Mutations affecting the alpha subunit of Bordetella pertussis RNA polymerase suppress growth inhibition conferred by short C-terminal deletions of the response regulator BvgA.

The effects of short deletions of the C terminus of the BvgA response regulator protein of the BvgAS two-component system were examined in Bordetella pertussis. When present as a single copy in the chromosome, deletions removing as few as two amino acids conferred a completely Bvg- phenotype. When provided in trans, on the broad-host-range plasmid pRK290, under the control of the native bvgAS promoter, deletions of two or three amino acids conferred a profound growth inhibition which was dependent on the integrity and activity of the wild-type chromosomal bvgAS locus. It is proposed that this phenotype was the result of an inappropriate interaction of the mutant BvgA protein with the RNA polymerase enzyme, specifically the alpha subunit. Mutant strains in which this growth inhibition was relieved were isolated and characterized. Although most of the suppressor mutations affected either the mutant plasmid copy or the wild-type chromosomal bvg locus, three mutations which affected the alpha subunit of B. pertussis RNA polymerase were also isolated. Two of these resulted in increased levels of the alpha subunit, and one caused a substitution of glycine for the aspartic acid residue at position 171, in the N-terminal domain. All three mutations also resulted in a differential phenotype in that expression of fha was essentially normal, but expression of ptx was greatly reduced.