Requirement of phosphatidylglycerol for flagellation of Escherichia coli

Rcs Phosphorelay Activation in Cardiolipin-Deficient Escherichia coli Reduces Biofilm Formation

Biofilm formation is a complex process that requires a number of transcriptional, proteomic, and physiological changes to enable bacterial survival. The lipid membrane presents a barrier to communication between the machinery within bacteria and the physical and chemical features of their extracellular environment, and yet little is known about how the membrane influences biofilm development. We found that depleting the anionic phospholipid cardiolipin reduces biofilm formation in Escherichia coli cells by as much as 50%. The absence of cardiolipin activates the regulation of colanic acid synthesis (Rcs) envelope stress response, which represses the production of flagella, disrupts initial biofilm attachment, and reduces biofilm growth. We demonstrate that a reduction in the concentration of cardiolipin impairs translocation of proteins across the inner membrane, which we hypothesize activates the Rcs pathway through the outer membrane lipoprotein RcsF. Our study demonstrates a molecular connection between the composition of membrane phospholipids and biofilm formation in E. coli and suggests that altering lipid biosynthesis may be a viable approach for altering biofilm formation and possibly other multicellular phenotypes related to bacterial adaptation and survival.IMPORTANCE There is a growing interest in the role of lipid membrane composition in the physiology and adaptation of bacteria. We demonstrate that a reduction in the anionic phospholipid cardiolipin impairs biofilm formation in Escherichia coli cells. Depleting cardiolipin reduced protein translocation across the inner membrane and activated the Rcs envelope stress response. Consequently, cardiolipin depletion produced cells lacking assembled flagella, which impacted their ability to attach to surfaces and seed the earliest stage in biofilm formation. This study provides empirical evidence for the role of anionic phospholipid homeostasis in protein translocation and its effect on biofilm development and highlights modulation of the membrane composition as a potential method of altering bacterial phenotypes related to adaptation and survival.

Polar localization of Escherichia coli chemoreceptors requires an intact Tol-Pal complex

Subcellular biomolecular localization is critical for the metabolic and structural properties of the cell. The functional implications of the spatiotemporal distribution of protein complexes during the bacterial cell cycle have long been acknowledged; however, the molecular mechanisms for generating and maintaining their dynamic localization in bacteria are not completely understood. Here we demonstrate that the trans-envelope Tol-Pal complex, a widely conserved component of the cell envelope of Gram-negative bacteria, is required to maintain the polar positioning of chemoreceptor clusters in Escherichia coli. Localization of the chemoreceptors was independent of phospholipid composition of the membrane and the curvature of the cell wall. Instead, our data indicate that chemoreceptors interact with components of the Tol-Pal complex and that this interaction is required to polarly localize chemoreceptor clusters. We found that disruption of the Tol-Pal complex perturbs the polar localization of chemoreceptors, alters cell motility, and affects chemotaxis. We propose that the E. coli Tol-Pal complex restricts mobility of the chemoreceptor clusters at the cell poles and may be involved in regulatory mechanisms that co-ordinate cell division and segregation of the chemosensory machinery.

Increased motility of Escherichia coli by insertion sequence element integration into the regulatory region of the flhD operon

The flhD operon is the master operon of the flagellar regulon and a global regulator of metabolism. The genome sequence of the Escherichia coli K-12 strain MG1655 contained an IS1 insertion sequence element in the regulatory region of the flhD promoter. Another stock of MG1655 was obtained from the E. coli Genetic Stock Center. This stock contained isolates which were poorly motile and had no IS1 element upstream of the flhD promoter. From these isolates, motile subpopulations were identified after extended incubation in motility agar. Purified motile derivatives contained an IS5 element insertion upstream of the flhD promoter, and swarm rates were sevenfold higher than that of the original isolate. For a motile derivative, levels of flhD transcript had increased 2.7-fold, leading to a 32-fold increase in fliA transcript and a 65-fold increase in flhB::luxCDABE expression from a promoter probe vector. A collection of commonly used lab strains was screened for IS element insertion and motility. Five strains (RP437, YK410, MC1000, W3110, and W2637) contained IS5 elements upstream of the flhD promoter at either of two locations. This correlated with high swarm rates. Four other strains (W1485, FB8, MM294, and RB791) did not contain IS elements in the flhD regulatory region and were poorly motile. Primer extension determined that the transcriptional start site of flhD was unaltered by the IS element insertions. We suggest that IS element insertion may activate transcription of the flhD operon by reducing transcriptional repression.

Characterization of the Staphylococcus aureus mprF gene, involved in lysinylation of phosphatidylglycerol

Lysylphosphatidylglycerol (LPG) is a basic phospholipid in which L-lysine from lysyl-tRNA is transferred to phosphatidylglycerol (PG). This study examined whether the Staphylococcus aureus mprF gene encodes LPG synthetase. A crude membrane fraction prepared from wild-type S. aureus cells had LPG synthetase activity that depended on PG and lysyl-tRNA, whereas the membrane fraction from an mprF deletion mutant did not. When S. aureus MprF protein was trans-expressed in wild-type Escherichia coli cells, LPG synthesis was induced, whereas it was not observed in E. coli pgsA3 mutant cells in which the amount of PG is significantly reduced. In addition, LPG synthetase activity and a 93 kDa protein whose molecular size corresponded to that of MprF protein were co-induced in the crude membrane fraction prepared from E. coli cells expressing MprF protein. The Km values of the LPG synthetase activity for PG and for lysyl-tRNA were 56 microM and 6.9 microM, respectively, consistent with those of S. aureus membranes. These results suggest that the MprF protein is LPG synthetase.

Systematic characterization of Escherichia coli genes/ORFs affecting biofilm formation

To understand the nature and function of bacterial biofilm and the process of its formation, we have performed systematic screening of a complete set of Escherichia coli genes/open reading frames (ORFs) to identify those that affect biofilm development upon over-expression. In contrast to the biofilm of strain AG1 used as a control, some of the genes/ORFs when over-expressed led to the formation of an abnormal biofilm such as thin, mat-like, filamentous or one easily detaching from various surfaces. Disruptants of selected genes were constructed in order to clarify their roles in the different stages of biofilm formation. Our results suggest that diverse metabolic pathways contribute to the development of biofilm.

Effects of lipoprotein biogenesis mutations on flagellar assembly in Salmonella

Flagellar assembly requires the expression of a large number of flagellum-specific genes. However, mutations in a number of other genes in Salmonella and Escherichia coli have been shown to have pleiotropic effects that affect flagellar assembly. FlgH (the L-ring subunit of the flagellar basal body) is a lipoprotein whose modification is important for L-ring assembly. We therefore tested whether the lack of motility of Salmonella mutants defective in lipoprotein biogenesis is a result of inability to modify FlgH. Our results show that temperature-sensitive apolipoprotein N-acyltransferase [lnt(Ts)] mutants are nonflagellate at 42 degrees C. However, the flagellar assembly defect occurs at a much earlier step in the pathway than L-ring assembly. These mutants failed to assemble even an MS ring, presumably because of the observed decrease in transcription of fliF. In contrast, temperature-sensitive diacylglycerol transferase [lgt(Ts)] mutants were motile at 42 degrees C, provided the strains carried an lpp (Braun lipoprotein) mutation to permit growth. We have isolated second-site mutants from an lgt(Ts) lpp(+) strain that grow but are nonflagellate at 42 degrees C. Thus, lipoprotein biogenesis is a factor that is important for flagellar assembly.

Immotile phenotype of an Escherichia coli mutant lacking the histone-like protein HU

The histone-like protein HU in Escherichia coli are encoded by the hupA and hupB genes. A hupA-hupB double deletion mutant has now been shown to express an immotile phenotype. The motility of hupA or hupB single mutants was similar to that of wild-type cells. SDS-polyacrylamide gel electrophoresis revealed that the amount of flagellin in the hupA-hupB double deletion mutant was markedly reduced compared with the wild-type strain suggesting that the immotile phenotype of the double deletion mutant is caused by a loss of flagella.

Unbalanced membrane phospholipid compositions affect transcriptional expression of certain regulatory genes in Escherichia coli

The amount of porin protein OmpF in the outer membrane of Escherichia coli was reduced to one-third by the pgsA3 mutation that diminishes the amount of phosphatidylglycerol and cardiolipin in the membrane, whereas a cls (cardiolipin synthase) null mutation had no effect. Osmoregulation of OmpF was functional in the pgsA3 mutant. As assessed by the beta-galactosidase activities of lacZ fusions, the ompF expression was not reduced at the transcriptional level but was reduced about threefold at the posttranscriptional level by pgsA3. This reduction was mostly restored by a micF null mutation, and the micF RNA that inhibits the ompF mRNA translation was present 1.3 to 1.4 times more in the pgsA3 mutant, as assayed by RNase protection and Northern blot analyses. Elevation of the level of micF RNA was not restricted to acidic-phospholipid deficiency: OmpF was hardly detected and micF RNA was present 2.7 to 2.8 times more in a pssA null mutant that lacked phosphatidylethanolamine. Other common phenotypes of pgsA3 and pssA null mutants, reduced rates of cell growth and phospholipid synthesis, were not the cause of micF activation. Salicylate, which activates micF expression and inhibits cell motility, did not repress the flagellar master operon. These results imply that an unbalanced phospholipid composition, rather than a decrease or increase in the amount of specific phospholipid species, induces a phospholipid-specific stress signal to which certain regulatory genes respond positively or negatively according to their intrinsic mechanisms.

Physiological and biochemical analyses of FlgH, a lipoprotein forming the outer membrane L ring of the flagellar basal body of Salmonella typhimurium

The FlgH protein of Salmonella typhimurium, from which the outer membrane L ring of the flagellar basal body is constructed, has a consensus motif (LTG C) for lipoylation and signal peptide cleavage. We have confirmed the previous finding (M. Homma, K. Ohnishi, T. Iino, and R. M. Macnab, J. Bacteriol. 169:3617-3624, 1987) that it is synthesized in precursor form and processed to a mature form with an apparent molecular mass of ca. 25 kDa. flgH alleles with an in-frame deletion or a 3' truncation still permitted processing. The deletion permitted partial restoration of motility in complementation tests, whereas the truncation did not. Globomycin, an antibiotic which inhibits signal peptide cleavage of prolipoproteins, caused accumulation of precursor forms of FlgH. When cells transformed with a plasmid containing the flgH gene were grown in the presence of [3H]palmitate, a 25-kDa protein doublet was found to be radiolabeled; its identity as FlgH was confirmed by shifts in mobility when the internally deleted and truncated alleles of the gene were used. Hook-basal body complexes from cells grown in the presence of [3H]palmitate demonstrated that FlgH incorporated into flagellar structure was also labeled. An in-frame fusion between the leader sequence of the periplasmic protein PeIB and the mature FlgH sequence, with the putative N-terminal cysteine replaced by glycine, resulted in production of a fusion protein that was processed to its mature form. With a low-copy-number plasmid, the ability of this pelB-flgH fusion to complement a flgH mutant was poor, but with a high-copy-number plasmid, it was comparable to that of the wild type. Although lacking the N-terminal cysteine and therefore being incapable of lipoylation via a thioether linkage, the mutant protein still incorporated [3H]palmitate at low levels, perhaps through acylation of the N-terminal alpha-amino group. We conclude that FlgH is a lipoprotein and that under normal physiological conditions the lipoyl modification is necessary for FlgH to function properly as the L-ring protein of the flagellar basal body. We suggest that the N terminus of FlgH is responsible for anchoring the basal body in the outer membrane and that the C terminus may be responsible for binding to the P ring to form the L,P-ring complex.

Control by phosphatidylglycerol of expression of the flhD gene in Escherichia coli

We reported elsewhere that mutation in the pgsA gene, responsible for the synthesis of phosphatidylglycerol, repressed the synthesis of flagellin and caused the loss of motility of Escherichia coli (Tomura et al., FEBS Letters 329, 287-290, 1993). We now describe evidence for a decrease in promoter activity of the flhD gene, a master gene for flagellum synthesis, in the pgsA3 mutant. We constructed a plasmid with a promoter region of the flhD gene connected with the structure region of the lacZ gene. The activity of beta-galactosidase in the extract prepared from the pgsA3 mutant harboring the fusion plasmid was 30% of that in the wild type cells. This result means that phosphatidylglycerol is likely to be required for the initiation of transcription of the flhD gene. We also found that the motility-less phenotype of the mutant was partially suppressed by elevating incubation temperature. This suppression is caused by restoration of transcription of the flhD gene by high temperature. As the content of phosphatidylglycerol did not increase by elevating incubation temperature, we proposed that this suppression is caused by alternation of a physical structure of phospholipid bilayers in cytoplasmic membranes.

Loss of flagellation in dnaA mutants of Escherichia coli

A dnaA46 mutant of Escherichia coli showed loss of motility at 37 degrees C, a permissive temperature for cell growth of this mutant. Other dnaA mutations near the middle of the gene also caused an immotile phenotype. The amount of flagellin was much less in the dnaA46 mutant than in the wild-type control, as was the promoter activity. DnaA protein may play an important role in expression of the fliC gene.