The gene encodes an integral outer-membrane protein (Tsx) that functions as a substrate-specific channel for deoxynucleosides and the antibiotic albicidin, and also serves as a receptor for bacteriophages and colicins. We cloned the structural genes of the Tsx proteins from and and expressed them in an mutant. The heterologous Tsx proteins fully substituted the Tsx protein with respect to its function in deoxynucleoside and albicidin uptake, and as receptor for colicin The Tsx proteins from and were also proficient as receptors for several Tsx-specific bacteriophages, whereas the corresponding protein from did not confer sensitivity against these phages. The nucleotide sequence of the genes from and was established. Each of the Tsx proteins is initially synthesized with typical bacterial signal sequence peptides and the predicted mature forms of the Tsx proteins have a calculated M of 30567 (265 residues), 31412 (272 residues) and 31477 (272 residues), respectively. Multiple sequence alignments between the Tsx proteins showed a high degree of sequence identity and revealed the presence of four hypervariable regions, which are thought to constitute segments of the polypeptide chain exposed at the cell surface. Most notable was a deletion of 8 amino acids in one of these hypervariable domains in the Tsx protein. When this deletion was introduced by site-directed mutagenesis into the corresponding region of the gene, the mutant Tsx-515 protein lost its phage receptor function but still served as a colicin K receptor and as a substrate-specific channel, indicating that the region between residues 198 and 207 might be part of the bacteriophage receptor area. Multiple sequence alignments, structural predictions and the properties of previously characterized Tsx missense mutants were taken into account to develop a two-dimensional model for the topological organization of the Tsx protein within the outer membrane.


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