A Phosphate-Bond-Driven Dipeptide Transport System in Streptococcus cremoris Is Regulated by the Internal pH

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The uptake of amino acids and peptides by Streptococcus cremoris is mediated by different highly specific transport systems. The leucine transport system has a high affinity only for leucine, isoleucine, and valine and no affinity for leucyl-peptides. The transport system for leucyl-leucine is strongly inhibited by several dipeptides with hydrophobic, neutral, N-terminal amino acids but not by leucine. The leucyl-leucine transport system has a high affinity for dipeptides containing β-methyl groups in the side chain; the C terminus of the dipeptide affects the affinity to a much lower extent. Leucyl-leucine transport in whole cells was studied as a function of the internal pH at different external pH values in the presence and absence of nigericin. The internal pH was shown to be an important controlling factor in leucyl-leucine uptake, but the ΔpH was not involved as a driving force. At increasing external pH values, the affinity of the transport system for leucyl-leucine decreased. Uptake of leucyl-leucine was also studied in the presence of arsenate, which inhibited ATP synthesis by substrate-level phosphorylation. The rate of leucyl-leucine transport appeared to be dependent on the intracellular ATP concentrations. These results indicate that the energy for the leucyl-leucine transport is directly supplied by ATP.

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