Porin Regulon of Escherichia coli

In Escherichia coli, the two porin proteins OmpF and OmpC form pores in the outer membrane that allow for the passive diffusion of small hydrophilic molecules across this hydrophobic barrier. Studies using gene and operon fusions to both ompF and ompC revealed that regulation of porin expression occurs at the transcriptional level. This work, combined with additional genetic analysis, led to the proposal of an early model to explain porin regulation. Importantly, strains merodiploid for envZ473 and either envZ247 or envZ250 exhibit intermediate levels of porin expression that are comparable with that normally seen in low osmolarity. OmpR can activate transcription of both the ompF and ompC genes and can also repress transcription of ompF. To regulate expression of the porin genes, OmpR must interact with regulatory regions of the ompF and ompC promoters in a manner that results in the activation and/or repression of transcription. Much study has been focused on elucidating the following: the regions of DNA to which OmpR binds and the nucleotide sequence of these regions; how OmpR recognizes these regions; and finally, how these OmpR-promoter interactions affect DNA topology, functional interactions with the polymerase, and ultimately transcriptional activation and/or repression. Importantly these conditions mimic those found within the bodies of animals. In external environment surroundings, nutrients are scarce, and the slightly wider OmpF channel enables their more efficient uptake. The smaller, more protective OmpC porin is no longer required in this very dilute environment.