The IclR-Type Transcriptional Repressor LtbR Regulates the Expression of Leucine and Tryptophan Biosynthesis Genes in the Amino Acid ProducerCorynebacterium glutamicum

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Abstract
The transcriptional regulator Cg1486 ofCorynebacterium glutamicumATCC 13032 is a member of the IclR protein family and belongs to the conserved set of regulatory proteins in corynebacteria. A defined deletion in the cg1486 gene, now designatedltbR(leucine and tryptophan biosynthesis regulator), led to the mutant strainC. glutamicumIB1486. According to whole-genome expression analysis by DNA microarray hybridizations, transcription of theleuBandleuCDgenes encoding enzymes of the leucine biosynthesis pathway was enhanced inC. glutamicumIB1486 compared with the wild-type strain. Moreover, the genes of thetrpEGDCFBAoperon involved in tryptophan biosynthesis ofC. glutamicumshowed an enhanced expression in the cg1486 mutant strain. Bioinformatics pattern searches in the upstream regions of the differentially expressed genes revealed the common 12-bp motif CA(T/C)ATAGTG(A/G)GA that is located downstream of the −10 region of the mapped promoter sequences. DNA band shift assays with a streptavidin-tagged LtbR protein demonstrated the specific binding of the purified protein to 40-mers containing the 12-bp motif localized in front ofleuB,leuC, andtrpE, thereby confirming the direct regulatory role of LtbR in the expression of the leucine and tryptophan biosynthesis pathway genes ofC. glutamicum. Genes homologous withltbRwere detected upstream of theleuCDgenes in almost all sequenced genomes of bacteria belonging to the taxonomic classActinobacteria. TheltbR-like genes ofCorynebacterium diphtheriae,Corynebacterium jeikeium,Mycobacterium bovis, andBifidobacterium longumwere cloned and shown to complement the deregulation ofleuB,leuCD, andtrpEgene expression inC. glutamicumIB1486.