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Introduction
Peptidoglycan (or murein) forms a mesh-like sacculus surrounding the cytoplasmic membrane of almost all eubacteria.1 Bacterial peptidoglycan maintains turgor pressure and cell shape in addition to functioning in cell division. The primary glycan chain structure of peptidoglycan consists of alternating β-1,4-linked N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc) residues. Neighboring glycan chains are interlinked by short peptides attached to MurNAc.
In the bacterial peptidoglycan biosynthetic pathway, GlcNAc-MurNAc-pentapeptide monomer units synthesized in the cytoplasm are transferred to the periplasm and cross-linked to pre-existing peptidoglycans by penicillin-binding proteins (PBPs). Peptidoglycan synthesis requires two activities: transglycosylase (TG) to polymerize the glycan chains and transpeptidase (TP) to catalyze peptide cross-linking between two adjacent glycan chains.2 Among the three classes (A, B, and C) of PBPs, the class A-type high molecular mass (HMM) PBPs are major enzymes for peptidoglycan synthesis. They are anchored in the cytoplasmic membrane by the transmembrane (TM) region at the N-terminus and have TG and TP domains in the periplasm (schematic representation of PBP from the cyanobacterium Anabaena in Fig. 2a). Class B-HMM PBPs have an N-terminal domain for cell morphogenesis in addition to the TP domain at the C-terminus, and class C-type low molecular mass (LMM) PBPs are monofunctional and related to cell separation, peptidoglycan maturation, and its recycling.2
Figure 2.
Complementation of ΔPpPbp with Anabaena PBP. (a) Schematic representations of PpPBP, AnaPBP, the N-terminus of PpPBP fused to Anabaena PBP (CP-AnaPBP), and domain swapped PpPBP. Boxes with horizontal lines or light green or light blue fill in PpPBP indicate putative plastid (chloroplast)-targeting sequence (CP), transglycosylase (TG), and transpeptidase (TP) domains, respectively. Anabaena PBP is shown as a gray box with the transmembrane region (red), and TG (dark green) and TP (dark blue) domains. AnaPBP with the plastid-targeting sequence (CP-AnaPBP) and domain swapped PpPBP (PpPBP-AnaTG and PpPBP-AnaTP) are also shown. The plasmids expressing these Pbp genes were transformed into ΔPpPbp . (b) Protonemal cells of the CP-AnaPBP#28, PpPBP-AnaTG#89, and PpPBP-AnaTP#22 transformants are shown. (c) The number of chloroplasts in subapical cells was counted.
It is now widely accepted that an endosymbiotic cyanobacterium evolved into the plastids of green plants. Although free-living bacteria typically have peptidoglycan in their cell walls, it is believed that the plastids of green plants lost peptidoglycan...