An extensive random mutagenesis study on the function of nine residues in the subunit binding site of FimC was conducted to evaluate the mol. mechanisms of subunit binding by FimC and the contribution of chaperone surface residues. Five plasmid libraries of FimC variants with amino acid replacements in one or two of the nine residues were constructed to transform an Escherichia coli FimC null mutant. The assembly of type 1 pili was affected by the amino acid replacements at the subunit site of FimC; these did not completely eliminate the chaperone function. The amino acid replacements affect the interaction with different pilus subunits. The changes in the subunit binding site of FimC not only affect the pilus length and pilus no., but also the subunit compn. of type 1 pili. These findings revealed a fine-tuned affinity of pilus chaperones for the individual pilus subunits which detd. both the initiation of pilus assembly, stoichiometry and morphol. [on SciFinder (R)]