BMC Genomics 2006, 7:21 http://www.biomedcentral.com/1471-2164/7/21 FGirgouwrteh1curves of the wild-type strain C. glutamicum ATCC 13032 and the dtxR mutant C. glutamicum IB2103 Growth curves of the wild-type strain C. glutamicum ATCC 13032 and the dtxR mutant C. glutamicum IB2103. The strains and plasmid-carrying derivatives were cultivated in CGXII medium and in low-iron CGXII medium, respectively. Growth was mon- itored with a nephelometer and is shown as relative turbidity. Values are means of measurements of six biological replicates. Abbreviations: A, C. glutamicum ATCC 13032 in CGXII medium; B, C. glutamicum IB2103 in CGXII medium; C, C. glutamicum ATCC 13032 in low-iron CGXII medium; D, C. glutamicum IB2103 in low-iron CGXII medium; E, C. glutamicum ATCC 13032 (pIB4000) in CGXII medium; F, C. glutamicum IB2103 (pIB4000) in CGXII medium; G, C. glutamicum ATCC 13032 (pEC- XK99E) in CGXII medium. Results Phenotypic characterization of the dtxR mutant strain C. glutamicum IB2103 The cg2103 (dtxR) gene of C. glutamicum ATCC 13032 encodes a transcriptional regulator with significant amino acid sequence similarity to the diphtheria toxin repressor DtxR of C. diphtheriae [7,11]. To explore the regulatory network controlled by the DtxR protein of C. glutamicum, a defined deletion was established in the coding region of dtxR by applying a gene replacement strategy [19] that finally resulted in the mutant strain C. glutamicum IB2103. Subsequently, growth assays with C. glutamicum IB2103 were performed in liquid CGXII minimal medium and growth of the cultures was monitored by nephelometry (Figure 1). Growth assays with the wild-type strain C. glutamicum ATCC 13032 served as control. The growth assays clearly showed that C. glutamicum IB2103 contain- ing the dtxR gene deletion was unable to grow in CGXII minimal medium. On the other hand, the dtxR mutant revealed the same growth characteristics as the wild-type strain when cultivated in low-iron CGXII medium (Figure 1). The growth deficiency of C. glutamicum IB2103 in CGXII minimal medium was complemented by transfor- mation with plasmid pIB4000 carrying the cloned dtxR gene. Control assays with C. glutamicum IB2103 contain- ing the empty cloning vector pEC-XK99E failed to com- plement the phenotype of the mutant (Figure 1). These data demonstrate that the defined deletion of dtxR in C. glutamicum IB2103 resulted in a conditionally lethal phe- notype apparently depending on the amount of iron that Page 3 of 19 (page number not for citation purposes)