LEUCINE ALTERS THE INTERACTION OF THE LEUCINE-RESPONSIVE REGULATORY PROTEIN (LRP) WITH THE FIM SWITCH TO STIMULATE SITE-SPECIFIC RECOMBINATION IN ESCHERICHIA-COLI

The leucine-responsive regulatory protein (Lrp) is a global regulator that controls the expression of numerous operons in Escherichia coil. Lrp can act as a repressor or as an activator of transcription with it s effects being potentiated, repressed or unaffected by the presence o f exogenous leucine. The phase variation of type 1 fimbria in E. coil provides a unique system in which to investigate the effects of leucin e on Lrp, as it is the only known example in which Lrp is a positive r egulator and leucine potentiates this effect. Previous studies determi ned that Lrp binds with high affinity to two sites within the fim swit ch (fim sites 1 and 2), and binding to these sites stimulates recombin ation. Here, it is shown that, even though leucine stimulates the fim switch in vivo, it nevertheless causes a slight decrease in Lrp bindin g to the fim switch in vitro. These contradictory results are explicab le by the finding that Lrp binding to a third region adjacent to fim s ites 1 and 2 inhibits recombination. According to this model, leucine stimulates recombination by selectively disrupting Lrp binding to this newly characterized region, while having little or no effect on Lrp b inding to firn sites 1 and 2.