TOPOLOGY OF THE PHOR PROTEIN OF ESCHERICHIA-COLI AND FUNCTIONAL-ANALYSIS OF INTERNAL DELETION MUTANTS

The PhoR protein of Escherichia coli K-12 belongs to a family of struc turally related sensor-kinases that regulate responses to environmenta l stimuli. These proteins are often located in the inner membrane with two membrane-spanning segments that are separated by a periplasmic do main, which is supposed to sense the environmental stimuli. However, t he hydrophobicity plot of PhoR suggests a somewhat different topology in which a large periplasmic domain is lacking and an extended cytopla smic domain is present besides the kinase domain. In protease-accessib ility experiments and by using phoR-phoA gene fusions, the topology of PhoR was investigated and the absence of a large periplasmic domain w as confirmed. Furthermore, the function of the extended cytoplasmic do main was studied by creating internal deletions. The mutations in this domain resulted in a constitutive expression of the pho regulon, indi cating that the mutant PhoR proteins are locked in their kinase functi on. We propose that this extended cytoplasmic domain functions by sens ing an internal signal that represses the kinase function of the PhoR protein.