Phospholipid changes in seqA and dam mutants of Escherichia coli

Effects of static magnetic fields on growth and membrane lipid composition of Salmonella typhimurium wild-type and dam mutant strains

This study was carried out to explore the adaptive mechanisms of S. typhimurium particularly, the implication of the Dam methyltransferase in the remodelling of membrane lipid composition to overcome magnetic field stress. With this aim, we focused our analyses on the increase in viable numbers and membrane lipid modifications of S. typhimurium wild-type and dam mutant cells exposed for 10h to static magnetic fields (SMF; 200 mT). For the wild-type strain, exposure to SMF induced a significant decrease (p<0.05) of CFU at 6h, followed by an increase between 8 and 10h. Growth of the dam mutant was significantly affected (p<0.05) after 6h and no recovery was observed until 10h, highlighting a different behavior of SMF stressed wild-type and dam mutant strains. SMF significantly affected the phospholipid proportions in the two strains. The most affected were those of the acidic phospholipids, cardiolipins (CL). In the dam strain the phospholipid response to SMF followed a globally similar trend as in the wild-type with however lower effects, leading mainly to an unusual accumulation of CL. This would in part explain the different behavior of the wild-type and the dam strain. Results showed a significant increase of membrane cyclic fatty acids Cyc17 and Cyc19 in the wild-type strain but only the Cyc17 in the dam strain and a meaningful increase of the total unsaturated fatty acids (UFAs) to total saturated fatty acids (SFAs) ratios of the exposed cells compared to controls from 3 to 9h (p<0.05) for both strains. The net increase of the total UFAs to total SFAs ratios seemed to result mainly from the increase of (C18:1) proportion (p<0.05) and to a lower extent from that of (C16:1) (p<0.05). These modifications of cyclic and unsaturated fatty acid proportions constitute an adaptive response to SMF stress in S. typhimurium wild-type and dam mutants to maintain an optimum level of membrane fluidity under SMF.

Proteomic analyses of nucleoid-associated proteins in Escherichia coli, Pseudomonas aeruginosa, Bacillus subtilis, and Staphylococcus aureus

Background

The bacterial nucleoid contains several hundred kinds of nucleoid-associated proteins (NAPs), which play critical roles in genome functions such as transcription and replication. Several NAPs, such as Hu and H-NS in Escherichia coli, have so far been identified.

Methodology/Principal Findings

Log- and stationary-phase cells of E. coli, Pseudomonas aeruginosa, Bacillus subtilis, and Staphylococcus aureus were lysed in spermidine solutions. Nucleoids were collected by sucrose gradient centrifugation, and their protein constituents analyzed by liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS). Over 200 proteins were identified in each species. Envelope and soluble protein fractions were also identified. By using these data sets, we obtained lists of contaminant-subtracted proteins enriched in the nucleoid fractions (csNAP lists). The lists do not cover all of the NAPs, but included Hu regardless of the growth phases and species. In addition, the csNAP lists of each species suggested that the bacterial nucleoid is equipped with the species-specific set of global regulators, oxidation-reduction enzymes, and fatty acid synthases. This implies bacteria individually developed nucleoid associated proteins toward obtaining similar characteristics.

Conclusions/Significance

Ours is the first study to reveal hundreds of NAPs in the bacterial nucleoid, and the obtained data set enabled us to overview some important features of the nucleoid. Several implications obtained from the present proteomic study may make it a landmark for the future functional and evolutionary study of the bacterial nucleoid.

Escherichia coli DnaA forms helical structures along the longitudinal cell axis distinct from MreB filaments

DnaA initiates chromosomal replication in Escherichia coli at a well-regulated time in the cell cycle. To determine how the spatial distribution of DnaA is related to the location of chromosomal replication and other cell cycle events, the localization of DnaA in living cells was visualized by confocal fluorescence microscopy. The gfp gene was randomly inserted into a dnaA-bearing plasmid via in vitro transposition to create a library that included internally GFP-tagged DnaA proteins. The library was screened for the ability to rescue dnaA(ts) mutants, and a candidate gfp-dnaA was used to replace the dnaA gene of wild-type cells. The resulting cells produce close to physiological levels of GFP-DnaA from the endogenous promoter as their only source of DnaA and somewhat under-initiate replication with moderate asynchrony. Visualization of GFP-tagged DnaA in living cells revealed that DnaA adopts a helical pattern that spirals along the long axis of the cell, a pattern also seen in wild-type cells by immunofluorescence with affinity purified anti-DnaA antibody. Although the DnaA helices closely resemble the helices of the actin analogue MreB, co-visualization of GFP-tagged DnaA and RFP-tagged MreB demonstrates that DnaA and MreB adopt discrete helical structures along the length of the longitudinal cell axis.

SeqA, the Escherichia coli origin sequestration protein, can regulate the replication of the pBR322 plasmid

The pBR322 plasmid origin replication and oriC show similar responses to adenine methylation. Both are subject to sequestration by membrane fractions. In fact, like the host origin oriC, the RNA II promoter region of pBR322 is regulated by methylation of three GATC adenine methylation sites. The SeqA gene product acts in the negative control of oriC by sequestration. We suggest that the role of SeqA protein in sequestration is similar to oriC region DNA. Hence, SeqA recognize the methylation state of the pBR322RNA II promoter region by direct DNA binding in vitro. Using the pOC42 plasmid, we show that SeqA binds exclusively to the hemimethylated form of the replication origin of the pBR322 plasmid. In addition, we suggested that the SeqA protein could modulate periodically the initiation of replication of the pBR322 plasmid. The later could be fixed by its origin sequence, on a hemimethylated state, during the initiation of the replication.

Reduced lipopolysaccharide phosphorylation in Escherichia coli lowers the elevated ori/ter ratio in seqA mutants

The seqA defect in Escherichia coli increases the ori/ter ratio and causes chromosomal fragmentation, making seqA mutants dependent on recombinational repair (the seqA recA colethality). To understand the nature of this chromosomal fragmentation, we characterized DeltaseqA mutants and isolated suppressors of the DeltaseqA recA lethality. We demonstrate that our DeltaseqA alleles have normal function of the downstream pgm gene and normal ratios of the major phospholipids in the membranes, but increased surface lipopolysaccharide (LPS) phosphorylation. The predominant class of DeltaseqA recA suppressors disrupts the rfaQGP genes, reducing phosphorylation of the inner core region of LPS. The rfaQGP suppressors also reduce the elevated ori/ter ratio of the DeltaseqA mutants but, unexpectedly, the suppressed mutants still exhibit the high levels of chromosomal fragmentation and SOS induction, characteristic of the DeltaseqA mutants. We also found that colethality of rfaP with defects in the production of acidic phospholipids is suppressed by alternative initiation of chromosomal replication, suggesting that LPS phosphorylation stimulates replication initiation. The rfaQGP suppression of the seqA recA lethality provides genetic support for the surprising physical evidence that the oriC DNA forms complexes with the outer membrane.

The GATC-binding protein SeqA is required for bile resistance and virulence in Salmonella enterica serovar typhimurium

Disruption of the seqA gene of Salmonella enterica serovar Typhimurium causes defects similar to those described in E. coli: filament formation, aberrant nucleoid segregation, induction of the SOS response, envelope instability, and increased sensitivity to membrane-damaging agents. Differences between SeqA(-) mutants of E. coli and S. enterica, however, are found. SeqA(-) mutants of S. enterica form normal colonies and do not exhibit alterations in phage plaquing morphology. Lack of SeqA causes attenuation of S. enterica virulence by the oral route but not by the intraperitoneal route, suggesting a virulence defect in the intestinal stage of infection. However, SeqA(-) mutants are fully proficient in the invasion of epithelial cells. We hypothesize that attenuation of SeqA(-) mutants by the oral route may be caused by bile sensitivity, which in turn may be a consequence of envelope instability.

Effect of seqA mutation on Salmonella typhimurium virulence

It is well established that lack of SeqA protein leads to reinitiation at the same origin more than once per cell cycle, in E. coli. Aberrant cell division and filament formation in SeqA-deficient strains suggest that this protein might be involved in cell cycle process other than transient inhibition of replication initiation. The aim of our work was to examine the effect of lack of this protein on Salmonella typhimurium virulence. In the present study, Swiss albino mice were used for the determination of LD50, the competitive index and detection of bacteria in target organs. In vitro assays were used to determine the sensitivity of either wild-type and seqA mutant to hydrogen peroxide and bile salts. The seqA mutant strain of Salmonella typhimurium is attenuated for virulence in mice. seqA mutant is highly sensitive towards hydrogen peroxide and bile salts compared with the isogenic wild-type. The 50% lethal dose of seqA mutant were found to be significantly increased compared to the wild-type strain. In addition, enumeration of bacteria from target organs (spleen and liver) showed that the number of wild-type bacteria recovered from these organs was higher than SeqA-deficient cells during the infection. Also, competitive index demonstrated that seqA mutant was significantly out competed by the wild-type strain in both intraperitoneal and oral infections. In addition, our data showed that both adhesion and invasion of Salmonella typhimurium seqA mutant are reduced. According to these results, we can suggest that Salmonella typhimurium seqA mutant is attenuated for virulence in mice.