This work was conducted on Pinctada maxima nacre (mother of pearl) in order to understand its multiscale ordering and the role of the organic matrix in its structure. Intermittent-contact atomic force microscopy with phase detection imaging reveals a nanostructure within the tablet. A continuous organic framework divides each tablet into nanograins. Their shape is supposed to be flat with a mean extension of 45nm. TEM performed in the darkfield mode evidences that at least part of the intracrystalline matrix is crystallized and responds like a 'single crystal'. The tablet is a 'hybrid composite'. The organic matrix is continuous. The mineral phase is thus finely divided still behaving as a single crystal. It is proposed that each tablet results from the coherent aggregation of nanograins keeping strictly the same crystallographic orientation thanks to a hetero-epitaxy mechanism. Finally, high-resolution TEM performed on bridges from one tablet to the next, in the overlying row, did not permit to evidence a mineral lattice but crystallized organic bridges. The same organic bridges were evidenced by SEM in the interlaminar sequence.

Physiological properties of the murG gene product of Escherichia coli were investigated. The inactivation of the murG gene rapidly inhibits peptidoglycan synthesis in exponentially growing cells. As a result, various alterations of cell shape are observed, and cell lysis finally occurs when the peptidoglycan content is 40% lower than that of normally growing cells. Analysis of the pools of peptidoglycan precursors reveals the concomitant accumulation of UDP-N-acetylglucosamine (UDP-GlcNAc) and UDP-N-acetylmuramyl-pentapeptide (UDPMurNAc-pentapeptide) and, to a lesser extent, that of undecaprenyl-pyrophosphoryl-MurNAc-pentapeptide (lipid intermediate I), indicating that inhibition of peptidoglycan synthesis occurs after formation of the cytoplasmic precursors. The relative depletion of the second lipid intermediate, undecaprenyl-pyrophosphorylMurNAc-(pentapeptide)GlcNAc, shows that inactivation of the murG gene product does not prevent the formation of lipid intermediate I but inhibits the next reaction in which GlcNAc is transferred to lipid intermediate I. In vitro assays for phospho-MurNAc-pentapeptide translocase and N-acetylglucosaminyl transferase activities finally confirm the identification of the murG gene product as the transferase that catalyzes the conversion of lipid intermediate I to lipid intermediate II in the peptidoglycan synthesis pathway. Plasmids allowing for a high overproduction of the transferase and the determination of its N-terminal amino acid sequence were constructed. In cell fractionation experiments, the transferase is essentially associated with membranes when it is recovered.The biosynthesis of bacterial cell wall peptidoglycan is a complex process involving many different cytoplasmic and membrane steps (10,49). Conditional-lethal mutants altered at different levels of this metabolic pathway have been described previously, and most of the mutations have been mapped (5,43,49,50,52,56). In particular, the 2-min region of the Escherichia coli chromosome contains a large cluster of genes from pbpB to envA that code for proteins involved in different aspects of peptidoglycan synthesis and cell division. The organization of the genes in this cluster is now completely elucidated, and the nucleotide sequence of the whole 17-kbp region has been determined (11-13, 16, 37, 39, 41; see reference 35 for references before 1990). The genes, tightly packed and overlapping in many cases, appear in the following order: pbpB-murE-murF-ORF-Y-murD-ftsW-murGmurC-ddl-ftsQ-ftsA-ftsZ-envA. The murC, murD, murE, murF, and ddl gene products were identified previously as L-alanine, D-glutamic acid, meso-2,6-diaminopimelic acid (DAP), D-alanyl-D-alanine-adding enzymes, and D-alanine: D-alanine ligase, respectively (10, 26-28, 32, 35, 43), which are involved in the synthesis of the soluble nucleotide precursors from UDP-N-acetylmuramic acid (UDP-MurNAc) to UDP-MurNAc-L-Ala-y-D-Glu-meso-DAP-D-Ala-DAla (UDP-MurNAc-pentapeptide). These genes are flanked on the left by a cell division gene named pbpB (1, 52) or ...