Turnover and recycling of the murein sacculus in oligopeptide permease-negative strains of Escherichia coli: indirect evidence for an alternative permease system and for a monolayered sacculus

Abstract

Turnover of murein in oligopeptide permease-negative Escherichia coli cells appeared to be minimal or nonexistent. In one strain in which it was possible to measure turnover during the first generation of chase, it was found that the rate of turnover was constant throughout a chase of three generations. This result suggests that an "inside-to-outside" mode of growth of the sacculus does not occur in E. coli. Turnover, though minimal, was significantly higher from cells labeled uniformly than from cells labeled only in the lateral wall, suggesting that a significant portion of the observed turnover is related to cell separation. Actually, turnover only appeared to be minimal in opp mutant strains. Tripeptides were being released by turnover at a rate of about 509% per generation and then were efficiently recycled. This suggests that in addition to opp, a low-affinity uptake system for tripeptide derived from the sacculus may exist.For many years, it was thought that the Escherichia coli cell wall consisted of a single layer of murein and that during growth there was little or no release of wall material. Both of these assumptions have recently been questioned. Since 1984, several lines of evidence have suggested that the murein sacculus of exponentially growing E. coli cells might be multilayered, with a thickness of up to three layers. Electron microscopic observations based on new embedding techniques and freeze substitution revealed a periplasmic space filled with an electron-dense layer of uniform thickness which was several times thicker than the peptidoglycan layer observed in previous electron micrographs (9). The authors proposed that the peptidoglycan formed a gel consisting of a cross-linked outer shell with additional lesscross-linked material attached to the inner surface. Consistent with this view was the observation that staining of thin sections of E. coli by the Rambourg technique (phosphotungstic acid in 10% chromic acid) revealed a similar 15-nmthick layer in the periplasm apparently consisting primarily of peptidoglycan (13) Holtje and Glauner have reported the presence of trimers in muramidase digests of E. coli sacculi and have suggested that this is another indication that multiple layers may exist, since a trimer cannot be present in a monolayer. These authors went on to suggest that a multilayered wall, should one exist in E. coli, ought to be formed by the "inside-tooutside" mechanism that is known to occur in gram-positive bacilli (10).In the case of Bacillus subtilis, after the murein is labeled for a short pulse and then chased, there is a lag of about one generation before any radioactivity is released into the medium. After the first generation, about 50% of the radioactivity of the cylindrical wall is released per generation (15). This is interpreted to indicate that peptidoglycan is initially laid down next to the cytoplasmic membrane and eventually reaches the external surface, where it is degraded by autolytic enzymes. Since the principal enzymes used to polymerize and cross-link...

Modified peptidoglycan chemical composition in shape-altered Escherichia coli

“…However, on comparing and integrating his results with those of Labischinski et al (1991), Park (1993) also suggested speculatively that, while lateral wall PG may be monolayered, septal wall PG (and poles) may be triplelayered. Now, for the first time, biochemical evidence supporting this conclusion has been found in E. coli.…”

Peptidoglycan structure of Salmonella typhimurium growing within cultured mammalian cells

“…Their physiological functions are still uncertain, although they have been implicated in autolysis and/or peptidoglycan recycling (Parquet et al, 1983;Kitano et al, 1986;Goodell and Higgins, 1987;Shockman and Hö ltje, 1994). The relevance of AmiA and AmiB in peptidoglycan recycling has been questioned by the recent finding in E. coli of a muropeptide-permease, AmpG, and a cytosolic N-acetyl-muramyl-L-alanine amidase, AmpD, which apparently mediate the main recycling pathway (Park, 1993;Hö ltje et al, 1994;Jacobs et al, 1994;. Incidentally, a DNA fragment with extensive homology to E. coli ampD has also been identified in S. typhimurium (Hughes et al, 1993).…”

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

“…Cells release PG during growth (72,73), but the mechanism of PG turnover is not clear. It was proposed in the 'three-for-one' model that PG turnover is a result of a PG remodeling mechanism that replaces one existing strand with three new strands (12).…”