Biogenesis of E. coli Pap pili: papH, a minor pilin subunit involved in cell anchoring and length modulation.

The biogenesis of Escherichia coli Pap pili, encoded by the pap gene cluster, was studied. A novel gene, papH, was identified and found to encode a weakly expressed pilin-like protein. PapH was dispensable for digalactoside-specific binding and for formation of Pap pili. However, in papH deletion mutants 50%-70% of total pilus antigen was found free of the cells. We present evidence showing coregulation of papH and the adjacent gene, papA, which encodes the major pilin subunit. A decrease in the PapA to PapH ratio resulted in a large fraction of cells producing shortened pili, whereas overproduction of PapA relative to PapH resulted in cells with lengthened pili. The data show that PapH has roles in anchoring the pilus to the cell and in modulating pilus length.