Role of FtsEX in cell division of Escherichia coli: viability of ftsEX mutants is dependent on functional SufI or high osmotic strength

Abstract

In Escherichia coli, at least 12 proteins, FtsZ, ZipA, FtsA, FtsE/X, FtsK, FtsQ, FtsL, FtsB, FtsW, FtsI, FtsN, and AmiC, are known to localize to the septal ring in an interdependent and sequential pathway to coordinate the septum formation at the midcell. The FtsEX complex is the latest recruit of this pathway, and unlike other division proteins, it is shown to be essential only on low-salt media. In this study, it is shown that ftsEX null mutations are not only salt remedial but also osmoremedial, which suggests that FtsEX may not be involved in salt transport as previously thought. Increased coexpression of cell division proteins FtsQ-FtsA-FtsZ or FtsN alone restored the growth defects of ftsEX mutants. ftsEX deletion exacerbated the defects of most of the mutants affected in Z ring localization and septal assembly; however, the ftsZ84 allele was a weak suppressor of ftsEX. The viability of ftsEX mutants in high-osmolarity conditions was shown to be dependent on the presence of a periplasmic protein, SufI, a substrate of twin-arginine translocase. In addition, SufI in multiple copies could substitute for the functions of FtsEX. Taken together, these results suggest that FtsE and FtsX are absolutely required for the process of cell division in conditions of low osmotic strength for the stability of the septal ring assembly and that, during high-osmolarity conditions, the FtsEX and SufI functions are redundant for this essential process.

FIG. 1.

Growth and filamentation of ftsEX mutants. (A) Growth of the wild type (wt) and of ftsEX mutants on LB at different temperatures. (B) DIC micrographs of cells grown in LB broth at different temperatures.

FIG. 2.

Growth and filamentation of ftsEX mutants. (A) Growth of the wild type (wt) and ftsEX mutants on LBON plates supplemented with 0.4 M osmolytes. (B) Growth of MR10 (ΔftsEX210::Kan) in LBON broth (diamonds), LBON supplemented with either 0.4 M glucose (closed circles) or 0.4 M NaCl (open circles) at 30°C. OD₆₀₀, optical density at 600 nm. (C) DIC micrographs of cells grown in LB without NaCl or with added osmolytes at 30°C. (D) DIC micrograph of a single filament of the ftsEX mutant and the corresponding fluorescence image after DAPI staining.

FIG. 3.

Effect of multicopy suppressor plasmids on growth of ftsEX. (A) Cultures of ftsEX carrying vector pCL1920 or its derivatives with cloned ftsYEX, ftsN, ftsQAZ, and sdiA genes or plasmid pJK537 (pBR322 with dksA) are grown at 30°C in LB, and 10-μl aliquots of various dilutions (10⁻², 10⁻⁴, 10⁻⁵, 10⁻⁶, and 10⁻⁷) are applied on LBON plates and grown for 32 h at 30°C. (B) DIC micrographs of the same cells grown in LBON to midexponential phase at 30°C.

FIG. 4.

Effect of ftsEX deletion on division-defective mutations. (A) Growth of strain MR11 (ΔftsEX210::Kan/pMN6) and its mutant derivatives on LB-Amp-IPTG, LB, or LB supplemented with 0.4 M glucose at 30°C. (B) Growth of ftsEX and ftsEX ftsZ84 mutants. Cultures are grown in LB at 30°C, and 10 μl of various dilutions are spotted on LBON (the plate incubated at 37°C has 10⁻², 10⁻⁴, and 10⁻⁵ dilutions) or LB plates, followed by incubation at 30°C or 37°C for 32 h. (C) DIC micrographs of the cultures grown in LB at 37°C.

FIG. 5.

Interactions of ftsE with sufI. (A) Synthetic lethal mutants of ftsE. Growth of ftsE sufI or ftsE tatB mutants carrying plasmid pMN6. Cultures were grown with IPTG and dilutions were spotted on LB-Amp-IPTG, LB, or LB-glucose plates. (B) Effect of multicopy sufI on the growth and filamentation of ftsEX mutants.