Steady-state protein levels of wild-type and mutant TEM-1 β-lactamases as determined by immunoblot of E. coli periplasmic protein. Positions are: 1, wt TEM-1; 2,L201P; 3, M182T; 4, M182T:L201P; 5, L76N; 6, L76N:M182T; 7, L76N:L201P; 8, R244A; 9, M182T:R244A; 10, L201P:R244A; 11, I47Y:E48C; 12, I47Y:E48C:M182T; 13, I47Y:E48C:L201P; 14, H158S:V159S:T160H; 15, H158S:V159S:T160H:M182T; 16, H158S:V159S:T160H:L201P.