THE ESCHERICHIA-COLI FIS PROTEIN IS NOT REQUIRED FOR THE ACTIVATION OF TYRT TRANSCRIPTION ON ENTRY INTO EXPONENTIAL-GROWTH

The Escherichia coli DNA bending protein factor for inversion stimulat ion (FIS), is neither necessary nor responsible for the stimulation of transcription from the wild type promoter for the tyrT operon (encodi ng a species of tyrosine tRNA) that occurs upon resumption of exponent ial growth. This conclusion is unexpected given that the regulatory el ement required for optimal transcription of tyrT contains three bindin g sites for FIS protein. In addition, it is in apparent conflict with reports from other laboratories which have described FIS-dependent act ivation of the stable RNA promoters rrnB Pl and thrU(tufB) in vivo. Ho wever, tyrT transcription is stimulated in a FIS-dependent manner both in vivo and in vitro when promoter function is impaired by mutation o f the promoter itself or by the addition of the polymerase effector gu anosine 5'-diphosphate 3'-diphosphate. These conditions, which expose a requirement for activation of stable RNA synthesis by FIS, suggest t hat FIS serves an adaptive role permitting high levels of stable RNA t ranscription on nutritional shift-up when RNA polymerase levels are de pleted. In principle such a mechanism could confer a significant selec tive advantage thus accounting for the conservation of FIS binding sit es in the regulatory regions of stable RNA promoters.