Type 1 fimbriae of Escherichia coli: genetic regulation, morphogenesis, and role in pathogenesis.

Over the last several years a number of aspects related to the molecular biology of the type 1 fimbriae of Escherichia coli have been investigated with use of a variety of approaches, including molecular genetics, electron microscopy, and cell biology. With the help of these analytic tools, new insights have emerged. The on-and-off genetic regulation of expression, referred to as phase variation, has been found to be regulated at the level of transcription by the inversion of a 314-base-pair segment of DNA. Morphogenesis of the organelles, which depends on the assembly of processed subunits of 17,000-dalton mass, has been found to proceed from the addition of new subunits at the base, rather than at the tip, of the organelle. Although the precise role of type 1 fimbriae in the pathogenesis of E. coli infections is still somewhat obscure, new data indicate that they significantly potentiate the uptake of nutrients from and the delivery of toxins to eukaryotic cells.