The regulatory functions of the leader region preceding the operon have been studied by mutagenesis analysis. This leader presents striking similarity to ‘T-box’ leaders found upstream of many Gram-positive aminoacyl-tRNA synthetase genes and some amino acid biosynthesis operons, which are controlled by antitermination through interaction of the leader transcript with cognate uncharged tRNA. A region of the leader transcript also contains a series of (G/U)AG repeats which, in , are involved in the binding of the RNA-binding protein (TRAP) which controls transcription. A screen was developed for the isolation of regulatory mutants affected in the leader region. All spontaneous mutants contained deletions; point mutations were only obtained after UV-induced mutagenesis. All mutations affected the putative transcription terminator upstream of the operon, demonstrating that is indeed controlled by transcription antitermination.


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