Essential Nature of the mreC Determinant of Bacillus subtilis

Abstract

The mre genes of Escherichia coli and Bacillus subtilis are cell shape determination genes. Mutants affected in mre function are spheres instead of the normal rods. Although the mre determinants are not required for viability in E. coli, the mreB determinant is an essential gene in B. subtilis. Conflicting results have been reported as to whether the two membrane-associated proteins MreC and MreD are essential proteins. Furthermore, although the MreB protein has been studied in some detail, the roles of the MreC and MreD proteins in cell shape determination are unknown. We constructed a strain of B. subtilis in which expression of the mreC determinant is dependent upon the addition of isopropyl-␤-D-thiogalactopyranoside to the culture medium. Utilizing this conditional strain, it was shown that mreC is an essential gene in B. subtilis. Furthermore, it was shown that cells lacking sufficient quantities of MreC undergo morphological changes, namely, swelling and twisting of the cells, which is followed by cell lysis. Electron microscopy was utilized to demonstrate that a polymeric material accumulated at one side of the division septum of the cells and that the presence of this material correlated with the bending of the cell. The best explanation for the results is that the MreC protein is involved in the control of septal versus long-axis peptidoglycan synthesis, that cells lacking MreC perform aberrant septal peptidoglycan synthesis, and that lysis results from a deficiency in long-axis peptidoglycan synthesis.Bacteria have characteristic shapes, which are important taxonomic features. Most bacteria possess a fixed shape, although members of the genus Arthrobacter transition from rods to cocci depending on growth conditions (12). The bacterial cell wall is thought to be the major determinant of cell shape, as the purified murein sacculus retains the characteristic morphology of the cell, and specific changes in the chemical composition of the peptidoglycan have been noted in shape-altered mutants (25). However, shape determination is a complicated process, which involves not only the topology of peptidoglycan biosynthesis but also contributions of other components of the cell envelope as well as involvement of the cell division machinery. A mutation in the tagF (rodC) determinant of Bacillus subtilis gives rise to a shift from rod-shaped to spherical cells, indicating a contribution by teichoic acid to the determination of cell morphology (4,16,23).A model for bacterial shape regulation, the two-competingsites model, which relates cell shape to a balance between cell wall synthesis along the long axis of a rod-shaped cell and septum-specific peptidoglycan synthesis, has been proposed (21,24). This model proposes that shape determination in bacterial rods depends on the activity of two peptidoglycan synthesis reactions (sites) which are mutually exclusive. Consequently, the lateral wall is not extended during the septum formation step of cell division, and septal peptidoglycan synthesis does not occur during lat...

The Bacterial Actin-Like Cytoskeleton

“…mreC has also been shown to be essential in C. crescentus, where MreC-depleted cells also displayed morphological defects identical to those of cells depleted of MreB (or treated with A22) (40). Thus, it is currently believed that MreBCD proteins function in the same morphogenetic pathway (14,35,40,88,95,96). Consistently, MreC and MreD in B. subtilis (95) and MreC in C. crescentus (35,40) (Fig.…”

“…In both E. coli and B. subtilis, the three cell shape determinants encoded by mreBCD are essential and required for the maintenance of the rod shape of the cell, and depletion of each mre gene separately confers similar gross changes in cell morphology (24,88,95,96). mreC has also been shown to be essential in C. crescentus, where MreC-depleted cells also displayed morphological defects identical to those of cells depleted of MreB (or treated with A22) (40).…”

Coarse-grained simulations of bacterial cell wall growth reveal that local coordination alone can be sufficient to maintain rod shape

“…In our model, direct transfer of PG precursors from the cytoplasm to transglycosylases facilitates processivity and rapid re-initiation. Consistent with these ideas, the putative bridging proteins MreC, MreD, RodA, and RodZ have been shown to be essential for rod shape maintenance (21,74,(77)(78)(79)(80)(81)(82)(83) and interact with both cytoplasmic (84) and periplasmic enzymes (85,86). Also, treatment with A22 to depolymerize MreB dramatically reduces PG synthesis in the periplasm (87) and results in shorter glycan strands (88).…”

RodZ, a component of the bacterial core morphogenic apparatus

“…How this occurs is not fully understood, but other proteins of the core morphogenic apparatus, such as MreC, MreD, and RodA are likely to be involved. While the exact function of these membrane-bound or periplasmic proteins remains somewhat unclear, evidence suggests that they regulate PG growth by linking MreB to cell wall enzymes or by working in concert with MreB to spatially restrict cell wall activities (9)(10)(11)(12)(13)(14)(15)(16)(17).…”