The digestion was performed on β protein alone or in complex with DNA as
formed by the sequential addition of two complementary 33-mer oligonucleotides.
A, time course of limited trypsin digestion. The text box
shows for each of the major fragments the mass measured by LC-MS, the mass calculated from the
amino acid sequence, the results from N-terminal sequencing, and the corresponding residues of
β protein. Notice that formation of the DNA complex confers protease
resistance to residues 135–230 of β protein, as seen by the
appearance of bands B, C, and D. B, limited digestion with trypsin,
chymotrypsin, subtilisin, and thermolysin for 30 min in the presence and absence of DNA. The
identities of each of the fragments produced were determined by mass spectrometry and are
presented in supplemental Fig. S2.