Size Restriction on Peptide Utilization in Escherichia coli

Abstract

It has been shown by measuring the growth response of appropriate amino acid auxotrophs to several series of homologous oligopeptides that higher oligomers were unable to enter Escherichia coli. The exclusion did not always occur at the same chain length of peptide, for the oligomer at which exclusion was first observed varied with the nature of the amino acid residues. To investigate the possibility that over-all size may be important in regulating peptide entry, the relative hydrodynamic volumes of the peptides used in the above studies were measured by gel filtration on Sephadex G-15. These studies suggested that to be able to enter E. coli the hydrodynamic volume of a peptide could not exceed a certain critical value. To test the generality of this observation, the heterogeneous mixture of peptides present in the enzymatic protein hydrolysate Neopeptone was separated according to size on Sephadex G-15. On the basis of the previous measurements it proved possible to exactly predict which of the Neopeptone peptide fractions could enter the cell and which would be excluded. Lack of competition between peptides that can enter and peptides that are excluded, suggests that the exclusion may occur external to the peptide transport system. The speculation is raised that the cell wall may act as a molecular sieve preventing molecules above a certain size from crossing the cell envelope, the critical hydrodynamic volume being some measure of the pore size of such a sieve.