Steady-state levels of wild-type and mutant β-lactamases from a
wild-type strain of E. coli (XL1-Blue). Lane 1, control
of purified wild-type β-lactamase; lanes 2, 4, 6, and 8, whole cell
extracts; lanes 3, 5, 7, and 9, periplasmic extracts; lanes 2 and 3,
wild-type; lanes 4 and 5, L76N; lanes 6 and 7, M182T; and lanes 8 and
9, L76N/M182T. Total β-lactamase was determined from whole-cell
extracts boiled in sample buffer; periplasmic extracts were prepared as
described in Materials and Methods.