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Bacterial alkaline phosphatase clonal variation in some Escherichia coli K-12 phoR mutant strains

Several phoR alleles (phoR19, phoR20, phoR68, phoR69, phoR70, and phoR78) led to either a bacterial alkaline phosphatase (BAP)-constitutive phenotype or a variable behavior, depending upon the strain tested. Whereas Escherichia coli K10, MC1000, and XPh4 phoR mutants were constitutive, AB1157, BD792, MC4100, and W3110 phoR mutants displayed the metastable character. For the latter strains, constitutive mutants regularly segregated BAP-negative clones which yielded constitutive variants again at a high frequency. Indeed, the pattern of variation observed in BAP-variable phoR strains is phenotypically analogous to phase variation of the H1/H2 flagellum antigen type in Salmonella typhimurium and the molecular switch between the immune and sensitive states in bacteriophage lambda. The metastable behavior was not a general property of BAP-constitutive mutants, since several phosphate-specific transport-phoU mutations led to a constitutive (stable) phenotype regardless of the strain tested. But in phoR phosphate-specific transport-phoU mutants, the metastable character was epistatic (dominant), and such double mutants showed clonal variation in BAP-variable strains.