Peptides and Bacterial Growth

Energy transduction and solute transport in streptococci

Metabolic energy in lactic streptococci can be generated by substrate level phosphorylation and by efflux of end-products in symport with protons. During growth on lactose or glucose Streptococcus cremoris maintains a high proton motive force and phosphate potential. Both energy intermediates dissipate rapidly when the energy supply stops. In the initial phase of starvation the internal phosphoenolpyruvate (PEP) pool increases rapidly and this enables the organism for a prolonged period during starvation to accumulate the energy source via a PEP-dependent uptake system.

Stereospecificity of peptide transport by germinating barley embryos

The stereospecific requirements for peptide transport in the scutellum of germinating barley (Hordeum vulgare) embryos are described. Replacement of an L-amino acid residue in a peptide by its D-stereoisomer decreases the affinity of the peptide for the transport site, leading to a reduction in transport. Substitution of a second D-residue reduces affinity still further. The extent to which transport is inhibited depends upon the position of the D-residue in the primary sequence, with D-residues at the C-terminus of the peptide having the greatest effect. Competition between D- and L-peptides indicates that they both enter via the same transport system. Although D-amino acids can be accumulated when presented as a peptide, these same D-residues are not transported when supplied as the free amino acids. L-Leu-D-leu is accumulated intact against a concentration gradient, indicating the operation of an active transport mechanism that can function without the involvement of peptidase activity.