A NEW GENE LOCUS OF BORDETELLA-PERTUSSIS DEFINES A NOVEL FAMILY OF PROKARYOTIC TRANSCRIPTIONAL ACCESSORY PROTEINS

Recently, a novel type of regulatory mutation causing differential eff ects on the expression of virulence genes due to a slight overexpressi on of the RNA polymerase alpha subunit (RpoA) was found in Bordetella pertussis (N. H. Carbonetti, T. M. Fuchs, A. A. Patamawenu, T. J. Iris h, H. Deppisch, and R. Gross, J. Bacteriol. 176:7267-7273, 1994), To g ather information on the molecular events behind this phenomenon, we i solated suppressor mutants of the RpoA-overexpressing strains after ra ndom mutagenesis, Genetic characterization of these suppressor strains revealed the existence of at least three distinct groups of dominant alleles. Mutations occurred either in the rpoA locus itself, in the bv g locus, or in unknown gene loci, One mutant of the latter group was f urther characterized. By the introduction of a cosmid library containi ng genomic B, pertussis DNA into this suppressor strain, we isolated a cosmid which suppressed the phenotype of the suppressor strain, thus restoring the negative effect on transcription of the ptx and cya toxi n genes, Mutagenesis of the cosmid with Tn5 led to the identification of the gene locus responsible for this phenomenon, Its DNA sequence re vealed the presence of an open reading frame (ORF) consisting of 2,373 bp coding for a hypothetical 86-kDa protein with extensive sequence s imilarities to ORFs with not yet identified functions of Escherichia c oli, Haemophilus influenzae, and Neisseria meningitidis. The new gene, termed tex, for toxin expression, seems to be an essential factor for B, pertussis, as it cannot be deleted from the bacterial chromosome, All members of this new protein family show significant sequence simil arities with the mannitol repressor protein MtlR and with the presumpt ive RNA-binding domains of the Pnp and ribosomal S1 proteins of E. col i in their N- and C-terminal parts, respectively, These sequence simil arities and the fact that the tex gene was isolated by virtue of its e ffects on gene expression in B. pertussis indicate that the members of this new protein family may play an important role in the transcripti on machinery of prokaryotic organisms.