under high osmotic pressure when the synthesis of the former is repressed (Section 12.2.9). PhoE is synthesized under phosphate-limited conditions and is specific for anions. The pore size of porins varies depending on the species, and is regulated by the molecular arrangement around it. Nutrients With a higher molecular weight cross the OM through receptor proteins. The speed of diffusion across the OM through non-specific porins and receptors is dependent on the concentration gradient, but the speed increases only up to the saturation concentration through the poiins and receptors specific for that nutrient. OmpA functions as a receptor for various amino acids and peptides and for F-pili. Various other receptors are identified for nucleosides, vitamins and other nutrients. Lipoprotein stabilizes OM structure. The lipid end of the molecule is embedded in the lipid area of OM, while the protein end is covalently bound to murein. A lipoprotein in a plant pathogenic bacterium, Erwinia chrysanthemi, has pectin methylesterase activity. Like other microbes, many Gram-negative bacteria can use polymers as their carbon and energy sources (Section 7.1) and excrete hydrolyzing enzymes into their external environment. The OM has proteins responsible for the translocation of extracellular proteins (Section 3.8.2). Cytochromes and ferric reductase are located in the OM in Fe(III)—reducing bacteria such as Shewanella putrefaciens and Geobacter sulfiirreducens (Section 9.2.1).