Cell Wall Structures and their Phylogenetic Implications

The distribution of the various cell wall structures and polymers found in the living world fits together very well with Woese's concept, based on 16S ribosomal RNA sequence similarity, that 3 urkingdoms have emerged from a common ancestor. It is assumed that the common ancestor did not contain a rigid cell envelope, whereas the primitive eubacterial ancestor developed a peptidoglycan sacculus, characterized by muramic acid and a sequence of alternating L- and D-amino acids. This so-called murein occurs in two major structural modifications - monomolecular layer (Gram-negatives) or multilayer (Gram-positives) - and more than 100 chemotypes in all Eubacteria except the Mycoplasmatales. Each chemotype is more or less typical of certain taxa of bacteria. Within the Archaebacteria the cell wall is probably of polyphyletic origin, since no common sacculus or envelope polymer is found. Only after the cell wall-lacking primitive archaebacteria had branched into various physiologically different lines were cell walls independently developed within some of the lines. In the Methanobacteriales a peptidoglycan with a chemical structure quite different from that of murein is found, the so-called pseudomurein, which contains talosaminuronic acid instead of muramic acid and in which D-amino acids are missing. The genus Halococcus exhibits a sacculus consisting of a highly sulfated hetero-polysaccharide containing gulosaminuronic acid, whereas the sacculus of Methanosarcina consists of a different heteropolysaccharide lacking sulfate residues and gulosaminuronic acid. The archaebacteria of all the other groups do not possess a rigid sacculus, but an envelope of protein or glycoprotein subunits or a fibrillary protein sheath enclosing short chains of cells. Only Thermoplasma has not developed any kind of cell envelope.

Within the Eucaryotes the cell wall is probably also of polyphyletic origin. Whereas the animals lack a cell wall, plants, fungi and algae are characterized by rigid cell walls consisting of cellulose, chitin and a variety of heteropolysaccharides, respectively. A diagram demonstrating the correlation between the 16S rRNA-relatedness and the distribution of cell wall structures and polymers within the 3 urkingdoms is presented.