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A cryptic proline permease in Salmonella typhimurium

Min-Ken Liao

Department of Microbiology, University of Illinois, B103 Chemical and Life Sciences Laboratory, 601 S. Goodwin Ave, Urbana, IL 61801, USA

Author for correspondence: Stanley Maloy. Tel: + 1 217 333 3122. Fax: + 1 217 244 6697. e-mail: s-maloy@uiuc.edu

ABSTRACT

Summary: Wild-type Salmonella typhimurium expresses three proline transport systems: a high-affinity proline transport system encoded by the putP gene, and two glycine betaine transport systems with a lower affinity for proline encoded by the proP and proU 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 proY, which allows proline utilization in a putP genetic background and does not require the function of any of the known proline transport systems, was cloned and characterized. The suppressor gene, designated proY, maps at 8 min on the S. typhimurium linkage map, distant from any of the other characterized proline transport genes. The DNA sequence of the proY 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 proY⁺ gene or both a single copy of the proY⁺ gene and a proZ mutation. These results indicate that the proY gene is the structural gene for a cryptic proline transporter that is silent unless overexpressed on a multicopy plasmid or activated by a proZ mutation.

Present address: Department of Biology, Hope College, Holland, Ml 49422, USA.

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