Influence of FIS on the transcription from closely spaced and non‐overlapping divergent promoters for an aminoacyl‐tRNA synthetase gene (gltX ) and a tRNA operon (valU ) in Escherichia coli

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Abstract
The gltX gene, encoding the glutamyl‐tRNA synthetase (GluRS), and the valU operon, whose transcripts contain three tRNAVal/UAC and one tRNALys/UUU, are adjacent and divergently transcribed. It is the only known case of adjacent genes encoding an aminoacyl‐tRNA synthetase and a tRNA precursor in Escherichia coli. The gltX promoters (P1, P2 and P3) direct the synthesis of transcripts non‐overlapping with and divergent from the one initiated at the valU promoter. We report that their promoter region (250 bp) contains three binding sites for the factor for inversion stimulation (FIS), centred at positions −71, −91 and −112 from the valU transcription initiation site, and that the destruction of any of these sites does not prevent the binding of FIS to the others. As FIS is one of the major positive regulators of stable RNA operons, we have studied its role on gltX and valU transcription. FIS stimulates valU transcription in vitro and about twofold in vivo during steady‐state exponential growth. In contrast, gltX transcription is repressed by the presence of FIS in vitro and about twofold in vivo during growth acceleration when a decrease in GluRS concentration was observed. Under all conditions tested, most of the gltX transcripts start at the P3 promoter. Nested deletions of this regulatory region reveal that the FIS‐dependent repression of the gltX‐P3 promoter is abolished after the removal of the valU promoter, and is not altered by the additional removal of the FIS binding sites; moreover, in vivo transcription from gltX‐P1 and/or gltX‐P2 present on some of these regulatory region variants is modulated by the nature of the upstream region by FIS and is sometimes stronger than that from gltX‐P3. These results show that the strength and the site of gltX transcription initiation are influenced by the upstream region up to and including the valU promoter; furthermore, they indicate that although these adjacent genes are involved in the first step of protein biosynthesis and share cis and trans regulatory elements, their transcription is non‐co‐ordinate.