Peptidoglycan and FtsZ
Peptidoglycanis the characteristic strengthening component of the cell membrane of virtually all bacteria. The Bacterial kingdom may conveniently be divided according to how they stain with the Gram stain. Gram positive bacteria differ from Gram negative bacteria in that they contain a higher level of peptidoglycan. Peptidoglycan consists of long carbohydrate (sugar) chains of N-acetyl glucosamine and N-acetyl galactosamine cross linked by peptide (amino acid) bridges to form a complete bag (a sacculus) around the bacterium. This enables bacteria, particularly Gram positive ones, to survive their higher than ambient internal osmotic pressure without "exploding". The disadvantage is that this peptidoglycan 'strait-jacket' has to be remodelled when bacteria divide in two.
Penicillins and cephalosporins work by preventing the formation of appropriate peptide cross bridges. The medical journal Lancet captured the essence of this process in an editorial many years ago entitled wittily "How bacteria knit their overcoats and how penicillin makes them drop their stitches". Chlamydiae are unusual in that numerous laboratory studies indicate that they contain little or no peptidoglycan in their cell wall. However whole genome sequencing indicates that chlamydiae do have a full set of genes for the biosynthesis of peptidoglycan. How may this paradox be resolved?
There is no doubt that chlamydiae have penicillin binding proteins and that penicillin, which targets peptidoglycan, certainly has an inhibitory effect on chlamydiae, with the exception of the Simkaniaceae. Just one among several studies found trace amounts of peptidoglycan in C. trachomatis elementary bodies [Su et al., 1985]. However, the elementary body has a quite refractory structure, designed uniquely for chlamydial survival and transit outside the host cell. Reticulate bodies are the most likely developmental stage to have peptidoglycan, as they are the most closely analogous to a typical gram negative bacterium, undergoing division, necessitating the remodelling of any trace amounts of peptidoglycan thereby making them vulnerable to penicillin. Reticulate bodies are osmotically fragile, as would be expected if they lack significant peptidoglycan. However as they are osmotically protected by the host cell they might have little need of peptidoglycan. Osmotic fragility outside the host makes reticulate bodies difficult to isolate pure. Consequently there has been only one attempt, and that was unsuccessful, to demonstrate peptidoglycan in chlamydial reticulate bodies [Barbour et al., 1982].
It seems probable that reticulate bodies have and require only trace amounts of peptidoglycan. This trace amount might be degraded by amidase enzymes (whose genes have been identified in the chlamydial genome) during the normal differentiation of elementary bodies from reticulate bodies [Chopra et al., 1998]. This hypothesis is consistent with the presumed lack of a structural requirement for peptidoglycan in elementary bodies; the outer envelope of elementary bodies is thought to gain most of its strength from the presence of a ?? of unknown composition and from cross linking of its cysteine-rich outer envelope proteins [see: cysteine rich proteins] with disulphide bonds. Less plausibly, it is suggested that the role of peptidoglycan in reticulate bodies might be to substitute for the FtsZ ring-shaped structures involved in cell envelope constriction and division in most bacteria. Unusually, the gene for this protein is missing in the chlamydial genome.
An alternative possibility is that chlamydiae synthesise a glycan-less peptidoglycan, effectively a polypeptide [Ghuysen & Coffin, 1999]. This stems from the observation that the C. trachomatis genome encodes two high molecular weight class B penicillin binding proteins, devoid of the enzymic (transglycolase) activity required for glycan (carbohydrate) chain elongation. Such an aberrant peptido (non)-glycan overcoat would be weaker, but this might again be compensated by other chlamydial outer envelope components.
Studies by McCoy et al., 2003 confirm the notion that peptidoglycan plays a crucial role in chlamydial development. They focussed on MurA, a UDP-N-acetylglucosamine enolpyruvyl transferase that catalyzes the first committed step of peptidoglycan synthesis and whose sequence is characteristic of all Chlamydiales [Griffiths & Gupta, 2002]. The murA gene from C. trachomatis serovar L2 was cloned and placed under the control of the arabinose-inducible, glucose-repressible ara promoter, then transformed into E. coli. After transduction of a lethal DeltamurA mutation into the strain, viability of the E. coli strain became dependent upon expression of the C. trachomatis murA, indicating that the chlamydial gene product was functional. DNA sequence analysis of murA from C. trachomatis revealed a cysteine-to-aspartate change in a key residue within the active site of MurA. In E. coli, the same mutation has previously been shown to cause resistance to fosfomycin, a potent antibiotic that specifically targets MurA. In vitro activity of the chlamydial MurA was resistant to high levels of fosfomycin. Growth of C. trachomatis was also resistant to fosfomycin. Moreover, fosfomycin resistance was imparted to the E. coli strain expressing the chlamydial murA. Conversion of C. trachomatis elementary bodies to reticulate bodies and cell division are correlated with the expression of murA mRNA. mRNA from murB, the second enzymatic reaction in the peptidoglycan pathway, was also detected during C. trachomatis infection. It was concluded that a functional peptidoglycan pathway exists in Chlamydia species and that it is essential for chlamydial development and cell division.
[See also:the evolution of FtsZ]
[MEW] June 2003
NEXT: C. trachomatis Chlamydial plasmids
Barbour, A. G., Amato, K.-I., Hackstadt, T et al., (1982). Chlamydia trachomatis has penicillin-binding proteins but not detectable muramic acid. Journal of Bacteriology 151, 420 - 428.
Chopra, I., Storey, C., Falla, T. J. & Pearce, J. H. (1998). Antibiotics, peptidoglycan synthesis and genomics: the chlamydial anomaly revisited. Microbiology 144, 2673 - 2678. [Review] Full article
Ghuysen, J. M. & Goffin, C. (1999). Lack of cell wall peptidoglycan versus penicillin sensitivity: new insights into the chlamydial anomaly. Antimicrobial Agents and Chemotherapy 43, 2339 - 2344. [Major review] Full article
Griffiths, E. & Gupta, R. S. (2002). Protein signatures distinctive of chlamydial species: horizontal transfers of cell wall biosynthesis genes glmU from archaea to chlamydiae and murA between chlamydiae and Streptomyces. Microbiology 148, 2541 - 2549.
McCoy, A. J., Sandlin, R. C. & Maurelli, A. T. (2003). In vitro and in vivo functional activity of Chlamydia MurA, a UDP-N-acetylglucosamine enolpyruvyl transferase involved in peptidoglycan synthesis and fosfomycin resistance. Journal of Bacteriology 185, 1218 - 1228.
Su, H., Zhang, Y. X. & Li, R. (1985). Presence of muramic acid in Chlamydia trachomatis proved by liquid chromatography-mass spectrometry. Kexue Tongbao 30, 695 - 699. [ Never been able to find this paper since!]
NEXT: C. trachomatis the chlamydial plasmid
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