INFLUENCE OF FIS ON THE TRANSCRIPTION FROM CLOSELY SPACED AND NONOVERLAPPING DIVERGENT PROMOTERS FOR AN AMINOACYL-TRANSFER-RNA SYNTHETASE GENE (GLTX) AND A TRANSFER-RNA OPERON (VALU) IN ESCHERICHIA-COLI

The gltX gene, encoding the glutamyl-tRNA synthetase (GluRS), and the valU operon, whose transcripts contain three tRNA(Val/UAC) and one tRN A(Lys/UUU), are adjacent and divergently transcribed. It is the only k nown case of adjacent genes encoding an amino-acyl-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 diver gent 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 -11 2 from the valU transcription initiation site, and that the destructio n of any of these sites does not prevent the binding of FIS to the oth ers, As FIS is one of the major positive regulators of stable RNA oper ons, we have studied its role on gltX and valU transcription, FIS stim ulates valU transcription in vitro and about twofold in vivo during st eady-state exponential growth, In contrast, gltX transcription is repr essed by the presence of FIS in vitro and about twofold in vivo during growth acceleration when a decrease in GluRS concentration was observ ed, Under all conditions tested, most of the gltX transcripts start at the P3 promoter. Nested deletions of this regulatory region reveal th at the FIS-dependent repression of the gltX-P3 promoter is abolished a fter the removal of the valU promoter, and is not altered by the addit ional removal of the FIS binding sites; moreover, in vivo transcriptio n from gltX-P1 and/or gltX-P2 present on some of these regulatory regi on variants is modulated by the nature of the upstream region by FIS a nd is sometimes stronger than that from gltX-P3, These results show th at the strength and the site of gltX transcription initiation are infl uenced by the upstream region up to and including the valU promoter; f urthermore, they indicate that although these adjacent genes are invol ved in the first step of protein biosynthesis and share cis and trans regulatory elements, their transcription is non-co-ordinate.