Expression of multiple flagellin-encoding genes of Proteus mirabilis.

The overproduction of flagella is a distinguishing characteristic of Proteus mirabilis swarmer cell differentiation. The synthesis of flagellin, the principal protein composing the flagellar filament, is coordinately regulated as part of a larger regulon of genes whose expression is a prerequisite in urinary pathogenesis. In this report, the regulation of expression of the flaA locus, comprising flaA and flaB, two tandemly linked and nearly identical copies of flagellin-encoding genes, is examined. Transcriptional expression studies reveal that flaA, but not flaB, is expressed by wild-type cells, and flaA transcription increases eightfold during differentiation. The flaA transcriptional start site for both swimmer and swarmer cells was determined to be located at a guanine, 8 bases downstream of the flaA sigma 28 promoter. FlaA- mutants are nonmotile and undifferentiated and do not synthesize flagellin, while FlaB- mutants are wild type, thus verifying that FlaA is the sole flagellin produced by wild-type cells and that flaB is silent. FlaA- mutants frequently revert to a Mot+ phenotype that is antigenically distinct from that of wild-type cells. Southern blot analysis of the flaA Mot+ revertants reveals a deletion of between 2 and 7kb in the flaA locus. Biochemical analyses of revertant flagellin indicate major changes in protein size and composition but conservation of the first 28 N-terminal residues. The result of this process is to produce an antigenically distinct flagellum that may be significant in ensuring the survival of P. mirabilis during pathogenesis.