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The two-component regulatory system, OmpR-EnvZ, of Escherichia coli K-12 regulates the expression of the major outer membrane porin protein, OmpF. OmpR is a DNA-binding protein which acts as both an activator and a repressor to control ompF transcription. In this article, we describe a new OmpR-binding site that is located between 384 to 351 bp upstream from the ompF start point of transcription. Inactivation of this site by insertion of a 22-bp fragment prevents the repression of ompF expression conferred by the dominant negative mutation, envZ473. On the basis of the location of this binding site, the presence of bent DNA in the ompF regulatory region (T. Mizuno, Gene 54:57-64, 1987), and the fact that mutations altering integration host factor result in constitutive ompF expression (P. Tsui, V. Helu, and M. Freundlich, J. Bacteriol. 170:4950-4953, 1988), we propose that the negative regulation of ompF involves a DNA loop structure.

The two-component regulatory proteins OmpR and EnvZ of Escherichia coli K-12 regulate expression of the major outer membrane porin protein, OmpF. OmpR is a DNA-binding protein that is involved in both the positive and negative control of ompF transcription. EnvZ is a histidine kinase that phosphorylates OmpR in response to environmental signals. We used DNA migration retardation analysis to examine the interactions of OmpR and the phosphorylated form of OmpR (OmpR-P) with the regulatory region immediately upstream of the ompF promoter. Our results indicate that the binding of OmpR to this regulatory region is cooperative and that phosphorylation significantly stimulates these cooperative interactions. Moreover, although phosphorylation increases the intrinsic binding of OmpR to a single OmpR-binding site, the primary role of phosphorylation in ompF regulation is to facilitate cooperative interactions between OmpR molecules bound at adjacent sites. Based on these results, we propose a model to explain how the phosphorylation of OmpR could stimulate the occupancy of specific sites in the ompF regulatory region, thereby resulting in the activation or repression of ompF transcription under the appropriate environmental conditions. Expression of the major outer membrane porin proteins OmpF and OmpC of Escherichia coli K-12 is regulated in response to a wide variety of environmental signals (reviewed in refs. 1-3). The most extensively studied of these signals is the osmolarity of the growth medium. At low osmolarity, the OmpF porin is preferentially expressed and only low levels of the OmpC porin are found. Conversely, at high osmolarity, OmpC is preferentially expressed and only low levels of OmpF are found. This fluctuation in porin expression is controlled at the transcriptional level by the two-component regulatory system, EnvZ and OmpR. EnvZ is a membrane-bound histidine kinase that modulates the activity of the DNA-binding protein OmpR via phosphorylation (refs. 2 and 3 and references therein). The phosphorylated form of OmpR (OmpR-P), which is the active form of the protein, is a transcriptional activator at the ompC promoter and can function as either an activator or a repressor at the ompF promoter (4).According to the current model (3), at low osmolarity the cellular concentration of OmpR-P is low. This level is sufficient for OmpR-P to interact with the sites in the ompF regulatory region that are responsible for activating ompF transcription. At high osmolarity, the cellular concentration of OmpR-P is much greater. As a result of this increase, OmpR-P is now capable of occupying the sites in the ompC regulatory region responsible for activating ompC transcription, and the sites at ompF responsible for repressing ompF transcription. Thus, this model predicts that the regulation of ompF and ompC is a direct consequence of the level of OmpR-P in the cell and is dependent on the way in which OmpR-P interacts with sites in the ompF and ompC regulatory regions.A number of studies have addressed the basi...

Type I interferon induced MxA has antiviral activity against RNA viruses. Here we demonstrate a previously unrecognized MxA isoform induced by herpes simplex virus‐1 (HSV‐1) in human fibroblasts. Transcript encoding MxA splice variant has a deletion in Exons 14–16 which encode a central interactive domain associated with recognition of viral nucleocapsids and is predicted to encode a novel peptide sequence at the C‐terminal region as compared with IFN ‐induced MxA prototype. Polyclonal antiserum raised in rabbit against C‐terminal half of MxA splice variant demonstrated that HSV‐1 induced variant MxA undergoes nuclear translocation associated with viral replication compartment. Infection of variant MxA‐overexpressing fibroblasts with HSV‐1 resulted in the enhancement of viral yields but the yields were decreased in MxA‐knockdown cell by siRNA, suggesting that HSV‐1 induced MxA splice variant enhances viral replication. Both IFN‐ α and HSV‐1 are able to activate the promoter of MxA in luciferase reporter assay; however, only the MxA variant is translated in virus‐infected cells. Thus, differential expression of MxA controlled by IFN‐α or HSV‐1 could result in the increase of host resistance or favoring viral replication, depending on stimulating event. In summary, modulation of cellular mRNA could be a novel mechanism for human alphaherpesviruses to avoid innate immune detection.