Summary: Wild-type expresses three proline transport systems: a high-affinity proline transport system encoded by the gene, and two glycine betaine transport systems with a lower affinity for proline encoded by the and genes. Although proline uptake by the ProP and ProU transport systems is sufficient to supplement a proline auxotroph, it is not efficient enough to allow proline utilization as a sole source of carbon or nitrogen. Thus, the PutP transport system is required for utilization of proline as a carbon or nitrogen source. In this study, an overexpression suppressor, designated , which allows proline utilization in a genetic background and does not require the function of any of the known proline transport systems, was cloned and characterized. The suppressor gene, designated , maps at 8 min on the linkage map, distant from any of the other characterized proline transport genes. The DNA sequence of the gene predicts that it encodes a hydrophobic integral membrane protein, with strong similarity to a family of amino acid transporters. The suppressor phenotype requires either a multicopy clone of the gene or both a single copy of the gene and a mutation. These results indicate that the gene is the structural gene for a cryptic proline transporter that is silent unless overexpressed on a multicopy plasmid or activated by a mutation.


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