Thermoregulation of Shigella and Escherichia coli EIEC pathogenicity. A temperature-dependent structural transition of DNA modulates accessibility ofvirF promoter to transcriptional repressor H-NS

The expression of plasmid-borne virF of Shigella encoding a transcriptional regulator of the AraC family, is required to initiate a cascade of events resulting in activation of several operons encoding invasion functions. H-N S, one of the main nucleoid-associated proteins, controls the temperature-d ependent expression of the virulence genes by repressing the in vivo transc ription of virF only below a critical temperature (similar to 32 degrees C) , This temperature-dependent transcriptional regulation has been reproduced in vitro and the targets of H-NS on the virF promoter were identified as t wo sites centred around -250 and -1 separated by an intrinsic DNA curvature . H-NS bound cooperatively to these two sites below 32 degrees C, but not a t 37 degrees C. DNA supercoiling within the virF promoter region did not in fluence H-NS binding but was necessary for the H-NS-mediated transcriptiona l repression. Electrophoretic analysis between 4 and 60 degrees C showed th at the virF promoter fragment, comprising the two H-NS sites, undergoes a s pecific and temperature-dependent conformational transition at similar to 3 2 degrees C. Our results suggest that this modification of the DNA target m ay modulate a cooperative interaction between H-NS molecules bound at two d istant sites in the virF promoter region and thus represents the physical b asis for the H-NS-dependent thermoregulation of virulence gene expression.