Molecular cloning of the wild-type phoM operon in Escherichia coli K-12

A metastable bacterial alkaline phosphatase (Bap) phenotype is seen in phoR mutants, which alternately express a Bap-constitutive or -negative phenotype. The alteration is affected by mutations in the phoM region near 0 min. By molecular cloning of the wild-type phoM operon onto a multicopy plasmid and recombining onto the plasmid the pho-510 mutation that abolishes variation, the phoM operon, rather than some nearby gene, was shown to control variation. Complementation tests indicated that the wild-type phoM allele is dominant to the pho-510 mutation when both are in single copy, but whichever allele is present in higher copy appears as dominant when multicopy plasmids are examined. The alternating phenotypic variation of BAP synthesis was not seen in phoR+ cells with multicopy wild-type phoM plasmids, thus showing that the variation is associated with phoM-dependent Bap expression. The alternation acted at the level of phoA transcription; it was also recA independent. BAP clonal variation is phenotypically similar to Salmonella phase variation, which is controlled by a DNA rearrangement. No evidence was found for a DNA change near the phoM operon that might be responsible for the variable Bap phenotype.