Figure 3.

Cumulative plots of tolerant, partially tolerant and intolerant sites in Lac repressor against the solvent accessibility. In the experiment (17), 12 or 13 mutations (depending on the identity of the wild-type residue) were tested at 124 sites. We defined the tolerance at each site as follows: tolerant, <5 of the mutations cause loss of function (45 sites); intolerant, >8 of the mutations cause loss of function (69 sites); and partially tolerant, 5–8 of the mutations cause loss of function (10 sites). The solvent accessibility was calculated using the program ASC (19) with a protein–DNA complex form (PDB:1EFA) or a tetrameric form (PDB:1LBI), depending on the site considered.