The Metabolic Inertness of the Pole Wall of a Gram-negative Rod

During the growth cycle of Escherichia coli, elongation takes place by insertion of new murein into the thin cylindrical side wall. Where cell division is to take place, the cell inserts new wall at a high rate. However, it is concluded here that after separation of sisters incorporation of new murein into the mature poles wall is negligibly small. By reanalysis of published high-resolution electron microscope autoradiographic studies of the pulse incorporation of tritium-labeled diaminipimelic acid, earlier interpretations are shown to be incorrect. Reanalysis of these studies is possible using a computer program that allowed the data, as published, to be corrected in two ways: (i) for the distribution of silver grains around the site of a tritium decay; and (ii) for the effects of the normalization of cell lengths which had been used in presenting the primary data. In addition, we have reanalyzed published work on LamB and lipoprotein incorporation and conclude that incorporation of LamB and the lipoprotein, like the murein layer, into the outer membrane of the completed pole is essentially nil. Thus, in contrast to our earlier papers, it is concluded that three components of the envelopes of the established poles are quite inert and therefore the poles could serve as a stable support for cylindrical side-wall elongation, consistent with the key assumption of the surface stress theory. The possibility of continued insertion of new cytoplasmic membrane in an established pole is not covered by the available experimental data. A second conclusion is that incorporation of murein may occur during the development of the constriction site of the nascent pole over a much larger area than just the tip of the constriction.