To assess the relative importance and the degree of influence of each substitution on bacterial interactions, degrees of receptor recognition were assigned several categories as shown in Fig. 10A . The data show that substitution I91A has a profound effect on the interactions of all strains and whilst the side chain of Ile-91 is best suited to all, Leu can effectively substitute for Ile for several strains, whilst the polar Thr is less well tolerated. However, some Nm Opa proteins can also tolerate I91T substitution. Tyr-34 is also required in most cases. Interestingly, Phe-34 is preferred in general at this position over the native Tyr. Substitutions at Gln-89 have a delete- For the native receptor, expression levels and bacterial binding correlated more frequently than for the mutated molecules. In the case of substitutions at position 89, bacterial binding could be seen at very low levels of receptor expression (arrows in Q89A and Q89N panels). The reverse was the case with I91A, where even at high receptor levels (arrowhead, left panel), bacterial binding (arrowhead middle panel) was relatively low. In sampling of 100 cells, c. 50% exhibited this phenomenon. Note that the receptor recognition by Kat4C was not affected by mutations in the N-domain as determined by immunoblotting or by immunofluorescence microscopy (Fig. 7) .