Two non-consensus Clp binding sites are involved in upregulation of the gum operon involved in xanthan polysaccharide synthesis in Xanthomonas campestris pv. campestris.
Biosynthesis of xanthan polysaccharide, a virulence factor of phytopathogenic Xanthomonas campestris pv. campestris (Xcc), involves the gum operon and the cyclic AMP receptor protein (CRP) homologue Clp. Clp was shown to have the same DNA binding specificity as the CRP at positions 5, 6, and 7 (GTG motif) of the left arm. Therefore, Clp binding sites (CBSs) have typically been identified by pattern searching of the Xcc genome using the consensus CRP binding sequence. Here, results of a reporter assay and electrophoretic mobility shift assay suggest that Clp upregulates the gum operon by binding to two non-consensus sites, in which a more conserved right arm may compensate for the lack of conservation in the left arm, a high GC content in the central region (6 bp) may be important for binding, and binding may be enhanced if the GC-rich central region is palindromic. These suggest that atypical CBSs exist in Xcc promoters and that Clp, while retaining the capacity to bind typical CBSs, has evolved to bind atypical CBS because: 1) Clp shares only moderate homology with the CRP and is modulated by cyclic di-GMP; and 2) Xcc has a higher GC content (65%) than Escherichia coli (50%).
Chen
Chih-Hua
CH
Institute of Molecular Biology, National Chung Hsing University, Taichung 402, Taiwan.
Lin
Nien-Tsung
NT
Hsiao
Yi-Min
YM
Yang
Chiou-Ying
CY
Tseng
Yi-Hsiung
YH
D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D001483:Base Sequence; D001665:Binding Sites; D002373:Cyclic AMP Receptor Protein / Q000235:genetics / Q000378:metabolism; D004926:Escherichia coli; D020862:GC Rich Sequence; D015964:Gene Expression Regulation, Bacterial; D017930:Genes, Reporter; D016297:Mutagenesis, Site-Directed; D009876:Operon; D010957:Plasmids; D011135:Polysaccharides, Bacterial / Q000096:biosynthesis* / Q000235:genetics; D011401:Promoter Regions, Genetic; D012045:Regulatory Sequences, Nucleic Acid; D017384:Sequence Deletion; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D024363:Transcription Initiation Site; D015533:Transcriptional Activation; D015854:Up-Regulation / Q000235:genetics; D037521:Virulence Factors / Q000235:genetics; D016959:Xanthomonas campestris / Q000235:genetics* / Q000378:metabolism / Q000472:pathogenicity
D001426:Bacterial Proteins; C436751:Clp protein, Xanthomonas campestris; D002373:Cyclic AMP Receptor Protein; D011135:Polysaccharides, Bacterial; D014157:Transcription Factors; D037521:Virulence Factors; C002563:xanthan gum