an OD 660 of 0.05. A total of 148 l of culture was dispensed per well in a 96-well plate. To each well, 2 l of water (control) or ⌽Cr30 (final concentration, 5 ϫ 10 5 PFU/ml) was added. To prevent evaporation, each well was overlaid with 100 l of mineral oil. OD 660 was recorded every 20 min on a BMG Labtech CLARIOstar plate reader with incubation at 30°C with continuous shaking.
Phage adsorption and burst size quantification. Phage adsorption and burst size were measured essentially as previously described (25). To measure phage adsorption, cells were grown in HIGG-1 M phosphate and diluted to an OD 660 of 0.2. Cells (1 ml) were aliquoted into glass culture tubes, and 10 5 PFU of ⌽Cr30 was added. Cultures were incubated with shaking at 30°C; at various time points, 10 l of culture was removed and diluted into 1 ml of water-chloroform (9:1 [vol/vol]). Ten microliters of this mixture was used to titer the unbound phage as described above. To measure viral burst size, cells were grown in HIGG-1 M phosphate and diluted to an OD 660 of 0.1. Cells (0.5 ml) were infected with 0.5 ϫ 10 5 PFU of ⌽Cr30 and incubated at 30°C for 15 min. The culture was diluted 1,000-fold into HIGG-1 M phosphate; a 200-l aliquot was removed every 15 min for titering as described above.
SDS-PAGE and protein staining. E. coli strains were grown overnight in LB medium with 1 mM IPTG to induce protein expression. A total of 500 l of each strain was collected by centrifugation (6,000 ϫ g for 2 min) and the pellet was resuspended in 100 l of sample buffer. Protein samples were resolved on a 12% SDS-PAGE gel and stained with Coomassie blue for visualization.
LPS purification and analysis. Lipopolysaccharide (LPS) was purified essentially as previously described (45). Briefly, 5 ml of C. crescentus cells grown in HIGG-1 M phosphate (OD 660 ϭ 0.5) was collected and washed once in 10 mM HEPES (pH 7.2). Cells were resuspended in 250 l of TE buffer (10 mM Tris, 1 mM EDTA [pH 7.2]) and frozen overnight at Ϫ20°C. Cells were thawed, treated with 1 l of DNase (0.5 mg/ml), 20 l of lysozyme (10 mg/ml), and 3 l of MgCl 2 (1 M), and incubated at room temperature for 15 min. For each sample, 36.25 l was mixed with 12.5 l of 4ϫ SDS sample buffer and boiled at 100°C for 10 min. After cooling to room temperature, 1.25 l of proteinase K (20 mg/ml) was added and samples were incubated at 60°C for 1 h. LPS samples were resolved on a 12% SDS-PAGE gel and stained with a Pro-Q Emerald 300 LPS stain kit according to the manufacturer's protocol (Thermo Scientific). Images were acquired on a Bio-Rad ChemiDoc MP using UV excitation and a 530-nm emission filter.
S-layer (RsaA) purification and analysis. RsaA was purified essentially as previously described (46). Briefly, cells were grown overnight in HIGG-1 M phosphate and 5 ml (OD 660 ϭ 0.6) was collected by centrifugation. The cell pellets were washed twice in 5 ml of 10 mM HEPES (pH 7.2), resuspended in 200 l of 100 mM HEPES (pH 2), and incubated at room temperature for 10 min. Cells were pelleted (10 min at 5,000 ϫ g), and the supernatant containing RsaA was collected and neutralized with 2.8 l of 10 N NaOH. RsaA samples in 1ϫ sample buffer were resolved on a 7.5% SDS-PAGE gel without heat denaturing and stained with Krypton protein stain (Thermo Scientific). Images were acquired on a Bio-Rad ChemiDoc MP using green light-emitting diode (LED) excitation and a 605-nm emission filter.