H-pilus assembly kinetics determined by electron microscopy.

The kinetics of pilus outgrowth were examined for Escherichia coli containing pDT1942, a TnlacZ insertion derivative of the IncHI1 plasmid R27, which was derepressed for transfer. IncHI1 plasmids are thermosensitive for transfer. The pili specified by pDT1942 were examined by transmission electron microscopy after the pilus had been labeled with the H-pilus-specific bacteriophage Hgal, which had been inactivated with RNase A. H pili were extended by extrusion from the cell surface and not by the addition of pilin subunits to the pilus tip. After pili were removed by vortexing, the outgrowth of full-length pili (2 microns long) required 20 min. H pili expressed at the transfer optimal temperature (27 degrees C) remained stable after incubation at the transfer inhibitory temperature (37 degrees C), but the formation of mating aggregates was inhibited at 37 degrees C. Within 1 min of exposure of the host cell to a heat stimulus of 50 degrees C, pili vanished. Pili were observed in straight and flexible forms with a field emission scanning electron microscope, which may indicate a dynamic role for the pilus in conjugation.