Effects of bfp mutations on biogenesis of functional enteropathogenic Escherichia coli type IV pili

Enteropathogenic Escherichia coli expresses a type IV fimbria known as the bundle-forming pilus (BFP) that is required for autoaggregation and localiz ed adherence (LA) to host cells. A cluster of 14 genes is sufficient to rec onstitute BFP biogenesis in a laboratory strain of E. coli. We have underta ken a systematic mutagenesis of the individual genes to determine the effec t of each mutation on BFP biogenesis and LA. Here we report the constructio n and analysis of nonpolar mutations in sis genes of the bfp cluster, bfpG, bfpB, bfpC, bfpD, bfpP, and bfpH, as well as the further analysis of a pre viously described bfpA mutant strain that is unable to express bundlin, the pilin protein. We found that mutations in bfpB, which encodes an outer mem brane protein; bfpD, which encodes a putative nucleotide-binding protein; a nd bfpG and bfpC, which do not have sequence homologues in other type IV pi lus systems, do not affect prebundlin expression or processing but block bo th BFP biogenesis and LA. The mutation in bfpP, the prepilin peptidase gene , does not affect prebundlin expression but blocks signal sequence cleavage of prebundlin, BFP biogenesis, and LA. The mutation in bfpH, which is pred icted to encode a lytic transglycosylase, has no effect on prebundlin expre ssion, prebundlin processing, BFP biogenesis, or LA For each mutant for whi ch altered phenotypes were detected, complementation with a plasmid contain ing the corresponding wild-type allele restored the wild-type phenotypes. W e also found that association of prebundlin or bundlin with sucrose density flotation gradient fractions containing bath inner and outer membrane prot eins does not require any accessory proteins. These studies indicate that m any bfp gene products are required for biogenesis of functional type IV pil i but that mutations in the individual genes do not lead to the identificat ion of new phases of pilus assembly.