Classic Spotlight: Staying in Shape and Discovery of the mrdAB and mreBCD Operons

EDITORIAL

How nonspherical bacteria maintain their shape is a long-standing question in bacteriology. That the peptidoglycan (PG) sacculus is critical to maintain both cell shape and integrity was established early on when treatment of bacterial rods with lysozyme was found to convert them into protoplasts that are both spherical and osmotically unstable (1). The first evidence that cell shape is also controlled by factors that are not required for building a sacculus per se came from a famous “large-scale” screen of 400 temperature-sensitive Escherichia coli mutants at the Pasteur Institute. Some of these formed spherical cells at 42°C. Moreover, these cells were osmotically stable, indicating that they still possessed an intact but spherical PG sacculus (2). The next decade saw the isolation of additional cell shape mutants of both E. coli and Bacillus species by various laboratories. Many of these propagated as osmotically stable spheroids in a nonconditional fashion, and corresponding mutations in E. coli mapped to two regions of the chromosome: near lipA at ∼14 min (mrd, rodA, and rodX alleles) and near rpsL at ∼73 min (mre, mon, envB, and rodY alleles). Meanwhile, the β-lactam amdinocillin (compound FL1060) became available as an important drug and tool (3). The drug causes E. coli to phenocopy spherical mutants (3), and Spratt and Pardee identified its target as a penicillin-binding protein that they named PBP2 (4). Several groups then showed that selection for amdinocillin resistance (Mec^r) frequently yields mutants with a spherical cell phenotype very similar to those described earlier. In a series of 1980s Journal of Bacteriology papers, Michio Matsuhashi and colleagues used this Mec^r selection to (i) obtain a set of isogenic cell shape mutants, (ii) map mutations on the chromosome, and (iii) isolate and study the affected genes. Mutations in the spherical Mec^r mutants again mapped to either of the mrd or mre regions identified before (5). Work by Matsuhashi, Spratt, and others then first established that the mrd region contains two genes in an operon that are essential for rod shape maintenance. One was shown to encode PBP2 and is called pbpA or mrdA, and the other is designated rodA or mrdB (5, 6). The subsequent identification and isolation of the relevant cell shape genes in the mre region started with a classic paper from 1987 (7). The authors isolated a specialized transducing λ phage that could correct the shape defect of mre mutants, one of which contained a sizeable chromosomal deletion (Δmre-678). Phage DNA was then used to construct a set of plasmids carrying different fragments of the mre locus, and cell shape complementation assays with the plasmids implied that the locus contains at least two genes required for maintenance of rod shape (7). This was confirmed soon after by additional complementation results, DNA sequencing, and in vitro translation assays, demonstrating that the protein products of three contiguous genes in the locus (mreB, mreC, and mreD) are all required to convert spherical Δmre-678 cells back to rod shape (8, 9). The discoveries of the mrdAB and mreBCD operons form the basis of all subsequent work on understanding how the proteins they encode collaborate to dictate a cylindrical shape to the PG sacculus. Of course, the discovery of MreB (7, 8) was also more broadly important, as it eventually led to the realization that bacteria invented not just tubulin (FtsZ) but actin (MreB) as well.

The views expressed in this Editorial do not necessarily reflect the views of the journal or of ASM.