ABSTRACT
Bacteria must frequently adapt to drastic changes in their environment. For example, Escherichia coli released from the gut can survive for some period of time in soil or water. Adaptation includes induction of transport systems for the uptake of available substrates, and the synthesis of enzymes required for substrate metabolism. The transport mechanism for any substrate into bacteria is not known at the atomic level. Only such detailed knowledge enables us to understand the molecular events that take place when a polar substrate crosses the hydrophobic layer of a biological membrane with the aid of a protein. Actual transport against a concentration gradient, and stereochemical recognition between the substrate and the transport proteins occurs in the cytoplasmic membrane. Transport of many amino acids, peptides, certain sugars and anions follows a so-called binding protein-dependent transport mechanism in which proteins in the periplasmic space are involved. The citrate-inducible iron transport system is of particular interest with regard to its regulation.
