The excC and excD genes of Escherichia coli K-12 encode the peptidoglycan-associated lipoprotein (PAL) and the TolQ protein, respectively

The tol genes of Escherichia coli K-12 are involved in the import of macromolecules such as group A colicins or filamentous phage DNA, and in the maintenance of cell envelope integrity (for a review, see Webster, 1991). Four genes, tolQRAB, located at min 17 in the genetic map have been identified and sequenced (Sun & Webster, 1987; Levengood & Webster, 1989). Mutants altered in the tolQRA genes are tolerant to groupA colicins and resistant to filamentous phages while mutants in the tolB gene are only tolerant to colicin A, E2 and E3. In addition, tol mutants are hypersensitive to drugs and release most of the periplasmic proteins into the extracellular medium.