Activation of Escherichia coli leuV transcription by FIS

The transcription factor FIS has been implicated in the regulation of sever al stable RNA promoters, including that for the major tRNA(Leu) species in Escherichia coli, tRNA(1)(Leu). However, no evidence for direct involvement of FIS in tRNA(1)(Leu) expression has been reported. We show here that FIS binds to a site upstream of the leuV promoter (centered at -71) and that i t directly stimulates leuV transcription in vitro. A mutation in the FIS bi nding site reduces transcription from a leuV promoter in strains containing FIS but has no effect on transcription in strains lacking FIS, indicating that FIS contributes to leuV expression in vivo. We also find that RNA poly merase forms an unusual heparin-sensitive complex with the leuV promoter, h aving a downstream protection boundary of similar to-7, and that the first two nucleotides of the transcript, GTP and UTP, are required for formation of a heparin-stable complex that extends downstream of the transcription st art site. These studies have implications for the regulation of leuV transc ription.