SEQUENCING AND CHARACTERIZATION OF THE XYL OPERON OF A GRAM-POSITIVE BACTERIUM, TETRAGENOCOCCUS-HALOPHILA

The xyl operon of a gram-positive bacterium, Tetragenococcus halophila (previously called Pediococcus halophilus), was cloned and sequenced. The DNA was about 7.7 kb long and contained genes for a ribose bindin g protein and part of a ribose transporter, xylR (a putative regulator y gene), and the xyl operon, along with its regulatory region and tran scription termination signal, in this order. The DNA was AT rich, the GC content being 35,8%, consistent with the GC content of this gram-po sitive bacterium. The xyl operon consisted of three genes, xylA, encod ing a xylose isomerase, xylB, encoding a xylulose kinase, and xylE, en coding a xylose transporter, with predicted molecular weights of 49,40 0, 56,400, and 51,600, respectively. The deduced amino acid sequences of the XylR, XylA, XylB, and XylE proteins were similar to those of th e corresponding proteins in other gram-positive and -negative bacteria , the similarities being 37 to 64%. Each polypeptide of XylB and XylE was expressed functionally in Escherichia coli. XylE transported D-xyl ose in a sodium ion-dependent manner, suggesting that it is the first described xylose/Na+ symporter, The XylR protein contained a consensus sequence for binding catabolites of glucose, such as glucose-6-phosph ate, which has been discovered in glucose and fructose kinases in bact eria. Correspondingly, the regulatory region of this operon contained a putative binding site of XylR with a palindromic structure. Furtherm ore, it contained a consensus sequence, CRE (catabolite-responsive ele ment), for binding CcpA (catabolite control protein A). We speculate t hat tbe transcriptional regulation of this operon resembles the regula tion of catabolite-repressible operons such as the amy, lev, xyl, and gnt operons in various gram-positive bacteria. We discuss the signific ance of the regulation of gene expression of this operon in T. halophi la.