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When a solution of binding protein and its ligand is dialyzed against a large volume of ligand-free medium the rate of exit of the ligand from the protein-containing compartment can be extremely slow, much slower than the rate observed in the absence of protein. This is what we call retention of ligand by protein. A simple calculation demonstrates that when the protein concentration is in large excess over the total ligand concentration, the exit of ligand follows quasi-first-order kinetics, the half-life being proportional to (1 + (P)/Kd), where (P) is the concentration of binding sites, and Kd the dissociation constant characteristic of the equilibrium between the ligand and the protein. Experimental verification of this relation is provided in the case of the periplasmic maltose-binding protein of Escherichia coli; The implications of the retention effect in biochemical techniques are discussed, as well as its possible significance in biological phenomena, such as bacterial chemotaxis and transport, mechanism of hormone action, or transmission of the nerve impulse.

To find out if there are changes in membrane potential during bacterial chemotaxis, we measured the membrane potential of Escherichia coli indirectly by use of the permeating, lipid-soluble cation triphenylmethylphosphonium. Addition of attractants or repellents to the bacteria brought about a hyperpolarizing peak (as well as additional, later changes in membrane potential). This peak was shown to be a part of the chemotactic mechanism based on the following evidence: (i) All attractants and repellents tested gave this peak while chemotactically inert chemicals did not. (ii) Mutants lacking galactose taxis failed to give the peak with galactose but did with another attractant and with repellents. (iii) Methionine, required for chemotaxis, is also required for production of this peak. (iv) A mutant in a control gene )flaI), unable to synthesize flagella and cytoplasmic membrane proteins related to motility and chemotaxis, failed to give the peak. (v) Paralyzed (mot) mutants gave little or none of the peak. Generally nonchemotactic (che) mutants, on the other hand, did give this peak. Very likely there are ion fluxes that bring about this change in membrane potential. We discuss the possible role of the mot gene product as an ion gate controlled by a methylation-demethylation process in response to attractants and repellents acting through their chemoreceptors.

Mutants affected in lamB, the structural gene for phage lambda receptor, are unable to utilize maltose when it is present at low concentrations (less than or equal 10 muM). During growth in a chemostat at limiting maltose concentrations, the lamB mutants tested were selected against in the presence of the wild-type strain. Transport studies demonstrate that most lamB mutants have deficient maltose transport capacities at low maltose concentrations. When antibodies against purified phage lambda receptor are added to a wild-type strain, transport of maltose at low concentrations is significantly reduced. These results strongly suggest that the phage lambda receptor molecule is involved in maltose transport.